General Histopathological Features

Light microscopic examination of hematoxylin and eosin-stained sections is the basis of most classification and grading systems for nervous system tumors. The approach to nervous system neoplasia follows the standard approach to classifying human tumors from all parts of the body; thus, many neuropathological terms come from the discipline of general pathology. This lexicon includes the terms most frequently used in clinical practice.

Palisading

Palisading, or lining up of tumor cells, is characteristic of schwannomas, in which collections of palisaded nuclei are termed Verocay bodies. The term describes a collection of neoplastic cells arranged in parallel arrays, or palisades. In glioblastomas, palisading (also known as pseudopalisades) occurs around necrosis, with serpiginous zones of necrosis lined by hypercellular viable tumor nuclei (Fig. 52B.1).

Fig. 52B.1 Glioblastoma. Nuclear pseudopalisading surrounding foci of central necrosis (hematoxylin-eosin stain, ×100).

Rosettes

The term rosette is confusing because of its application to several histological structures. The two most commonly encountered rosettes are Homer Wright rosettes and perivascular rosettes. A Homer Wright, or neuroblastic, rosette is a ring of cells surrounding neuropil (i.e., delicate fibrillary processes) and represents axon formation by primitive neuronal elements. Because these Homer Wright rosettes are identical to those encountered in the neuroblastomas of the peripheral nervous system, their presence is often taken as evidence of neuroblastic differentiation. They are associated with primitive neuroectodermal tumors (PNETs) such as medulloblastoma and pineoblastoma, where they are typically a focal finding. Sometimes, the terms central or cerebellar neuroblastomas are used in reference to CNS tumors with extensive Homer Wright rosette and neuropil formation.

Characteristic of perivascular rosettes is a peripheral ring of tumor nuclei surrounding a central blood vessel with a nuclear-free eosinophilic zone in between (Fig. 52B.2). The nuclear-free zone derives from tapering cellular processes that radiate from the tumor cells to the vessel. Perivascular rosettes are most typical of ependymomas, and glial fibrillary acidic protein (GFAP) immunostains highlight the perivascular processes. Occasionally, similar axon-bearing structures appear in neuronal tumors as well (e.g., neurocytomas, pineocytomas, various forms of PNET).

Fig. 52B.2 Ependymoma. Perivascular pseudorosette characterized by a fibrillary-appearing perivascular nuclear-free zone (hematoxylin-eosin stain, ×200).

Microvascular Proliferation

Microvascular (endothelial and pericytic/smooth muscle cell) proliferation represents an important and interesting response to tumor-associated and hypoxia-associated angiogenesis factors. It occurs in either benign or malignant CNS neoplasms, although it is more common in the malignant. In pilocytic astrocytoma, the new vessels may take on a delicate glomeruloid or “wicker-works” configuration, usually with a single-layered flat endothelial lining. In malignant tumors such as glioblastoma, these vessels are also glomeruloid but additionally display multilayering, with enlarged or “hyperplastic” endothelial cells (Fig. 52B.3). In either case, these neovascular structures lack the tight junctions normally contributing to the blood-brain barrier. Therefore, they typically leak intravascular contrast material, thus accounting for the tumor-associated enhancement detected radiologically.

Fig. 52B.3 Glioblastoma. Focus of endothelial hyperplasia with glomeruloid multilayered vessel (hematoxylin-eosin stain, ×400).

Frozen Sections and Touch Imprints/Smears

Intraoperative frozen sections are more difficult to interpret than fixed tissue sections, but their use has the advantage of speed, requiring only a few minutes to prepare. This technique is requested for several reasons, including simple curiosity and a desire to provide rapid feedback to patients and their families. However, the most important reasons for performing this technique are to ensure that the pathologist obtains a representative sample for permanent sections and to provide information that may alter the surgical procedure. For example, a neurosurgeon may opt to stop at a limited biopsy for a suspected lymphoma, aggressively resect for an ependymoma, or send additional material to the microbiology laboratory for an abscess. Because the process of tissue freezing produces significant cytological artifacts, another popular technique has been the preparation of touch imprints, or smears, from fresh tissue. Although the underlying architecture is lost, the tissue requirements are minimal, and the cytological preservation is excellent (Fig. 52B.4). This may be critical for diagnoses based primarily on nuclear cytology, such as reactive gliosis versus low-grade diffuse glioma. It is also important to realize that the frozen section technique results in substantial tissue loss and morphological artifacts in the thawed residual specimen. Therefore, it is best to omit its use in limited biopsies for which the surgical procedure will not be altered. In other words, it is imperative to save at least some optimally fixed nonfrozen tissue for final diagnosis on sections from the fixed embedded material.

Fig. 52B.4 Diffuse astrocytoma on an intraoperative smear. Cytological preservation is superior to that of a typical frozen section, and cytoplasmic processes are easier to discern. The oval- to spindle-shaped hyperchromatic nuclei are typical of diffuse astrocytoma (hematoxylin-eosin stain, ×400).

Electron Microscopy (Ultrastructural Pathology)

Electron microscopy has been virtually supplanted by immunohistochemistry in most pathology laboratories. Nevertheless, it can be extremely valuable in the diagnosis of certain tumors and remains the gold standard for a few. For example, it may be the only method capable of proving ependymal differentiation. Ultrastructural pathology is used to gain insight into forms of differentiation primarily by visualizing specific organelles and other components of the cytoplasm and cell surface (e.g., intermediate filaments, neurosecretory granules, synapses, pinocytotic vesicles, intercellular junctions, cilia, microvilli, basement membrane) and is therefore of particular utility in the diagnosis of poorly differentiated neoplasms.

Immunohistochemistry

Immunohistochemistry is an ancillary diagnostic technique for detecting protein expression within tumor nuclei, cytoplasm, and cell membranes. It is used most commonly to determine lines of cellular differentiation, proliferation indices, oncogene overexpression, or losses of tumor suppressor expression. Monoclonal antibody technology and recently improved antigen-retrieval methods have greatly expanded the versatility of this technique in routine formalin-fixed paraffin-embedded tissue, although a number of pitfalls remain, and considerable experience is necessary to make accurate determinations. Commercial antibodies vary greatly in terms of sensitivities, specificities, and clinical utilities. Most laboratories use the immunoperoxidase staining technique with horseradish peroxidase and a brown diaminobenzidine dye because staining is permanent and the reaction is visible by conventional light microscopy. The application of several antibodies is usual to study the more common CNS tumors.

Methods of Assessing Cell Proliferation

The simplest and least expensive method for estimating cellular proliferation is the mitotic index, usually expressed as either the average or the maximal attainable count per 10 high-power fields (HPF). Difficulties stem from pyknotic nuclei mimicking mitoses in poorly preserved specimens, variable field sizes among microscopes, and lack of uniform methods for counting. Underestimates of the proliferation index are common as well, given that the cells undergoing mitosis represent only a small fraction of cycling cells within a tumor.

Most proliferation antigens are nuclear constituents that are manifested during one or more proliferative phases in the cell cycle, allowing the recognition of cycling cells by routine immunohistochemistry. These markers are already valued in the diagnosis and management of brain tumors. The monoclonal antibody, Ki-67 (also known as MIB-1), binds to a human nuclear protein in the growth fraction (G1, S, G2, and M phases of the cell cycle). In most tumor types, the Ki-67 or MIB-1 labeling index increases with increasing degree of malignancy.

Molecular Diagnostics

As stated, tumors arise when alterations occur in key growth regulatory molecules. The expectation is that measurements of such molecular alterations will change the way tumors are classified. Molecular changes are detectable at the genomic DNA, messenger ribonucleic acid (mRNA), and protein levels. Some tumors are associated with “signature” cytogenetic abnormalities such as chromosomal translocations and deletions. However, abnormalities at the submicroscopic or molecular levels (e.g., gene rearrangements, single-gene defects) that may affect oncogenes and tumor suppressor genes are undetectable by routine karyotyping (conventional cytogenetics). The most common and practical approaches to detect deletions of chromosomal regions or specific tumor suppressor genes include loss of heterozygosity (LOH), fluorescence in situ hybridization (FISH), array comparative genomic hybridization (aCGH), and quantitative polymerase chain reaction (PCR) techniques. For detection of oncogene amplification, FISH and quantitative PCR are the most practical methods. Thus far, no specific translocations of fusion transcripts associated with primary CNS neoplasms exists, as is the case in hematopoietic and soft-tissue tumors.

Techniques such as aCGH and oligonucleotide or cDNA expression profiling are useful for surveying entire genomes for abnormalities. However, the primary use for these techniques is research rather than diagnostic pathology.

Few molecular diagnostic tests have become routinely used or standard of care in neuro-oncology. The most notable and most widely used is chromosome 1p and 19q testing as a prognostic/management tool for patients with oligodendroglial tumors. FISH has the advantage of simplicity, morphological preservation, minimal tissue and purity requirements, and a lack of necessity for microdissection or matching blood/nonneoplastic tissue. However, it requires some experience for accurate interpretation, especially in cases with aneuploid populations of tumor cells. It also utilizes large probes (100–300 kb) and is insensitive to very small deletions. In selected cases, FISH has been used to evaluate epidermal growth factor receptor (EGFR) amplification to distinguish small cell glioblastoma from anaplastic oligodendroglioma. It is likely that many more applications of molecular diagnostics will become incorporated into diagnostic neuropathology laboratories in the near future, but it is also important to note that specific immunohistochemical assays may “substitute” for molecular diagnostic approaches in certain situations. For example, immunohistochemical absence of the product of the INI1 gene on chromosome 22q is now widely accepted as a sensitive and specific marker for atypical teratoid/rhabdoid tumors (AT/RTs) (Judkins et al., 2004). Similarly, because IDH1 is a frequently mutated gene in lower-grade gliomas and secondary glioblastoma (Yan et al., 2009), antibodies to the most common mutant form of the IDH1 protein have demonstrated clinical utility in the pathological assessment of infiltrating glioma (Camelo-Piragua et al., 2009).

Diffuse Astrocytoma (WHO Grade II)

Referred to loosely as well-differentiated, low-grade, diffuse, grade II, or simply astrocytoma, this slow-growing tumor regularly undergoes malignant progression to anaplastic astrocytoma and glioblastoma. Despite often-quoted survivals of 5 to 10 years, a wide range exists among individual patients. The median age at the time of diagnosis is approximately 35 years, and young patients have a considerably better prognosis than older patients. Radiographically, these tumors are ill-defined nonenhancing cerebral masses. A much smaller proportion of diffuse astrocytomas occur in the spinal cord, cerebellum, and brainstem. Brainstem gliomas deserve special mention because these tumors typically present in childhood, expand the pons, and have a uniformly poor prognosis regardless of the histological grade.

Grossly, diffuse astrocytomas are poorly circumscribed tumors in which the CNS parenchyma variably expands, but the overall disruption of anatomy is minimal. Given its diffusely infiltrative nature and general lack of solid mass formation, large portions of the tumor may appear invisible to the naked (or radiographic) eye (i.e., microscopic disease). Also, this infiltrative growth pattern makes therapy a challenge because it has not been possible to treat microscopic foci of disease adequately with focal therapies such as surgery and radiation. Particularly widespread infiltration involving multiple lobes and even the brainstem (gliomatosis cerebri) is rare and tends to have a poor prognosis regardless of histological grade.

Microscopically, a slight to moderate increase in cellularity occurs, and the primary distinction from reactive astrocytosis rests on the finding of cytological atypia. Diffuse astrocytoma cells can take on various morphological appearances, including fibrillary, gemistocytic, and protoplasmic types. Fibrillary morphology is most common and consists of irregular elongated hyperchromatic nuclei, either appearing “naked” in an otherwise fibrillary background or displaying discernible cytoplasmic processes (see Fig. 52B.4). GFAP immunostains highlight the latter, although this is neither absolutely sensitive nor specific, because (1) some astrocytomas harbor minimal quantities of GFAP-positive cytoplasm, (2) interpretation may be difficult owing to staining of nonneoplastic astrocytic elements or high background in general, and (3) other gliomas display GFAP immunoreactivity. By definition, grade II astrocytomas lack mitotic activity, microvascular proliferation, and necrosis. Ancillary staining for MIB-1 (Ki-67) generally reveals a low proliferative index as well.

Recently, a large proportion of low-grade gliomas (including 90% of grade II astrocytomas) were found to have mutations in IDH1 and IDH2, even though the mutation was initially described in 12% of (largely secondary) glioblastomas (Parsons et al., 2008). IDH1 encodes isocitrate dehydrogenase, a component of the citric acid cycle. The role of IDH1 in glioma genesis remains uncertain, although recent work suggests that this loss of function mutation may cause accumulation of substrate, triggering HIF signaling pathways (Zhao et al., 2009). Others have proposed that tumorigenesis may be a consequence of 2-hydroxyglutarate formation (Dang et al., 2009). IDH1 mutations in low-grade glioma are tightly correlated with earlier age at diagnosis and tumor grade, appear to confer prognostic benefit beyond tumor grade, and are frequently associated with O⁶-methylguanine-DNA methyltransferase (MGMT) methylation (Sanson et al., 2009).

Chromosome 17p losses and mutations of TP53 are common molecular genetic events in diffuse astrocytomas, seen in approximately half of cases (Louis, 2006). Those with gene mutation usually display strong nuclear immunostaining for p53 protein, although the association is imperfect, since the protein is stabilizable by other mechanisms, and many immunopositive cases lack TP53 mutations. Those cases that evolve to glioblastoma (secondary glioblastoma) typically retain evidence of TP53 mutation and may even show additional TP53 gene mutations or clonal expansion of the tumor cells with mutation. On the other hand, de novo, or primary, glioblastomas rarely display such mutations. Other common alterations involve the overexpression of platelet-derived growth factor receptor alpha (PDGFα) in astrocytomas of all grades and losses of chromosome 22.

Anaplastic Astrocytoma (WHO Grade III)

The clinical presentation and radiographic features of anaplastic astrocytoma are similar to those of grade II astrocytoma, except that the mean age at presentation is approximately a decade later, and some cases show contrast enhancement. Average survival is reduced to 3 years, although as with grade II astrocytoma, great individual variability exists; patient age is one of the most powerful prognostic variables.

Histologically, anaplastic astrocytomas are more cellular than grade II astrocytomas and display a greater degree of proliferation. The presence of mitotic figures primarily defines these tumors. One astrocytoma variant that commonly presents at the anaplastic or grade III level is the gemistocytic (“stuffed cell”) astrocytoma. Characterized by strongly GFAP-positive cells with eccentric bellies of eosinophilic cytoplasm, this tumor type has a high incidence of progression to glioblastoma. Interestingly, the small-cell astrocytoma elements in the background primarily constitute the proliferating element. Another feature that may help distinguish anaplastic astrocytomas from grade II astrocytomas is the generally higher Ki-67 or MIB-1 proliferative index on immunostains (Giannini et al., 1999a).

Anaplastic astrocytomas share the high frequency of TP53, IDH1, and IDH2 mutations seen in grade II astrocytomas. Inactivation of the retinoblastoma protein (Rb) cell-cycle regulatory pathway is also common and may be primarily responsible for the increased proliferation observed histologically. Homozygous deletion of the CDKN2A/p16 gene on 9p is the most common mechanism for disabling this pathway, although deletions of the RB gene and CDK4 amplifications are alternative aberrations (Louis, 2006). Amplifications of the EGFR gene and losses of PTEN on 10q occur less frequently in anaplastic astrocytomas than in glioblastomas and are virtually absent in tumors containing an IDH1 mutation (Sanson et al., 2009). When present, EGFR amplification may suggest either a worse prognosis or the possibility of undersampling/undergrading in a glioblastoma (Smith et al., 2001).

Glioblastoma (WHO Grade IV)

Glioblastoma (previously known as glioblastoma multiforme) is the most common malignant primary brain tumor in adults, accounting for approximately 50% of all gliomas. The peak age at onset is 50 to 60 years, roughly a decade later than anaplastic astrocytoma. Unfortunately, several decades of basic and clinical research have had little impact on the clinical outcomes associated with glioblastoma, and the average survival remains at approximately 1 to 1.5 years following radiation therapy and chemotherapy.

Glioblastoma most commonly occurs in the deep white matter, basal ganglia, or thalamus and is rarely found in the cerebellum or spinal cord. The gross and microscopic appearance is heterogeneous. Replacing the affected portion of the brain is a single mass that grossly may appear deceptively well circumscribed but microscopically infiltrates widely, often spreading to the opposite hemisphere via the corpus callosum (butterfly lesion). Multifocal tumors may occur and in most cases likely represent separate regions of malignant transformation within a widely disseminated lower-grade astrocytoma such as gliomatosis cerebri. In advanced stages, the tumor may extend into the meninges or the ventricle. Seeding of the neuraxis as multiple implants on the brain or ventricular surfaces is an atypical growth pattern, and extracranial metastases are extremely rare. The cut surface has a variegated appearance (Fig. 52B.5) characterized by central yellow or white zones of necrosis and hemorrhage surrounded by a hyperemic ring (endothelial hyperplasia) and “edematous” brain with variable mixtures of vasogenic edema, gliosis, and tumor infiltrates. Microscopically characterizing glioblastomas are all the features of anaplastic astrocytoma plus endothelial hyperplasia or necrosis. The endothelial hyperplasia is thickened or glomeruloid vessels with multilayering (see Fig. 52B.3), representing a form of tumor-induced angiogenesis, a potential target for novel therapies. The necrosis is often associated with a characteristic serpiginous distribution and associated nuclear pseudopalisading (see Fig. 52B.1). Several histological variants exist, including giant cell glioblastoma, small cell glioblastoma, and gliosarcoma. Characteristic of the latter is a sarcomatous element, currently felt to arise most likely from mesenchymal metaplasia within a glioblastoma. No significant clinical differences are identifiable in these variants when compared with conventional glioblastoma, although gliosarcomas more frequently occur in the temporal lobes.

Fig. 52B.5 Glioblastoma (cut in axial imaging plane). This patient’s right hemispheric glioblastoma demonstrates central necrosis, a deceptively demarcated hyperemic rim (endothelial hyperplasia), and an infiltrative component that extends well beyond the grossly discernible margins (note expansion of right thalamus, with blurring of gray/white junctions).

Glioblastomas are among the best genetically characterized CNS tumors and often harbor numerous cytogenetic and molecular genetic aberrations (Louis, 2006). Mutations of TP53, IDH1, and IDH2 are less common than in diffuse astrocytomas or anaplastic astrocytomas and occur more often in secondary glioblastomas. Arising in younger patients after malignant transformation of a lower-grade precursor, TP53 mutations occur at greater frequency in giant cell glioblastomas (Louis et al., 2001). In contrast, 30% to 40% of glioblastomas harbor EGFR amplifications, often with an associated constitutively activating mutation, and these occur mainly in the primary, or de novo, glioblastoma (Louis et al., 2001). Defining the latter is the lack of a precursor lesion, typically in an older patient with rapid clinical onset. EGFR amplifications are also particularly common (≈70%) in the small-cell variant. These tumors are composed of small round cells with minimal cytoplasm and a remarkably brisk mitotic index. Owing to the rounded, uniform nuclei, a misdiagnosis of anaplastic oligodendrogliomas may occur, although small cell glioblastoma share the demographic features and poor prognosis of glioblastomas in general and do not harbor the 1p and 19q deletions seen in oligodendrogliomas. Alterations of the p16/CDK4/RB pathway are nearly universal in glioblastoma, and many cases harbor chromosome 10 losses (mostly primary glioblastomas). The PTEN gene located on chromosome 10q23 mutates in a subset of these and may have prognostic significance in certain patient populations. Additional tumor suppressor genes on both the long and short arms of chromosome 10 are suspected in the remaining group of PTEN wild-type tumors with demonstrable LOH. The list of additional glioblastoma-associated alterations is growing rapidly. Recent large-scale sequencing studies have shown that glioblastomas have somatic mutations of multiple genes (Parsons et al., 2008; TCGA Network, 2008). The studies have confirmed our knowledge of the incidence of mutated and/or amplified genes such as EGFR, PTEN, RB1, and PIK3CA and showed higher prevalence for mutations in TP53, NF1, ERBB2, and PIK3R1 (TCGA Network, 2008) as well as new candidate genes involved in tumorigenesis, such as IDH1 (Parsons et al., 2008). Nevertheless, the focus of intensive research is on mechanisms of invasion, cellular migration, proliferation, angiogenesis, apoptosis deregulation, and therapeutic resistance (Louis, 2006).

Early evidence suggests that molecular analysis of glioblastomas may have prognostic and predictive relevance. For example, methylation of the MGMT gene promoter, leading to down-regulation of MGMT expression, appears prognostic of improved survival in patients with glioblastoma treated with the alkylating agent, temozolomide (Hegi et al., 2005). However, emerging data suggest that prognostic benefit associated with MGMT methylation in glioblastoma may be unrelated to the mode of therapy administered (Rivera et al., 2010). Furthermore, resistance to alkylating chemotherapy in glioblastoma is associated with therapy-related inactivation of MSH6, a DNA mismatch repair enzyme, allowing increased mutagenesis and the emergence of a hypermutated phenotype (Cahill et al., 2007; Yip et al., 2009).

Molecular approaches to global-expression profiling potentially provide powerful means to distinguish glioblastomas from other malignant gliomas that follow distinct clinical courses (Nutt et al., 2003). Moreover, gene-expression profiling of glioblastoma has progressed from augmenting information provided by histological examination to characterization of molecular subtypes based on different gene expression patterns in each group (Sulman et al., 2009). For example, The Cancer Genome Atlas (TCGA) Research Network published a molecular classification of glioblastoma based on expression of signature genes (Verhaak et al., 2010). Of the four subgroups identified (named based on prior work and genes expressed as proneural, neural, mesenchymal, and classic subtypes), the proneural group showed distinctive clinical characteristics including overrepresentation among younger patients, mutations in IDH1, and prior or concomitant areas with low-grade histology—indicating that these are secondary glioblastomas (Verhaak et al., 2010). There also appears to be a CpG island methylator phenotype (CIMP) of glioma belonging to the same proneural molecular subgroup that is common among low-grade gliomas, with a high proportion showing mutations in IDH1 (Noushmehr et al., 2010). While perhaps allowing more coherent selection of potential targets for molecular therapy and validating the role of surrogate markers such as IDH1 for identifying prognostically significant subgroups of glioma, molecular profiling remains largely investigative to date.

Circumscribed (“Favorable Prognosis”) Astrocytomas

Pilocytic astrocytoma, pleomorphic xanthoastrocytoma (PXA), and subependymal giant cell astrocytoma (SEGA) are examples of circumscribed or “favorable prognosis” astrocytomas that grow slowly, have relatively discrete demarcation, and are associated with significantly better prognoses than the diffuse astrocytomas. These tumors are more common in children and young adults.

Pilocytic Astrocytoma (WHO Grade I)

Usually, pilocytic astrocytomas are well-circumscribed tumors, both grossly and radiographically, although limited degrees of infiltration are usual microscopically. They are more common in the cerebellum, hypothalamus, third ventricle, optic nerve, spinal cord, and dorsal brainstem but may also involve the cerebrum. Nonspecific clinical terms such as cerebellar astrocytomas, optic nerve glioma, and tectal glioma generally refer to pilocytic astrocytomas but should be avoided because diffuse gliomas can also present in these locations. Likewise, the adjective juvenile often added to pilocytic astrocytoma is misleading because most adult cases are histologically and clinically indistinguishable from their pediatric counterparts. Outcome depends on the surgical accessibility of the tumor but is usually excellent (80% 20-year survival), and many are curable with resection alone. The gross appearance varies somewhat with anatomical location. Cerebellar tumors, which are often hemispheric, are typically composed of a large fluid-filled cyst with an enhancing mural nodule. Hypothalamic and optic nerve tumors are usually solid. Optic nerve gliomas appear as a focal segmental nerve swelling. Both unilateral and bilateral optic nerve gliomas are particularly common in NF1 patients, a setting in which most tumors are indolent and do not progress to a point requiring surgical intervention.

The distinctive histological feature of pilocytic astrocytoma is a biphasic pattern with compact pilocytic areas interspersed with microcystic, spongy, or loose areas (Fig. 52B.6). Dense portions contain piloid (hairlike) or bipolar astrocytes with long spindle-shaped processes. Rosenthal fibers are common. These are masses of intracellular astrocytic filaments, fusiform or corkscrew shaped, with a hyaline appearance (see Fig. 52B.6, A). In addition, mulberry-shaped eosinophilic granular bodies also occur in most cases (see Fig. 52B.6, B). Although not entirely specific, both Rosenthal fibers and eosinophilic granular bodies are generally signs of an indolent process and represent important diagnostic clues that distinguish pilocytic from diffuse astrocytomas. NF1 gene inactivation occurs in NF1-associated pilocytic astrocytomas but not in sporadic examples. A KIAA1549:BRAF fusion gene at 7q34 characterizes the majority of pilocytic astrocytomas (Jones et al., 2008). The fusion protein is a constitutively active mutant BRAF isoform with activity levels similar to or higher than wild-type BRAF, with demonstrable transforming potential in transfected cells.

Fig. 52B.6 Pilocytic astrocytoma. Classic examples are biphasic with dense (A) and loose (B) regions. Helpful markers include bright red corkscrew-shaped Rosenthal fibers (A) and mulberry-shaped eosinophilic granular bodies (B) (hematoxylin-eosin stain, ×200).

Pilomyxoid astrocytoma is a variant of pilocytic astrocytoma, with more aggressive behavior corresponding to WHO grade II. This tumor mainly affects children younger than 3 years of age and occurs predominantly in the hypothalamic region. The tumor lacks a biphasic appearance and Rosenthal fibers but shows piloid cells, often with an angiocentric arrangement in a myxoid matrix. Similarly, eosinophilic granular bodies are often rare or absent. On re-resection years later, these tumors may show features of classic pilocytic astrocytoma, a phenomenon presumed to represent “maturation.”

Oligodendroglioma (WHO Grade II or III)

As opposed to astrocytomas, most of which are already high grade at the time of diagnosis, oligodendrogliomas commonly present at grade II. These tumors typically occur in young to middle-aged adults and are uncommon in children. Most are hemispheric masses, with the frontal lobe representing a favored location. The prognosis for grade II oligodendrogliomas is significantly better than for astrocytomas, with average survival times of 10 years or more and improved chemosensitivity profiles. As with astrocytomas, individual variability is substantial in time to progression and overall survival.

The classic and uniformly accepted oligodendroglial features include uniformly round nuclei, bland chromatin, clear perinuclear haloes imparting a “fried egg” appearance, and a rich branching capillary network reminiscent of “chicken wire” (Fig. 52B.7). Less specific findings include cortical involvement, microcalcifications, mucin-rich microcystic spaces, and perineuronal satellitosis. Although helpful, the so-called fried-egg appearance is a formalin fixation artifact that is neither necessary for diagnosis nor encountered in frozen sections or rapidly fixed specimens. The morphological spectrum includes two strongly GFAP-positive cells: minigemistocytes (or microgemistocytes) and gliofibrillary oligodendrocytes. The former are gemistocyte-like cells with small bellies of eosinophilic cytoplasm, round bland nuclei resembling those of classic oligodendroglioma nuclei, and no cytoplasmic processes. The latter are histologically identical to classic oligodendroglioma cells but exhibit a thin perinuclear rim of GFAP immunoreactivity. These two cell types (1) occur commonly in otherwise classic-appearing oligodendrogliomas, (2) are reminiscent of normal GFAP-positive oligodendroglial precursor cells, and (3) do not impact negatively on prognosis (although the presence of numerous microgemistocytes should prompt a careful search for anaplastic features). Hypercellularity, numerous mitoses, and microvascular proliferation defines anaplastic oligodendroglioma (grade III). Some oligodendrogliomas have regions that are histologically similar to diffuse astrocytoma, which can cause diagnostic difficulty because currently no specific oligodendroglioma markers exist.

Fig. 52B.7 Oligodendroglioma. Cells have uniform round nuclei with bland chromatin and a clear perinuclear halo, producing a “fried-egg” appearance. The rich, branching capillary network has been likened to chicken wire (hematoxylin-eosin stain, ×200).

The genetic characterization of oligodendrogliomas has provided some of our most important clues thus far in the diagnosis and treatment of diffuse gliomas. A characteristic loss of chromosomal arms 1p and 19q occurs in 50% to 80% of cases. More importantly, this genetic signature has been associated with both prolonged survival and a favorable response to chemotherapy or radiation therapy in both anaplastic (WHO grade III) (Cairncross et al., 1998, 2006; Van den Bent et al., 2006) and low-grade (grade II) oligodendrogliomas (Kaloshi et al., 2007). Based on these findings, ancillary testing for 1p and 19q status is a common request by both clinicians and patients; many centers now perform genetic testing routinely (Abrey et al., 2007). The most commonly utilized techniques include FISH and LOH, each with advantages and disadvantages (Perry et al., 2003; Yip et al., 2008). A subset of 1p/19q co-deleted tumors display polysomies or evidence of polyploidy. Patients with these tumors have shorter progression-free survival, although the overall survival is not affected (Snuderl et al., 2009). IDH1 (and IDH2) mutations occur very at high frequency in oligodendroglioma (84%) and anaplastic oligodendroglioma (94%) (Yan et al., 2009). Additional progression-associated alterations, such as those commonly seen in astrocytomas, sometimes occur in anaplastic oligodendrogliomas as well and are typically associated with worse prognosis.

Oligoastrocytoma (WHO Grade II or III)

As a group, oligoastrocytoma patients have survival rates intermediate between those of astrocytomas and oligodendrogliomas, although there is great variability. Of all the gliomas, oligoastrocytomas remain the most difficult to define and are the most likely to receive discordant diagnoses from expert neuropathologists. Cases are most often characterized by the presence of some rounded nuclei, some irregular hyperchromatic nuclei, and many cells with intermediate or indeterminate features that are difficult to characterize (i.e., morphologically ambiguous). Nonetheless, these tumors are usually graded and treated in the same fashion as pure oligodendrogliomas.

Genetic studies of the biphasic variant, in which microdissection is feasible (Maintz et al., 1997), have identified the same alterations in both components, consistent with a monoclonal process rather than a collision tumor (i.e., coexistence of two neoplastic clones). Therefore, most biphasic oligoastrocytomas have genetically resembled either pure oligodendroglioma or astrocytoma, with only rare cases showing mixed patterns. One study suggested that WHO grade II and grade III oligoastrocytomas with an oligodendroglioma genotype (i.e., 1p and 19q loss) follow clinical courses similar to those of more typical oligodendrogliomas, whereas those with an astrocytic genotype (i.e., 17p loss) are more akin to astrocytomas in their behavior (Eoli et al., 2006).

Ependymoma (WHO Grade II or III)

Ependymomas comprise 4% of all brain tumors and are the third most common CNS tumors in children. They may occur at any age but are most frequent in the first decade; patients younger than age 3 years have a significantly worse prognosis. Ninety percent of tumors are in the brain, with an infratentorial site twice as common as a supratentorial site, and 10% are in the spinal cord, often in adults. The typical infratentorial ependymoma occupies the fourth ventricle. Noncommunicating hydrocephalus develops when tumors are large enough to obstruct the flow of cerebrospinal fluid (CSF). On the other hand, these tumors may be periventricular or may not have any obvious association with native ependyma (e.g., spinal cord, cerebral hemispheres).

Ependymomas are usually well-circumscribed masses that tend to compress rather than infiltrate the adjacent parenchyma. As such, some cases may be surgically curable, and the extent of resection constitutes a much more important prognostic variable in ependymomas than in diffuse gliomas. Cystic tumors more likely occur in the cerebrum. Ependymomas in contact with CSF pathways may seed the subarachnoid space and generate drop metastases in approximately 5% of cases, a condition associated with a poor prognosis. The characteristic features of ependymomas are sheets of cells interrupted by perivascular rosettes, nuclear free zones surrounding a central blood vessel (see Fig. 52B.2). True ependymal rosettes (i.e., containing a central lumen) and canals (i.e., slitlike structures resembling small ventricles) are even more specific but only occur in approximately 10% of cases.

Ependymoma is one of the few remaining tumors in which ultrastructural examination verifies the diagnosis in morphologically ambiguous cases. Electron microscopy shows a combined glial and epithelial-like appearance with intermediate filaments (GFAP), microvilli, zipper-like intercellular junctions, intracellular lumina, cilia, and their basal attachments, known as basal bodies or blepharoplasts. Immunohistochemistry for GFAP may be particularly helpful in highlighting the thin processes radiating toward vessels in pseudorosettes, and dot-like cytoplasmic inclusions may be highlighted with EMA, CD99, and D2-40 immunostains, although none of these markers are entirely specific. Establishing the diagnosis of anaplastic ependymoma (grade III) requires hypercellularity, increased mitotic activity, and microvascular proliferation. Regions of infarct-like necrosis are common in otherwise low grade–appearing tumors, and this is not a reliable grading criterion. In fact, the predictive value of histological grading of ependymomas is questionable because it has prognostic significance in some series but not in others.

Ependymomas are often aneuploid with complex, albeit nonspecific, alterations. Chromosome 22q deletions are among the most common, with associated NF2 mutations primarily restricted to spinal ependymomas, a fact that fits with the spinal location of most ependymomas in NF2 patients (Singh et al., 2002). Other 22q tumor suppressors are likely, and the existence of many additional ependymoma-associated genes is expected. Ependymomas may arise from radial glial cells (Taylor et al., 2005). Further molecular analysis of these tumors suggests that the gene-expression profile of ependymomas may differ according to site in the CNS, suggesting these are different entities at a molecular level (Taylor et al., 2005). In childhood tumors, 1q gain appears to be a frequent event and is possibly associated with aggressive behavior (Zacharoulis and Moreno, 2009). Gains at 9qter, specifically 9q33-44, appear associated with progression and up-regulation of the Notch pathway (Puget et al., 2009).

Myxopapillary Ependymoma (WHO Grade I)

Myxopapillary ependymoma is a distinct variant that is virtually restricted to the filum terminale. These tumors may also occur in the presacral soft tissue. Myxopapillary ependymomas are more common in adults than in children, tend to be red because of their rich vascularity, and are sometimes frankly hemorrhagic and gelatinous in their gross appearance. Typically surrounding the tumor is a thin collagenous capsule. These tumors have a variable papillary architecture, with numerous hyalinized vessels surrounded by mucin and an outside layer of tumor cells. The prognosis is excellent, particularly when the collagenous tumor capsule is intact intraoperatively. Tumors taken out piecemeal with mucin spillage have a higher likelihood of subsequent recurrence. Soft-tissue myxopapillary ependymomas may metastasize to the lungs or other sites, despite their benign appearance. Other tumors that may present as cauda equina masses include meningiomas, schwannomas, metastatic lesions (either hematogenous or via the CSF), and paragangliomas. Paraganglioma is a neuroendocrine neoplasm arising from the autonomic nervous system, rarely in the filum terminale but often in other systemic sites. Usually these tumors present in the cauda equina, with symptoms related to mass effect. Histologically they may mimic ependymoma and metastatic carcinoma but have a characteristic immunohistochemical profile (with chromogranin staining of the packeted cells and S100 positivity of the surrounding sustentacular cells), permitting accurate distinction. Complete excision is usually curative.

Subependymoma (WHO Grade I)

Subependymomas are slow-growing tumors often detected by MRI in the absence of clinical manifestations; 90% of subependymomas occur in adults, where they are small and incidental. Subependymomas appear as glistening, pearly white, lobulated, intraventricular protuberances, most often in the fourth ventricle. Large tumors may obstruct the ventricle and cause noncommunicating hydrocephalus. Clusters of bland rounded nuclei embedded in a fibrillary matrix with microcysts and foci of calcification characterize these tumors microscopically. The proliferative index is typically low. Rare cases contain foci of classic ependymoma; these are named mixed ependymoma/subependymoma. These cases are graded and treated according to the potentially more aggressive ependymoma component. The histogenesis of subependymomas is still a matter of debate, with candidates including subependymal glia, astrocytes, ependymal cells, or some mixture of these. Genetic changes are largely unknown.

Choroid Plexus Tumors

Choroid Plexus Papilloma (WHO Grade I)

Choroid plexus papillomas (CPP) comprise approximately 0.5% of all intracranial tumors. As a rule, the location of these tumors is solely in the portion of the ventricular system that contains normal choroid plexus. Approximately half are in the fourth ventricle, often occupying the cerebellopontine angle, typically in adults. Tumors in the lateral ventricle are more common in children and may cause hydrocephalus by a combination of outflow obstruction and excess CSF production. The onset of symptoms is usually in the first decade and may occur at birth (i.e., congenital). In most cases, papillomas are surgically curable tumors.

Choroid plexus papillomas have a pink or red, highly vascular, polypoid or cauliflower appearance, often with chalky calcifications (Fig. 52B.8). Large tumors in the third or fourth ventricle may occlude or even distend the ventricle, causing noncommunicating hydrocephalus. Histologically, CPP resemble normal choroid plexus in terms of a well-formed papillary structure with true fibrovascular cores and, in most cases, a single-layered epithelial covering. However, the lining of papillomas lacks the cobblestone-like appearance of normal choroid plexus and tends to form instead a uniform layer of tall cuboidal to columnar cells without intervening spaces. Calcifications and clear intracytoplasmic vacuoles are common. The mitotic index is low.

Fig. 52B.8 Choroid plexus papilloma. Polypoid or cauliflower-like lesion with a nodular and partially calcified surface, taken from the fourth ventricle.

Immunohistochemically, these tumors are consistently positive for cytokeratin, sometimes revealing a paranuclear ball-like pattern of staining. Immunoreactivity for transthyretin/prealbumin is also typical. A subset of CPP expresses GFAP focally; this reflects ependymal differentiation.

The majority of CPP are well-differentiated neoplasms (WHO grade I). The 2007 WHO classification also includes atypical CPP, which are distinguished from CPP by increased mitotic activity (more than 2 mitotic figures per 10 high-power fields). Other atypical histological features may be present but are not required for diagnosis (Louis et al., 2007).

Neuronal/Glioneuronal Tumors

Ganglioglioma/Gangliocytoma (WHO Grade I or II)

Most gangliogliomas occur before age 21 and comprise 4% to 8% of all pediatric brain tumors. Gangliogliomas grow slowly and tend to show benign biological and clinical behavior. The most common site of involvement is the temporal lobe, and seizures are a typical presentation. Other lobes of the cerebral hemispheres, cerebellum, and spinal cord are less often affected. The tumors are often cystic and well circumscribed, extending to the surface of the brain. The solid portions are firm, gray, and gritty due to calcium deposits that are evident on CT scans.

Portions of the tumor resemble a low-grade astrocytoma, either pilocytic or fibrillary in nature. Unlike native entrapped neurons within an infiltrative glioma, some of the tumor ganglion cells have a dysmorphic appearance, as evidenced by their lack of polarity, clustering, cytoplasmic vacuolation, increased nuclear pleomorphism, or multinucleation (Fig. 52B.9). Binucleate or multinucleate neurons are particularly helpful for diagnostic purposes when present. Otherwise, the most useful features to distinguish this tumor from diffuse gliomas include relative circumscription and eosinophilic granular bodies. Perivascular lymphocytic cuffing, microcystic spaces, and fibrosis with collagen deposition are other common findings. Rosenthal fibers are common, particularly at the edges of the lesion. Those without an obvious astrocytic component are sometimes referred to as gangliocytoma, although it is not yet clear that this distinction has any clinical relevance. The term ganglion cell tumor is less specific and incorporates both entities. Rare cases demonstrate signs of anaplasia (grade III), most often in the glial component, but the grading criteria and their predictive value have yet to be firmly established. GFAP activity is abundant in the astrocytic component, whereas the ganglion cell component expresses most markers of mature neurons such as synaptophysin, chromogranin, neurofilament, and NeuN. The molecular basis of these tumors remains poorly understood, with systematic investigation of altered gene expression hampered by their cellular complexity.

Fig. 52B.9 Ganglioglioma. Haphazardly arranged and clustered dysmorphic ganglion cells, including the binucleated form seen centrally (hematoxylin-eosin stain, ×400).

Central Neurocytoma (WHO Grade II)

Central neurocytomas are slow-growing tumors located in the lateral or third ventricle near the foramen of Monro, frequently involving the septum pellucidum (Schild et al., 1997). Age at diagnosis is usually in the second or third decade. They are usually sharply demarcated, sometimes lobulated masses that fill the ventricular space without significant infiltration of the surrounding brain.

The main histological feature is a proliferation of uniformly round tumor cells that mimic oligodendroglioma. Unlike oligodendrogliomas, these tumors often display neurocytic rosettes, exaggerated or irregular Homer Wright–like rosettes with central axon-rich neuropil. These rosettes are indistinguishable from the pineocytic rosettes encountered in pineocytoma, another tumor of small mature neurons, with similar cytological features to those of central neurocytomas. Ultrastructurally, their cytoplasm contains microtubules, synapses, and neurosecretory granules, belying their neuronal nature. Further, central neurocytomas are generally immunoreactive for the neuronal markers, synaptophysin and NeuN. Central neurocytomas with elevated proliferative indices (e.g., >2%) and vascular hyperplasia tend to have a higher rate of recurrence and are sometimes referred to as atypical neurocytomas, although grading criteria have not been firmly established. Rare examples of extraventricular neurocytomas and liponeurocytomas (with fat metaplasia) occur in the cerebral hemispheres, cerebellum, and spinal cord, but it is yet to be determined whether these represent the same family of tumors. Extraventricular neurocytoma is now recognized as a similarly behaving counterpart to central neurocytoma (Louis et al., 2007). The WHO also recognizes the cerebellar liponeurocytoma as a distinct entity (see later discussion). The genetic alterations of neurocytoma are largely unknown; as expected, central neurocytomas do not harbor the 1p and 19q deletions seen in oligodendrogliomas (Fuller and Perry, 2005), although rare extraventricular neurocytomas have been noted with 1p and 19q loss (Mrak et al., 2004). Further studies suggest this co-deletion may occur in as many as a quarter of the tumors and is associated with atypical histological features (Rodriguez et al., 2009). The relationship of these tumors to oligodendrogliomas with 1p/19q deletions remains to be clarified.

Embryonal Tumors/Primitive Neuroectodermal Tumors

The precise definition of primitive neuroectodermal tumor has long been debated, but essentially it refers to an extracerebellar “small blue cell tumor” that otherwise resembles medulloblastoma and in most cases shows evidence for primarily neuronal differentiation, albeit immature. Glial, mesenchymal, or melanotic elements may occur as well. Sometimes referred to as cerebellar PNET, medulloblastoma therefore represents the prototype and most common member of the PNET family. Adding further confusion to the nomenclature is the existence of a peripheral nervous system PNET (pPNET), which is felt to represent a completely different small blue cell tumor type with homologous genotypic and phenotypic features to those of Ewing sarcoma—t(11;22)(q24;q12) with EWS-FLI1 or variant fusion products. Other than medulloblastomas, the CNS variants are relatively uncommon and may include CNS PNET (cPNET), pineoblastoma, central neuroblastoma, ependymoblastoma, and medulloepithelioma. Despite the morphological and immunohistochemical overlap among these entities and the convenience of using the umbrella term PNET for all of them, this is likely overly simplified. We know, for example, that cPNETs have a significantly worse prognosis than medulloblastomas and seem to differ from them genetically (Reddy et al., 2000). Therefore, the overall heading of CNS primitive neuroectodermal tumors may be preferable to discuss this group of tumors and has been utilized in the 2007 WHO classification together with medulloblastoma and AT/RT to form the group of embryonal tumors.

Medulloblastoma (WHO Grade IV)

The name medulloblastoma is misleading because it is doubtful that any cell identifiable as a medulloblast exists during histogenesis. Instead, medulloblastomas likely arise either from the external granular layer (e.g., desmoplastic variant) or the subependymal matrix cells of the fourth ventricle (e.g., classic variant), or both. Much progress has been made in the treatment of medulloblastomas, with 5-year survival rates now as high as 70% to 80%.

Medulloblastoma is the most common form of PNET. More than 50% of medulloblastomas occur in children younger than age 10. A second smaller frequency peak occurs between ages 18 and 25. Medulloblastoma, by definition, originates in the cerebellum. It is generally well defined, soft, friable, and focally necrotic. Medulloblastomas have a proclivity to invade the ventricle and disseminate along CSF pathways. Affirmation of their potential aggressiveness are rare reports of metastases to bone, lymph nodes, and other extracranial sites.

Medulloblastomas consist of small immature cells with hyperchromatic round to carrot-shaped nuclei with minimal cytoplasm, as well as numerous mitoses and apoptotic bodies. These tumors typically display limited degrees of neuronal maturation, with neuropil formation, synaptophysin immunoreactivity, and occasional Homer Wright (neuroblastic) rosettes (Fig. 52B.10). Sometimes the terms cerebellar neuroblastomas or ganglioneuroblastomas apply to tumors with extensive rosette formation or neuronal maturation, respectively. The 2007 WHO Classification recognizes several medulloblastoma variants, some of which are clinically useful. These include the anaplastic/large-cell variant which shows widespread cellular anaplasia and has an aggressive clinical course with a high frequency of metastasis. The desmoplastic/nodular variant is recognized by the presence of pale nodular areas representing foci of neuronal differentiation surrounded by proliferating cells with hyperchromatic nuclei, which elaborate intercellular reticulin. When strictly applied, these histological features are associated with a better prognosis than “classic” medulloblastoma. Some evidence exists that the nodular or desmoplastic variant is encountered most frequently in adults, in the lateral cerebellar hemispheres, and in patients with Gorlin nevoid basal cell carcinoma syndrome (NBCCS) where there is loss of the tumor suppressor gene, PTCH, on chromosome 9q (see later discussion). The extensively nodular variant occurs in infants and features marked expansion of the pale areas described in desmoplastic medulloblastoma with elaboration of neuropil-like tissue. It is associated with much better survival than classic medulloblastoma (Eberhart et al., 2002).

Fig. 52B.10 Medulloblastoma. Tumor composed of small, poorly differentiated cells with Homer Wright (neuroblastic) rosette formation (hematoxylin-eosin stain, ×400).

Medulloblastoma with myogenic differentiation (previously medullomyoblastoma) and medulloblastoma with melanotic differentiation (previously melanotic medulloblastoma) have genetic alterations similar to classic forms of medulloblastoma and are no longer regarded as unique entities but rather as variant patterns of differentiation.

Familial forms of medulloblastoma have provided important clues regarding the inherited and sporadic forms of the tumor (Ellison, 2002). These include the hedgehog/patched signaling pathway implicated from studies of NBCCS and the APC/Wnt pathway associated with a form of Turcot syndrome (polyposis coli and brain tumors). Unfortunately, medulloblastoma appears to be a heterogeneous genetic tumor, with no single alteration accounting for the majority of cases. Cytogenetically, a characteristic isochromosome 17q occurs in one-third to one-half of cases, but mutations in the TP53 gene are rare. Both MYCC and MYCN gene amplifications have been associated with particularly aggressive medulloblastomas and encountered more commonly in the anaplastic/large-cell variant, whereas 6q loss or monosomy has been associated with Wnt pathway activation and a favorable prognosis (Northcott et al., 2010). Expression of the neurotrophin receptor, TrkC, is associated with a significantly better prognosis. Interestingly, genomic screening with expression profile microarrays that characterize thousands of genes simultaneously may be a useful method for predicting biological behavior (Pomeroy et al., 2002). Further work has suggested the presence of clinically relevant subgroups with anomalies and activation of different molecular pathways that may, as for glioblastoma, guide therapy in the future (Kool et al., 2008). Therefore, the routine diagnostic workup of medulloblastoma may eventually involve several histopathological and genetic techniques for further stratification and customized therapy.

Meningioma (WHO Grade I)

Meningiomas comprise 20% to 25% of all intracranial tumors. They are most prevalent after age 50. The female-to-male ratio is 2 : 1 in adults, nearly 10 : 1 in the spinal cord, and 1 : 1 in pediatric or malignant forms. Although most are benign (roughly 80%), a subset is aggressive with high-grade histology, high recurrence rates, or substantial morbidity and mortality. Even some of the histologically benign meningiomas are associated with disfigurement, neurological deficits, and major therapeutic challenges, particularly when located in sites at the skull base that prevent complete resection. Because growth is typically slow, recurrences many years after primary resection are common, and long follow-up times are required to determine the achievement of a surgical cure. Generally, the extent of surgical resection and histological grade represent the most important prognostic variables. For example, the 5-year recurrence rates are approximately 5% for gross totally resected versus 30% for subtotally resected benign meningiomas. In contrast, this figure rises to 40% in atypical meningiomas, even after gross total resection.

The locations of meningiomas, in descending order of frequency, are the cerebral convexity, parasagittal region, sphenoid wing, parasellar region, and spinal canal. Posterior fossa and lateral ventricle locations are more common in children. Multiple meningiomas suggest the possibility of NF2, although forms not associated with NF2 also occur. Interestingly, a number of such cases show identical mutations in each of the meningiomas arising from a single patient, suggesting that dural dissemination may account for multifocality in some patients, despite a histologically benign appearance.

Benign meningiomas demarcate well and compress rather than invade the adjacent brain or spinal cord. Nevertheless, bone and soft-tissue invasion may occur and is typically associated with hyperostosis. Notably, this type of invasion does not constitute evidence for malignancy, and those that are grossly totally resected share the same excellent prognosis as those without invasion. Meningioma en plaque is a pattern of diffuse carpet-like tumor spread along the dural surface. Meningiomas are generally firm in consistency and often gritty because of the presence of sandlike calcifications, referred to as psammoma bodies.

Meningiomas are histologically heterogeneous; the most recent WHO classification includes 13 morphological types (Louis et al., 2007). Four rare variants are considered more aggressive by definition: clear cell (grade II), chordoid (grade II), papillary (grade III), and rhabdoid (grade III). The other nine subtypes are considered benign unless they fulfill additional criteria for atypical (grade II) or anaplastic (grade IIII) meningioma. The majority of meningiomas have two basic histological patterns: meningothelial or fibroblastic, with the transitional variant having features of both. Meningothelial tumors are composed of arachnoidal epithelioid cells arranged in lobules, often with prominent whorls and psammoma bodies (Fig. 52B.11), which represent laminated calcifications of degenerated meningothelial whorls. Fibroblastic meningiomas are distinguished by their spindled appearance, fascicular or storiform architecture, and abundant collagen deposition. The most helpful immunohistochemical marker is EMA, which is detectable at least focally in the vast majority of meningiomas.

Fig. 52B.11 Meningioma. Typical meningothelial nests and whorls with focal hyalinization, the first step in the formation of concentrically laminated calcifications, or psammoma bodies (hematoxylin-eosin stain, ×400).

More than half of meningiomas are associated with losses of chromosome 22 or portions thereof. The tumor suppressor gene primarily involved in most of these cases is NF2, a finding that correlates well with the fact that meningiomas are the second most common tumor type in NF2 patients. A second gene with a high degree of homology, DAL1, or protein 4.1B on 18p11.3, has been implicated as well (Gutmann et al., 2000). Losses of both NF2 and protein 4.1B are common in meningiomas of all grades, suggesting that these alterations are early genetic events. Progesterone receptors are present in more than 50% of cases. Their significance is unclear, other than the fact that meningiomas can enlarge dramatically with pregnancy and regress after delivery.

Atypical Meningioma (WHO Grade II)

The intermediate-grade category of atypical meningioma defines a meningioma type that carries a considerable increased risk of recurrence, even after achieving gross total resection. These tumors are also associated with a slight but statistically significant increase in mortality when compared with age- and sex-matched controls. Atypical meningiomas account for 15% to 20% of all meningiomas.

In most pathology series, the mitotic or proliferative index is the most powerful predictor of outcome. Based on a large Mayo Clinic series, the presence of at least 4 mitoses/10 HPF, even focally, qualifies for the diagnosis of atypical meningioma (Perry et al., 1999). With fewer mitoses, the presence of at least three of five additional parameters (sheeting architecture, hypercellularity, macronucleoli, small cell formation, and necrosis) also suffices. The issue of brain invasion has been debatable; although once considered the ultimate manifestation of malignancy, recent studies suggest that in the absence of frank anaplasia, these tumors have similar recurrence and mortality rates as those of atypical meningioma. Similar to mitotic counts, MIB-1 (Ki-67) labeling indices may be helpful for predicting the risk of recurrence, particularly in borderline atypical cases. Meningioma grade is also inversely proportional to progesterone receptor expression, so that in general, fewer atypical meningiomas are progesterone receptor immunoreactive than their benign counterparts.

A growing number of cytogenetic alterations have been associated with malignant progression of meningiomas, though the genes are unidentified. Most common in atypical meningiomas are deletions of 1p, 6q, 10, and 14q (Weber et al., 1997). Recently, aCGH analysis has found 59% of tumors with 1q gain, and that 1q gain is associated with shorter progression-free survival (Gabeau-Lacet et al., 2009).

Anaplastic Meningioma (WHO Grade III)

With the omission of brain invasion as a criterion for anaplastic meningioma, these tumors have become quite rare, accounting for no more than 1% to 2% of all cases. Many of these tumors start as benign or atypical meningiomas and progress over time, although de novo presentations also occur. As a group, anaplastic meningiomas are highly aggressive, rapidly growing, and highly infiltrative, with a median survival of less than 2 years (Perry et al., 1999). Nevertheless, extent of resection remains important, and long-term survival is still possible in a subset of patients. Histologically, anaplastic meningiomas are defined by the presence of excessive mitotic activity (>20/10 HPF) and/or frank anaplasia with a carcinoma-like or sarcoma-like appearance. These tumors are highly cellular, with extensive sheeting, necrosis, and nuclear atypia. Often, lower-grade elements, more easily recognizable as meningioma, occur. For those lacking this feature, immunohistochemistry or electron microscopy is often necessary to exclude hemangiopericytoma or other tumors such as dural-based sarcoma, metastatic carcinoma, or melanoma.

Anaplastic meningiomas share the genetic features of lower-grade meningiomas but additionally harbor chromosome 17q gains/amplifications and 9p/p16 losses in many cases. In most, the MIB-1 (Ki-67) labeling index is markedly elevated, and there is no discernible progesterone receptor expression.

Hemangiopericytoma and Solitary Fibrous Tumor (WHO Grade II or III)

Hemangiopericytoma (HPC), once considered an angioblastic variant of meningioma, is now generally accepted to be a highly vascular dural-based sarcoma, analogous to those encountered in soft-tissue sites and of uncertain histogenesis. Solitary fibrous tumors (SFT) were initially characterized as primary mesenchymal neoplasms of the pleura but have in recent years been described in many extrapleural sites including the meninges. Although still debated, it has been suggested that HPC and SFT may represent ends of the same biological spectrum. It is clear, however, that tumors fulfilling criteria for HPC typically have high rates of local recurrence (60% to 80%) and systemic metastasis (25%). In contrast, SFTs of the meninges generally behave in a more indolent fashion, one study demonstrating excellent local control through surgical resection alone (Tihan et al., 2003). SFTs with malignant morphological features and aggressive behavior are rarely described. Both tumors occur at all ages, peaking in the fourth to sixth decades. Unlike meningiomas, there is no female predilection, and there is no association with NF2 or any of the known meningioma-associated genetic alterations.

Histologically, HPC is a highly cellular reticulin-rich neoplasm with numerous branching (staghorn) thin-walled vessels, an architectural feature also shared by SFT. HPCs are formed of densely packed hyperchromatic cells that are oval to spindled and display variable proliferative indices. Tumors with more than 5 mitoses/10 HPF or hemorrhage/necrosis are considered high grade (grade III), although even the low-grade (grade II) examples are considered malignant. Prototypic SFTs are less cellular tumors that have a so-called patternless pattern lacking characteristic architectural details. The tumors are composed of spindled cells with indistinct cell borders and finely dispersed chromatin, with much collagen deposition with dense collagen bands.

Both SFT and HPC are EMA negative but positive for CD34, which is characteristically described as diffusely strong in SFT (Perry et al., 1997). Bcl2 is described as strongly staining cells in SFT, but HPC frequently also shows positive signal.

Schwannoma (Neurilemoma) (WHO Grade I)

The frequency of schwannomas peaks in the fourth and fifth decades. Most are located on the vestibular portion of the eighth cranial nerve. Other cranial nerves, particularly the trigeminal, are much less frequent sites of involvement. Bilateral vestibular schwannomas (acoustic neuroma) are diagnostic of NF2. Vestibular schwannomas erode the internal auditory meatus and occupy the cerebellopontine angle; with increasing size, the tumor mass may compress and deform the pons. Spinal schwannomas comprise about 30% of intraspinal tumors. Most arise from the dorsal roots, preferring sensory nerves like their cranial counterparts. Spinal schwannomas may extend through the dura or, in some cases, through the intervertebral foramen as a dumbbell-shaped mass that is partly within and partly outside the spinal canal.

As opposed to neurofibromas, schwannomas are pure Schwann-cell proliferations, typically arranged in two architectural patterns. Cellular dense zones, known as Antoni A areas, contain spindle-shaped cells arranged in nuclear palisades, termed Verocay bodies. Antoni B areas are myxoid and microcystic in appearance, with thin wavy cells and foci of collagenization highly reminiscent of neurofibromas. Degenerative changes are common and include hemorrhage, cystic breakdown, vascular hyalinization, and calcification. As opposed to neurofibromas, schwannomas typically have a discernible capsule and push the parent nerve aside rather than invading it. Immunohistochemical studies reveal strong and diffuse S100 immunoreactivity, diffuse collagen IV positivity, reflecting the rich network of Schwann cell–associated basement membranes, and a relative lack of neurofilament-positive entrapped axons.

The vast majority of both sporadic and familial schwannomas are associated with loss of expression for the NF2 protein product, merlin or schwannomin (Stemmer-Rachamimov et al., 1997). NF2 gene deletions and LOH are common, though other mechanisms may also be involved. INI1 itself may be altered in familial schwannomatosis (Hulsebos et al., 2007). INI1 immunohistochemistry shows a mosaic pattern of staining in tumors from familial and sporadic schwannomatosis patients, as well as in schwannomas from NF2 patients, contrasting with strong uniform nuclear signal in sporadic tumors. These findings suggest a role for INI1 in multiple schwannoma syndromes (Patil et al., 2008).

Neurofibroma (WHO Grade I)

Neurofibromas are less commonly encountered centrally than schwannomas. Involvement of multiple spinal nerve roots is virtually pathognomonic of NF1. They more typically originate from nerve terminals in the dermis and from large nerve trunks such as the brachial plexus. Unlike the eccentric globular growth pattern of schwannomas, neurofibromas grow within the substance of a nerve, generating a fusiform intraneural mass. A plexiform (“bag of worms”) growth pattern results from the involvement of multiple nerve fascicles and is virtually diagnostic of NF1. On gross inspection, neurofibromas are typically gray and gelatinous.

Histologically, bundles of thin wavy cells with thin wavy nuclei suspend haphazardly in a myxoid or mucin-rich stroma. There are variable degrees of collagenization, depending on the age of the lesion, and the resulting hyaline silhouettes have been likened to “shredded carrots.” As opposed to schwannomas, neurofibromas are a mixture of cell types that includes not only Schwann cells but also fibroblasts, perineural-like cells, mast cells, and entrapped elements from the parent nerve. Most cases are hypocellular; therefore, foci of marked cellularity, increased cell size, and mitotic activity should raise concern for malignant transformation to MPNST. Immunostains demonstrate patchy S100 and collagen IV expression because only a portion of the intratumoral cells are Schwann cells. Entrapped neurofilament-positive axons are usually present except in dermal neurofibromas. The MIB-1 (Ki-67) index is generally low, and p53 protein is negative except in foci of transformation to MPNST.

Studies suggest that the Schwann cell is the neoplastic component within neurofibromas, with remaining cell types likely representing reactive or entrapped elements (Perry et al., 2001). Deletions of the NF1 gene and losses of its protein product, neurofibromin, are detectable in a subset of both familial and sporadic neurofibromas.

Central Nervous System Lymphoma

CNS involvement by lymphoma may be either primary or secondary. Secondary CNS involvement occurs in 5% to 29% of systemic non-Hodgkin lymphomas but is exceptional in Hodgkin disease. Systemic lymphomas tend to infiltrate the leptomeninges and spare the parenchyma. The epidural spinal space is a favored site, and spinal compression is a common complication. In contrast, primary CNS lymphoma (PCNSL), a type of extranodal non-Hodgkin lymphoma, typically presents deep in the brain parenchyma (e.g., periventricular) and usually spares the meninges, which may explain why the CSF cytological examination contains tumor cells in only a minority of patients. The incidence of CNS lymphoma increases in immunodeficient patients, such as patients with acquired immunodeficiency syndrome and organ transplant recipients. Such cases are typically associated with Epstein-Barr virus (EBV), and CSF PCR studies take advantage of this common finding. Primary CNS lymphoma has also increased in the elderly immunocompetent population for poorly understood reasons. These cases are generally not associated with EBV. Survival without treatment typically is less than 1 year, but prolonged survivals with methotrexate-based chemotherapy regimens are reported. Primary CNS lymphoma is unique in that approximately half are multifocal tumor masses that may disappear or regress after corticosteroid therapy, and they often recur in a completely different CNS site from that of the initial lesion.

Of non-HIV-associated PCNSL, 90% are diffuse large B-cell type, with the remaining 10% being poorly characterized low-grade lymphomas, Burkitt lymphomas, or T-cell lymphomas. These tumors show a distinctive angiocentric pattern in which malignant cells surround and invade blood vessels in concentric layers (Fig. 52B.12). Reactive T cells are often numerous as well. Cases from immunosuppressed patients are characteristically necrotizing and EBV immunoreactive, whereas those from immunocompetent patients are not. Biopsies from patients treated preoperatively with corticosteroids are a common source of frustration and diagnostic difficulty for pathologists because the tumor cells often die, leaving behind a process that resembles either an inflammatory or demyelinating disorder. In such cases, an accurate diagnosis may await the time of recurrence. In addition, because sometimes the cellularity is low in comparison to lymph node biopsies and the typically mixed infiltrate, wherein reactive T cells may outnumber tumor cells, flow cytometry is often less sensitive for establishing the diagnosis than routine histology and may waste a considerable amount of limited tissue. Immunohistochemical expression of the germinal-center B-cell subgroup surrogate marker, BCL-6, occurs at a lower frequency in primary CNS lymphoma compared to nodal lymphoma and may be a favorable prognostic marker (Lin et al., 2006). Gene expression profiling of primary CNS lymphoma specimens have demonstrated that these tumors are distinguished from their nodal lymphoma counterparts by high expression of regulators of the unfolded protein response signaling pathway—the oncogenes c-Myc and Pim-1—and by regulators of apoptosis (Rubenstein et al., 2006).

Fig. 52B.12 CNS lymphoma. Infiltrative angiocentric (aggregating around and within the vessel walls) neoplasm composed of immature lymphoid elements with a high nucleus-to-cytoplasm ratio, vesicular nuclei, and prominent nucleoli (hematoxylin-eosin stain, ×400).

Germ Cell Tumors

Germ cell tumors of the nervous system are most common in children and are analogous to their counterparts in the gonads, retroperitoneum, and mediastinum. The pineal region is the most common site of involvement, followed by the suprasellar/hypothalamic region. Germ cell tumors as a group grow rapidly, with a propensity to seed the subarachnoid space. CSF examination for both cytology and marker levels may be diagnostic. Elevations of placental-like alkaline phosphatase (PLAP) are most suggestive of germinoma, alpha fetoprotein (AFP) of yolk sac tumor, and beta human chorionic gonadotropin (β-hCG) of choriocarcinoma or a syncytiotrophoblastic element. For reasons not understood, pineal germ cell tumors are virtually restricted to boys; in some cases, the clinical and radiographical features are so typical that a biopsy is not necessary before therapy. Pure germinoma is most common and is virtually 100% curable because of its radiosensitivity. Mature teratoma is rarely pure, is slow growing and cystic, and typically does not respond well to chemotherapy or radiation. However, demarcation is often excellent, and surgical cure is possible. Most of the remaining cases of germ cell tumor consist of mixed tumors with various malignant elements, such as embryonal carcinoma, yolk sac tumor, choriocarcinoma, and immature teratoma. Such cases have a significantly worse prognosis, but survival rates have improved with modern multiagent chemotherapy regimens. In occasional cases that are successfully treated, the only viable element remaining on “recurrence” is mature teratoma.

Histologically, germinomas are identical to testicular seminoma and ovarian dysgerminoma, with two distinct cell populations (Fig. 52B.13). The neoplastic element resembles primordial germ cells with abundant glycogen-rich clear cytoplasm and both PLAP and c-kit immunoreactivity. Immunoreactivity for the transcription factor, Oct4, is a useful diagnostic marker for the neoplastic cells. The stroma is often rich in reactive lymphocytes, primarily T cells. Sarcoid-like granulomas are also quite common and may obscure the diagnosis in biopsies where the reactive response overshadows the tumor cells. Embryonal carcinoma resembles a poorly differentiated carcinoma. Endodermal sinus tumor (yolk sac tumor) forms loose papillary epithelial structures known as Schiller-Duvall bodies. Combinations of mononucleated cytotrophoblasts and multinucleated syncytiotrophoblasts characterize choriocarcinoma. The tumor is highly vascular and particularly prone to hemorrhage. Teratomas differentiate into elements from all three germ layers. These tumors contain mature components such as teeth, hair, muscle, cartilage, and bronchial wall. These components arrange in a haphazard nonfunctional manner but are benign. Immature teratoma contains the same elements but has a fetal rather than mature appearance. Foci of immature brain with neural tubelike structures are particularly common.

Fig. 52B.13 Pineal germinoma. The dual cellular population consists of numerous small reactive lymphocytes and large clear tumor cells which resemble primordial germ cells (hematoxylin-eosin stain, ×400).

Hemangioblastoma (WHO Grade I)

Hemangioblastomas are benign vascular tumors of uncertain histogenesis. Approximately 10% of patients with hemangioblastoma have von Hippel-Lindau disease; the rest are sporadic. The age at diagnosis ranges from adolescence to the sixth decade, with the peak frequency at 40 years. Hemangioblastomas are more common in males. The tumor usually presents as a cyst with an enhancing mural nodule in the cerebellum and is the most common primary cerebellar neoplasm in adults. Hemangioblastomas are sometimes located in the retina, brainstem, spinal cord, or paraspinal nerve roots, sites more commonly involved in patients with von Hippel-Lindau disease.

Hemangioblastomas are cystic and sharply demarcated, often allowing complete surgical resection. The cyst contents are usually clear to yellow; a rusty color indicates previous bleeding. Solid portions of the tumor are dark red because of a rich vascular supply, which predisposes to spontaneous hemorrhage. Histological examination shows abundant capillaries coursing throughout the tumor mass, though the actual tumor cell is believed to be the foamy lipid-laden stromal cells in between. These cells stain consistently with inhibin S100 protein and neuron-specific enolase. Use of additional markers such as α-inhibin, D2-40, and EGFR immunohistochemistry may aid in the distinction between hemangioblastoma and potential mimics such as metastatic renal cell carcinoma, especially in the context of von Hippel-Lindau disease. Patchy GFAP expression occurs occasionally. An erythropoietin-like substance is identifiable in the cyst fluid of 20% of tumors. It may be associated with pure red cell hyperplasia in the patient and extramedullary hematopoiesis in the tumor.

The VHL gene on 3p25-26 is a growth regulator that behaves as a classic tumor suppressor gene, with von Hippel-Lindau patients harboring a germline mutation (“first hit”). In these patients, it predisposes to a variety of tumors and malformative lesions, but the majority of the morbidity and mortality result from renal cell carcinomas, CNS hemangioblastomas, and in a subset of patients, pheochromocytomas.

Craniopharyngioma (WHO Grade I)

Craniopharyngiomas comprise 2% to 5% of CNS tumors. Most become symptomatic in the first 2 decades but can occur at any age. Craniopharyngiomas probably arise from cell rests of the Rathke pouch, an evagination of the primitive stomodeum. Rathke pouch remnants may be identifiable as nests of squamous epithelium on the anterior surface of the infundibulum and the pars tuberalis in infants and adults. Craniopharyngiomas may be intrasellar or (more frequently) suprasellar in location, often involving the hypothalamus and the optic nerve or chiasm. The expanding mass causes hydrocephalus by encroaching on the third ventricle.

The advancing margins of the craniopharyngioma may appear deceptively sharp, but microscopic finger-like extensions into surrounding tissue are common. Both solid and cystic areas are intermingled. The cysts can become large and typically fill with a dark, viscous, cholesterol-rich fluid likened to motor oil. Irregularly shaped calcium deposits, varying in size from grains of sand to fine gravel, occur in approximately 75% of cases. The microscopic appearance is comparable to that of adamantinoma of the tibia and ameloblastoma of the jaw. Therefore, sometimes the classic craniopharyngioma is named adamantinomatous craniopharyngioma. The tumor demonstrates benign-appearing epithelium with central cobweb-like loosening (stellate reticulum) and peripheral palisading. Squamoid foci may occur, and the pattern of keratinization with ghostlike nests of keratinocytes is called wet keratin. Wet keratin differs from the dry, flaky keratin of epidermoid and dermoid cysts and is unique to craniopharyngioma. Therefore, it is diagnostic on a biopsy, even without the presence of viable epithelium. A rare variant is the papillary craniopharyngioma, which often presents in the third ventricle of adults. A characteristic feature is a true nonkeratinizing squamous lining over fibrovascular cores. Goblet cells are another feature. It is uncertain whether or not this variant has a better prognosis. Aberrant nuclear accumulation of β-catenin occurs in 94% of adamantinomatous craniopharyngiomas. In 77% of tumors, the dysregulation of the Wnt signaling pathway causing this phenomenon is due to genetic mutations which appear to target the GSK3β phosphorylation site of the β-catenin gene necessary for protein degradation (Buslei et al., 2005). Mutations in other components of the pathway such as APC were not found. This finding may help discriminate adamantinomatous from papillary craniopharyngiomas, which do not show nuclear translocation of β-catenin. It may also help distinguish adamantinomatous craniopharyngioma from other sellar tumors in the event of a small biopsy where diagnostic histological features are not readily apparent.

Epidermoid and Dermoid Cysts

Presumably, epidermoid and dermoid cysts are implantation or sequestration cysts derived from misplaced ectoderm. These cysts may be congenital or acquired. The cause of the congenital type is inclusion of ectodermal tissue during embryonic closure of the neural groove or during coalescence of epithelial fusion lines in the cranium. Sequestration cysts accompany dysraphism, such as spina bifida, and may communicate with the skin surface through a sinus tract.

Epidermoid cysts occur in young adults and are usually found in the cerebellopontine angle or skull. Dermoid cysts are more common in children and tend to occur near the midline in the cerebellar vermis, parasellar or parapontine region, and spinal canal, especially in the lumbosacral region. A fibrous capsule that has a glistening white surface (like mother-of-pearl) envelops intact tumors called pearly tumors. The lining and contents of the epidermoid are composed of keratinizing squamous epithelium that may attenuate or focally stratify. The thin wisps of flaky intraluminal keratin are dry keratin, in contrast to the type seen in craniopharyngiomas. The adjacent collagenous wall often partially calcifies, producing a linear or speckled pattern in CT images. The inner layer of a dermoid cyst is also composed of squamous epithelium, but the presence of hair follicles and other skin appendages distinguishes dermoid cysts from epidermoid cysts. The cyst contents, once introduced into the meninges by spontaneous rupture or during surgery, can incite chemical meningitis.

Neuroenteric, Colloid, and Rathke Cleft Cysts

Cysts lined by cuboidal to columnar mucin-producing epithelial cells, resembling those of respiratory or enteric lining, may occur in several sites throughout the CNS. Such cysts are referred to as a Rathke cleft cyst in the sella, colloid cyst in the third ventricle, and neuroenteric cyst (or enterogenous, bronchogenic, or neuroepithelial cyst) when they occur in the anterior spinal region or rarely at intracranial sites. The origin of these cysts, thought to be developmental rather than neoplastic, is unknown. Although the presumed embryology may differ at these sites, the histological appearance is otherwise similar.

Lipomas

Most lipomas are located in the midline and are sometimes associated with other developmental abnormalities such as agenesis of the corpus callosum. Typical locations include the dorsal aspect of the midbrain, cerebellar vermis, and spinal cord. The most common location is the surface of the corpus callosum. Lipomas of the spinal cord often become symptomatic by causing cord compression, but lipomas of the brain are usually asymptomatic and are incidental findings during neuroimaging studies and on postmortem examinations.

Lipomas are yellow and resemble normal fat. Tumors of the brain, particularly those adjacent to the corpus callosum, may seem to infiltrate the parenchyma but are benign in histological appearance and biological behavior. Lipomas of the spinal canal are usually in an epidural or subdural position and well demarcated from the adjacent spinal cord.