Miscellaneous CNS neoplasms and cysts

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Miscellaneous CNS neoplasms and cysts

Diverse neoplasms covered in this chapter are:

In addition, there is a section on heterogeneous cysts/neoplasm-like entities.

GLIAL TUMORS OF UNCERTAIN ORIGIN

Three entities are placed in a category labeled ‘other neuroepithelial tumors’ in the WHO classification (2007):

ASTROBLASTOMA

Though this rare neoplasm has yet to be defined nosologically, it is nonetheless a distinct entity. It occurs preferentially in the cerebral hemispheres of young adults and teenage children. Radiologically and at surgery, it is quite well circumscribed. The astroblastoma’s behavior is difficult to predict, though many have a relatively good prognosis, which accords with its circumscribed nature. A high mitotic count, microvascular proliferation, and necrosis do not necessarily connote an aggressive behavior, though these features, which have been regarded as ‘high-grade’ in some series, seem to be associated with a higher frequency of tumor recurrence. This neoplasm has yet to be allocated to a WHO grade.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

The astroblastoma is usually a solid uniform mass of firm, yet slightly mucoid tissue, though cyst formation can occur. It is characterized microscopically by two particular features: astroblastomatous rosettes (Fig. 45.1) and a focal hyalinization of its vascular network (Fig. 45.2). The astroblastomatous rosette is centered on a capillary, onto which slightly stubby cytoplasmic processes project from uniform cells with round or oval nuclei. The tumor cells may have either indistinct cytoplasmic borders or an epithelioid appearance. Sometimes, they show artifactual perinuclear clearing, reminiscent of cells in most oligodendrogliomas. The mitotic count is variable, as is the presence of necrosis. Vascular proliferation is rarely like that seen in glioblastomas. Instead, the astroblastoma’s angiogenesis may sometimes simulate that found in pilocytic astrocytomas.

Astroblastomas are GFAP-positive, though this reactivity can be focal, even in areas that contain rosettes. Focal immunoreactivity can also be found with S-100 antibodies.

ANGIOCENTRIC GLIOMA

This recently described WHO grade I tumor of children and young adults is frequently associated with a history of intractable epilepsy. It usually occurs in the superficial cerebral cortex. The angiocentric glioma shares some histologic features with ependymal neoplasms, yet infiltrates adjacent brain diffusely, quite unlike ependymomas. Despite such infiltration, surgical resection is generally curative, a situation probably explained by the neoplasm’s slow growth and early presentation with epilepsy.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Macroscopically, angiocentric gliomas expand and distort involved brain. Microscopically, they consist of uniform cells with oval or spindle-shaped nuclei. Generally, cytoplasmic processes or borders are poorly defined, though some cells appear epithelioid. Characteristically, tumor cells have a distinctive alignment around blood vessels of varying caliber (Fig. 45.3), either parallel to blood flow or as a rosette with nuclei pointing to the lumen. Tumor cells are not within the perivascular space, a location often invaded by other types of diffuse glioma. Invasion of cerebral tissues is observed, in the manner of other diffuse gliomas, including a tendency to congregate around neurons. Necrosis and angiogenesis are absent, and dystrophic calcification is rare. Mitotic figures are very rare.

Neoplastic cells are immunoreactive for GFAP and S-100. Intracytoplasmic immunoreactivity for EMA is reminiscent of that seen in ependymomas (Fig. 45.3c), and microlumina with microvilli and cilia are evident at the ultrastructural level.

CHORDOID GLIOMA OF THE THIRD VENTRICLE

This rare neoplasm occurs in the third ventricle of adult patients, and often presents with obstructive hydrocephalus. It may have an intraparenchymal component, but appears circumscribed on neuroimaging. Its biologic behavior is generally low-grade (WHO grade II), but uncontrollable recurrence because of involvement of periventricular structures may follow incomplete surgical resection. Ultrastructural features, such as microvilli, support an ependymal origin.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

The mucoid nature of this generally solid neoplasm may be apparent macroscopically. It is composed of cords or sheets of epithelioid cells variably set against a myxoid matrix, regions of which show microvacuolation and contain a lymphoplasmacytic infiltrate (Fig. 45.4). A few cells demonstrate fibrillary cytoplasmic processes, hinting at glial differentiation. Mitoses are absent or sparse. Tumor cells are GFAP-positive and immunoreactive for vimentin and CD34. Occasionally, a few cells are immunoreactive for EMA. Widespread immunoreactivity for EMA, but none for GFAP, distinguishes the chordoid meningioma from the chordoid glioma.

MESENCHYMAL NON-MENINGOTHELIAL NEOPLASMS

In this category, the WHO 2007 classification lists a range of bone and soft tissue neoplasms, the former in the skull and vertebrae and the latter generally located in the meninges, though exceptional parenchymal examples are recorded. Those discussed below are solitary fibrous tumor of the meninges; osteocartilaginous neoplasms of the meninges; lipoma; solitary fibrous tumor of the meninges; vasoformative neoplasms, and sarcomas.

LIPOMA

CNS lipomas are rare. They occur preferentially in the anterior part of the corpus callosum (usually in association with its partial agenesis, see Chapter 3), over the quadrigeminal plate, in the cerebellopontine angle, at the base of the brain, and in the spinal cord. Lipomas of the lumbosacral cord are sometimes associated with neural tube defects.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Lipomas are soft yellow masses (Figs 45.5, 45.6) and usually encroach upon adjacent structures. They are composed of mature lipocytes, but rarely they contain other ectopic tissues such as striated muscle and cartilage. Spinal examples may be poorly defined and occasionally contain prominent blood vessels (angiolipoma).

SOLITARY FIBROUS TUMOR OF THE MENINGES

This rare neoplasm has the same histologic features as solitary fibrous tumors of the pleura, yet its distinction from the fibrous meningioma was not acknowledged until the 2000 WHO classification of nervous system tumors. It is thought to arise from fibroblasts in the dura, rather than a cell in the leptomeninges. It generally occurs in adults, and most are intracranial.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

The solitary fibrous tumor looks just like a meningioma. It is usually affixed to dura, projecting away from this structure as a globular mass. Microscopically, the uniform spindle-shaped tumor cells form interweaving fascicles (Fig. 45.7), and bundles of collagen parallel the elongated nuclei. The collagen can become dense, diminishing the tumor’s cellularity. Mitoses are rare.

Immunohistochemistry reveals reactivities for CD34, CD99, factor XIIIa, and vimentin, but not EMA or S-100. Immunohistochemical and ultrastructural analyses help to confirm the non-meningothelial status of this tumor, and the use of these six antibodies allows the solitary fibrous tumor to be distinguished from meningioma and schwannoma (Table 45.1).

VASOFORMATIVE NEOPLASMS

Vasoformative neoplasms of the CNS include the hemangioma (Fig. 45.8), epithelioid hemangioendothelioma, hemangiopericytoma, and angiosarcoma. Hemangiomas that encroach on the CNS usually arise in the thoracic spine. Hemangiopericytomas account for less than 0.5% of primary CNS neoplasms, and epithelioid hemangioendotheliomas and angiosarcomas are very rare. Angiosarcomas can occur in the context of previous radiotherapy to the neuraxis.

HEMANGIOPERICYTOMA

CNS hemangiopericytomas occur mainly in the intracranial meninges and were once regarded as an angioblastic variant of meningioma. However, they share the histologic features of hemangiopericytomas outside the nervous system and do not have the genetic abnormalities characteristic of meningiomas, such as allelic loss on chromosome 22 or NF2 gene mutations. Unlike meningiomas, hemangiopericytomas show no clear predilection for either sex. They occur in a younger age group than meningiomas, and have a tendency to recurrence and late metastasis. CNS hemangiopericytomas are now considered to form a spectrum of disease with the solitary fibrous tumor; they share some histopathological features and have an overlapping immunophenotype. However, diagnostic distinction between the two entities should be maintained, because solitary fibrous tumors have a more indolent biological behavior.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Hemangiopericytomas are usually spherical and discrete, but may invade the skull. They have a firm and homogeneous texture. Microscopically, many hemangiopericytomas are characterized by a uniform histologic appearance with a sheet-like arrangement of cells having a high nuclear:cytoplasmic ratio (Fig. 45.9). The cells are interspersed with many tiny capillaries, and a few larger, slightly gaping, vascular channels that may have a branching appearance. Other characteristics include focal lobularity, paucicellular areas, and dense pericellular reticulin. Diffuse fibrosis is sometimes seen. Mitoses are readily found, but foci of necrosis are uncommon. Cytologic pleomorphism and a high mitotic count are usually associated with rapid growth and aggressive behavior. The classic hemangiopericytoma is WHO grade II, but tumors with at least 5 mitoses per 10 HPFs and some of the following features: necrosis, hemorrhage, and increased nuclear pleomorphism or cell density, are regarded as anaplastic and WHO grade III.

Immunohistochemistry reveals reactivity for vimentin, but no labeling with antibodies to epithelial membrane antigen, which can help in distinguishing these neoplasms from meningiomas. Antibodies to markers of vascular endothelium such as factor VIII and Ulex europaeus lectin label only the endothelium of the vascular channels. CD34 antibodies label many of the neoplastic cells.

SARCOMAS

Primary sarcomas of the CNS are rare, though diverse types, including angiosarcoma, fibrosarcoma, malignant fibrous histiocytoma, and unclassifiable sarcomas, have been described and listed in the WHO classification. Chondrosarcomas and osteosarcomas arising in the skeleton around the CNS may cause neurologic problems. Many sarcomas are associated with prior radiotherapy, usually 10–15 years earlier.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Sarcomas are generally composed of anaplastic spindle-shaped cells (Fig. 45.10). There are usually abundant mitoses and foci of necrosis. Individual histologic characteristics sometimes allow subclassification. For example:

Immunohistochemistry and electron microscopy may also identify patterns of differentiation, e.g. rhabdomyosarcoma.

Many sarcomas are based in the meninges. The term meningeal sarcomatosis is used when neoplastic cells infiltrate widely in the meninges. This process usually occurs in children and young adults.

An important entity in this group to recognize when considering the differential diagnosis of intracranial or spinal embryonal tumors is the peripheral PNET, which is synonymous with extra-skeletal Ewing sarcoma. Ewing sarcoma itself, arising in the skull or vertebra, may also impinge on the CNS. Histopathologically (Fig. 45.11a–c), this tumor usually appears as sheets of small round cells with a high nuclear:cytoplasmic ratio and nuclear hyperchromasia. Mitotic activity is readily detected. Intracytoplasmic glycogen is a frequent finding. Immunoreactivity for neuronal antigens is evident, along with strong membranous immunostaining for CD99. A pathognomonic cytogenetic abnormality involving fusion of EWSR1 with several gene partners, usually FLI1, should be sought (Fig. 45.11d).

MELANOCYTIC NEOPLASMS

Melanocytes are normally present in the leptomeninges (Fig. 45.12), and give rise to various neoplasms recognized by the WHO classification. These are:

Metastatic malignant melanomas greatly outnumber primary melanocytic neoplasms.

Diffuse proliferations of leptomeningeal melanocytes may be associated with large cutaneous nevi and a predisposition to malignant melanoma. This is known as neurocutaneous melanosis, and is often fatal in childhood

Melanin also occurs in a variety of CNS neoplasms, including schwannomas, ependymomas, embryonal neoplasms, and some pineal neoplasms.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Primary melanocytic neoplasms may be diffuse or nodular. Diffuse melanosis appears macroscopically as brown discoloration of the leptomeninges. However, widespread involvement of the meninges in this way may be part of a more malignant process (malignant melanomatosis) that is also characterized by meningeal thickening, discontinuous spread, and invasion of underlying brain. Hydrocephalus may result. Nodular neoplasms are most commonly sited in the posterior fossa, but may occur in any part of the neuraxis. Melanocytomas tend to be intraspinal.

Cells in diffuse melanosis and melanocytoma tend to be arranged in nests or sheets. They are uniform with oval or spindle-shaped nuclei (Fig. 45.13). Mitoses are rare, and necrosis is absent. Melanin may be abundant or patchy.

Malignant melanomas (Fig. 45.14) show cytologic atypia, an increased mitotic count, and necrosis, and invade adjacent structures. Neoplasms showing intermediate histology between those of melanocytoma and malignant melanoma are difficult to classify.

Melanocytic neoplasms are immunoreactive for vimentin, and most label with S-100 and HMB-45 antibodies. Ultrastructural examination reveals melanosomes. A pericellular basal lamina is lacking, and melanocytic neoplasms can be distinguished in this way from melanotic schwannomas.

The prognosis of meningeal malignant melanomas is grave. The behavior of melanocytomas may be relatively benign, though reports of recurrent or metastatic melanocytomas are found, and the prognosis of melanocytomas should be regarded as unpredictable.

HEMANGIOBLASTOMA

The hemangioblastoma is a neoplasm of uncertain histogenesis. It is associated with von Hippel–Lindau syndrome (VHL). Hemangioblastoma has a predilection for the posterior fossa, particularly the cerebellum, but may occasionally involve the cerebrum or spinal cord. Rare supratentorial examples are usually attached to the dura, and these neoplasms were formerly grouped with hemangiopericytomas, being termed ‘angioblastic meningiomas’. However, although microcystic meningiomas may resemble hemangioblastomas, there is no evidence that hemangioblastomas and meningiomas are related histogenetically or genetically.

MACROSCOPIC APPEARANCES

Cerebellar hemangioblastomas frequently present as cysts with a mural nodule (Fig. 45.15), and their vascular nature imparts a red color to the nodule. The mural nodule is composed of clusters of neoplastic cells and may be minute. Surgical specimens of cyst wall should be examined carefully for neoplastic cells.

Other hemangioblastomas are globular. The discrete nature of most of these neoplasms facilitates complete resection, though some hemangioblastomas recur even when surgery appears to have been successful.

MICROSCOPIC APPEARANCES

Hemangioblastomas comprise two cell populations, which are combined in varying proportions:

Mitoses are uncommon. Mast cells may be present.

Hemangioblastomas are associated with marked reactive changes in the surrounding brain. Reactive astrocytosis and Rosenthal fiber formation are evident in the cyst wall and may divert the pathologist into considering a diagnosis of pilocytic astrocytoma.

Immunohistochemistry does not reveal the origin of the interstitial cells, which fail to label with antibodies to either vascular or epithelial markers. However, all interstitial cells are vimentin-positive and express N-CAM (CD56). They also express vascular endothelial growth factor, while its receptors are found on the endothelial cells. Ultrastructural examination has also failed to identify distinctive cytologic features that would establish a specific histogenesis.

CYSTS

Benign cysts occur at several sites in and around the CNS (Fig. 45.16) and the cysts considered in this chapter are: epidermoid and dermoid cysts; colloid cyst of the third ventricle; endodermal (enterogenous) cyst; ependymal (neuroglial) cyst, and arachnoid cyst. The pineal cyst is included in Chapter 39.

EPIDERMOID AND DERMOID CYSTS

Epidermoid cysts are mainly intracranial. The cerebellopontine angle, parasellar region, and cranial diploë are frequent sites. Intraspinal examples are uncommon, accounting for only 5% of cases.

Dermoid cysts are less common than epidermoid cysts (1:4). They are midline cysts, usually occurring in the region of the fontanelle, fourth ventricle, or cauda equina.

MICROSCOPIC APPEARANCES

Both epidermoid (Fig. 45.17) and dermoid (Fig. 45.18) cysts are lined by stratified squamous epithelium. The central eosinophilic material is formed from degenerate keratinocytes plus secretions from sebaceous glands in the case of the dermoid cyst. The dermoid cyst is distinguished by the presence of adnexal appendages such as sebaceous glands and hair follicles.

COLLOID CYST OF THE THIRD VENTRICLE

The origin of this cyst is unknown; however, the colloid cyst shares an immunohistochemical profile with cysts that show endodermal differentiation (endodermal/enterogenous cyst). It is considered to be congenital, but usually presents in people aged 20–50 years.

Colloid cyst of the third ventricle is a cause of recurrent headache, because it intermittently obstructs the flow of CSF at the level of the foramen of Monro. It occasionally causes sudden death if the obstruction does not resolve spontaneously.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Nearly all colloid cysts are found in the anterior part of the third ventricle (Fig. 45.19), though some examples are located in the posterior part of the third ventricle, and rare examples have been reported in the lateral and fourth ventricles. They are usually lined by a single layer of columnar cells, some of which are ciliated or contain mucin (Fig. 45.19c). Like the endodermal cyst, the colloid cyst has an epithelium with a cytokeratin-positive and EMA-positive immunophenotype.

ENDODERMAL CYST

This rare cyst has also been called a neurenteric or enterogenous cyst. Most are intradural, extramedullary, intraspinal cysts, though very rare examples are intracranial. Some intraspinal examples are associated with developmental abnormalities, for example bony defects in the lumbosacral vertebrae and duplications in the gastrointestinal tract. There is a male preponderance.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Endodermal cysts are simple fluid-filled sacs (Fig. 45.20a) Their thin walls are lined by a columnar or cuboidal epithelium, which covers a connective tissue stroma. The cyst epithelium may show features of a gastrointestinal or respiratory epithelium (Fig. 45.20b). Goblet cells are readily found, but ciliation is a variable feature (Fig. 45.20c). Squamous metaplasia is sometimes seen. The underlying stroma may contain glands, lymphoid tissue or even glial elements.

The epithelium shows immunoreactivities for cytokeratins and EMA, but not GFAP.

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