Miscellaneous CNS neoplasms and cysts

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Miscellaneous CNS neoplasms and cysts

Diverse neoplasms covered in this chapter are:

glial tumors of uncertain origin

mesenchymal non-meningothelial neoplasms, including vasoformative neoplasms

primary melanocytic neoplasms

hemangioblastoma.

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

angiocentric glioma

chordoid glioma of the third ventricle.

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.

45.1 Astroblastoma. (a) The astroblastomatous rosette forms around a capillary and is characterized by a corona of short fibrillary processes from perivascular tumor cells. (b) Tumor cells in astroblastomas tend to have a high nuclear:cytoplasmic ratio and monomorphic nuclei.

45.2 Astroblastoma. (a) Vascular hyalinization may be extensive. (b) Hyalinization tends not to be associated with rosette formation.

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.

45.3 Angiocentric glioma (a) At low power, perivascular and infiltrative tumor cell patterns can discerned, as can aggregates of subpial cells (b). EMA immunoreactivity is demonstrated by most tumor cells (c).

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.

45.4 Chordoid glioma of the third ventricle. (a) Cords of tumor cells, focal microvacuolation, and a lymphoplasmacytic infiltrate are characteristic. (b) A vague trabecular arrangement of moderately uniform cells is evident, along with microvacuolation and a myxoid background. (c) A mixture of epithelioid cells and cells with fibrillary cytoplasmic processes is present. Note the plasma cells (arrows) and the Russell bodies. (d) Cytoplasmic borders are apparent in this population of epithelioid cells.

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.

OSTEOCARTILAGINOUS NEOPLASMS OF THE MENINGES

Benign osteocartilaginous neoplasms of the meninges are rarely encountered, unlike ossification of the parasagittal meninges, which is a common incidental post-mortem finding. Discrete bony nodules composed of osteocytes are very rare and may contain cartilage (osteochondroma). Chondromas of the meninges usually present as a nodular mass over the cerebral convexities and are composed of well-differentiated chondrocytes. Osteoma and osteosarcomas may impinge on the CNS when arising in surrounding bony structures.

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).

45.5 Lipoma of the corpus callosum. (a) A mass of adipose tissue lies above the callosum. (b) Histology of the lipoma in (a), showing mature adipocytes. Note the thinning and typical distortion of the ventral part of the corpus callosum.

45.6 Lipoma of the spinal cord. Diffuse enlargement and yellow discoloration of the spinal cord are evident.

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.

45.7 Solitary fibrous tumor of the meninges. (a) The monomorphic cells have a fascicular arrangement. (b) Serpentine collagenous bands weave between elongated nuclei. (c) Tumor cells are immunoreactive for CD34.

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).

Table 45.1

Immunophenotypic variability across extra-axial spindle-cell tumors with similar morphology

+, immunopositive; ±, focal immunoreactivity; −, immunonegative.

^a Patchy immunolabeling.

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.

45.8 Hemangioma. Unusually, this epithelioid hemangioma was wrapped around the brain stem of a middle-aged man, causing multiple cranial nerve palsies. The vascular channels typically have plump endothelial cells. (Courtesy of Dr M Carey, University of Birmingham Hospital Trust, UK.)

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.