The astrocytic tumors, the most common forms of glioma, have been traditionally subdivided into diffuse well-differentiated astrocytoma (grade II), anaplastic astrocytoma (grade III), and glioblastoma (glioblastoma multiforme, or “GBM,” grade IV). These histologic grades represent a spectrum in terms of growth potential (degree of nuclear atypia, cellularity, mitoses, and vascular proliferation) and prognosis. In conventional neuropathologic terms, glioblastoma is largely defined by the features of necrosis and anaplasia of nonneural elements such as vascular proliferation and are set apart from anaplastic astrocytomas on the basis not only of their histology but also by a later age of onset than astrocytoma and a more rapid course. The grade I classification for astrocytomas has been reserved for the relatively benign group that includes pilocytic astrocytomas (well-differentiated tumors mostly of children and young adults); the pleomorphic xanthoastrocytoma (with lipid-filled cells), and the subependymal giant cell astrocytoma (associated with tuberous sclerosis). They have been set apart because of their different growth patterns, pathologic features, and better prognosis.
A distinct glioma type, distinct from the astrocytic line, is the oligodendroglioma. There are mixed types that combine the two and this has implications for treatment and prognosis. Moreover, the pathologic criteria of malignant astrocytoma do not apply to oligodendroglioma, for reasons elaborated further on. They are subdivided into tumors of pure oligodendroglial composition and those with mixed astrocytes and oligodendroglia.
However, as emphasized earlier and will be discussed in greater detail further on, modern classification depends as much on the genetic constitution of these tumors as it does on their histopathologic features. In part, this is the result of tentative data suggesting that treatment response, and potentially the prognosis, may be better aligned with molecular features than they are with the tumor’s appearance. In particular, the presence of acquired somatic mutations in the genes p53, 1p/19q codeletion, IDH1, TERT and in several other genes have led to reclassification of brain tumors, particularly those with genetic elements of both the astrocytic line and oligodendroglioma.
The ependymomas are subdivided into cellular, myxopapillary, clear cell, and mixed types; the anaplastic ependymoma and the subependymoma are given separate status.
Meningiomas are classified on the basis of their cytoarchitecture and genetic origin into several categories summarized in Table 30-3. Tumors of the pineal gland, which were not included in earlier classifications, comprise germ cell tumors, the rare pineocytomas, and pineoblastomas. The medulloblastoma has been reclassified with other tumors of presumed neuroectodermal origin, namely neuroblastoma, retinoblastoma, neuroepithelioma, and ependymoblastoma. Tumors derived from the choroid plexus are divided into several classes, pineocytoma: papillary and pineocytoma but metastatic carcinomas may also appear in the pineal body (Table 30-3).
Given separate status also are the intracranial midline germ cell tumors, such as germinoma, teratoma, choriocarcinoma, and also a group of gangliomas and neuroepithelial tumors. Another miscellaneous group comprises CNS lymphoma, hemangioblastoma, chordoma, and hemangiopericytoma.
Tumors of cranial and peripheral nerves differentiate into three main types: schwannomas, neurofibromas, and neurofibrosarcomas. Most are sporadic but the neurofibromas assume special importance in neurofibromatosis.
Biology of Nervous System Tumors