Malignant germ cell tumors occurring in locations other than the pineal body are usually found in the suprasellar space and rarely in the roof of the third ventricle. Germinoma, mentioned above, is the most common of this rare group of neoplasms, which also includes choriocarcinoma, embryonal cell carcinoma, endodermal sinus tumors, and malignant teratomas. Certain biochemical markers of these tumors are of interest and of clinical utility, because they may be detected in samples of the blood and CSF. The beta subunit of human chorionic gonadotropin (hCG) is elaborated by choriocarcinoma and alpha-fetoprotein, and by endodermal sinus tumors and immature teratomas. Typical germinomas have shown little elevation of either. Most often these markers indicate the presence of complex mixed germ cell tumors.
Gangliogliomas and mixed neuronal–glial tumors are special tumor types, more frequent in the young and of variable but usually low-grade malignancy. They are composed both of differentiated glial cells, usually astrocytes, and of neurons in various degrees of differentiation. The latter, which may resemble glial cells, can be identified by Nissl stains, silver stains, and immunochemical reactions for cytoskeletal proteins. Inflammation is common in the parenchyma and adjacent to these tumors; this has led to the erroneous diagnosis of a nonneoplastic inflammatory condition if only limited biopsy samples are taken.
Some of these developmental tumors are difficult to separate from hamartomas or from the tubers of tuberous sclerosis. In the case of hamartomas, it may be difficult to determine if the tumor or the associated developmental abnormality is the cause of seizures. Some of these tumors take the form of large, slowly growing cystic lesions.
The best characterized, albeit rare, type in this group is the gangliocytoma, a tumor that occurs in the adrenal gland, retroperitoneal and thoracic sympathetic chain, internal auditory canal, and in the spinal cord. One form is the dysplastic gangliocytoma of the cerebellum (Lhermitte-Duclos disease). This is a slowly evolving lesion that forms a mass in the cerebellum; it is composed of granule, Purkinje, and glia cells. Reproduced therein, in a disorganized fashion, is the architecture of the cerebellum with no clear plane from normally structured cerebellar tissue. The importance of distinguishing this disease from other cerebellar tumors is its lack of growth potential and favorable prognosis. It is, however, excised if symptomatic. The appearance on imaging is highly characteristic; a cerebellar hemisphere is occupied with an indistinct mass of “tiger stripe” appearance as a result of alternating layers of dysmorphic cerebellar cells (Fig. 30-16). Interest in this entity derives from its association with a germ line mutation in the PTEN gene that relates the disease to other gangliocytomas and to Cowden syndrome of multiple skin hamartomas and cancers of the gynecologic, breast, and thyroid glands (and which may include Lhermitte-Duclos as a component). Mice with PTEN knocked-out have abnormal synaptic structure and dysplasia of the cerebellar granule cells.
Other forms of gangliogliomas include the desmoplastic infantile ganglioglioma, some of the xanthoastrocytomas, and the dysembryoplastic neuroepithelioma tumor (DNET), not all of which are included in newer classifications.