This last of these tumors, DNET, is worthy of comment as it often causes seizures that may be difficult to control in children. We have encountered them mostly in young adults after a single seizure or as an incidental finding on MRI. Although the tumors may be located in any part of the brain, there is a proclivity for the superficial (juxtacortical or intracortical) lateral or medial temporal lobe. The radiologic appearance is usually of a nodule or small cyst, or more specific to this entity, a cluster of several adjacent small cystic lesions, generally not enhancing and hyperintense on T2-weighted MRI (Fig. 30-17). The lesion is very slow growing and, if appropriately situated, may remodel the bone of the orbital roof or calvarium. The tumors originate from dysplastic cells in the germinal matrix that become arrested during migration toward the cortex; they are often associated with an adjacent region of cortical heterotopia. The histologic appearance varies but has as its main element a collection of neuroepithelial cells and clusters of oligodendrocytes with multinodular architecture that create mucinous cysts in some cases. When the lesion is single and has a nonspecific radiologic appearance, a biopsy or resection is required to differentiate it from a low-grade glioma or oligodendroglioma. Biopsy alone may be misleading in showing only the adjacent inflammation, at times prominent enough to appear almost granulomatous. Resection is curative and often eliminates the seizures but it is not clear what the best course of action is for asymptomatic lesions.
Another tumor that may be considered in this group is the subependymal giant cell astrocytoma that is found in up to 20 percent of patients with tuberous sclerosis. These are very slowly growing tumors that arise mostly near the foramen of Munro. Repeated resection is sometimes required to treat the hydrocephalus caused by the location of the tumor at the point of egress of CSF from the lateral ventricle. Krueger and colleagues, and subsequently others, have reported that everolimus, an inhibitor of the mTOR complex, which is disrupted in this phakomatosis, reduces the size of the tumor and ameliorates seizures. The remaining tumors mentioned earlier are rare and affect children mostly; therefore, they are not discussed further. Descriptions are to be found in the monographs of Russell, of Rubinstein (1972), of Levine (both articles from 1993), and of Schmidek, and in the article by Zentner and coworkers.
Colloid Cyst and Other Tumors of the Third Ventricle
The most important of these is the colloid tumor, which is derived from ependymal cells of a vestigial third ventricular structure known as the paraphysis. The cysts formed in this structure are always situated in the anterior portion of the third ventricle between the interventricular foramina and are attached to the roof of the ventricle (Fig. 30-18). They vary from 1 to 4 cm in diameter, are oval or round with a smooth external surface, and are filled with a gelatinous material containing a variety of mucopolysaccharides. The wall is composed of a layer of epithelial cells, some ciliated, surrounded by a capsule of fibrous connective tissue. Although congenital, the cysts practically never declare themselves clinically until adult life, when they block CSF outflow through the foramen of Monro and produce an obstructive hydrocephalus.