Medulloblastoma (primitive neuroectodermal tumor of the posterior fossa) is highly malignant tumor composed of very primitive undifferentiated small round cells. About 40% of patients with medulloblastoma present within the first 5 years of life; 75% are seen in the first decade of life. The most common symptoms are nausea, vomiting, and headache. In children under 1 year of age, increased head size is frequent symptom. In older children and adults, ataxia is the most prominent symptom.

Approximately two-thirds of medulloblastomas in children are located in the vermis. They impinge on the roof of the fourth ventricle and cause partial or complete obstruction of flow of the cerebrospinal fluid. Invasion of the leptomeninges via dissemination along CSF pathways is frequent. Intracranially, when there is subarachnoid tumor spread, it is seen most prominent within the sylvian fissures, the suprasellar subarachnoid space, cisterns of the posterior fossa, or over the cerebral hemispheric convexities. Drop metastases to the spinal subarachnoid space and cauda equina are most commonly seen at the thoracic and lumbosacral levels.

The most frequent abnormality on skull radiography is sutural diastasis. On CT a medulloblastoma appears as a hyperdense or isodense mass, because it is composed of small, round cells with a high nuclear-to-cytoplasmic ratio. It arises in the vermis or cerebellar hemispheres and compresses and obstructs the fourth ventricle. Mild to moderate edema surrounds the tumor and there is hydrocephalus. The tumor usually enhances diffusely but sometimes patchily. Calcification is seen in 10-20% of cases of medulloblastoma and cystic or necrotic, nonenhancing regions are seen in close to 50%.

On MR, the appearance of medulloblastomas is variable and nonspecific. Tumor location and patient age are the most important factors in making the correct diagnosis. The most common appearance is that of a hypointense mass compared with normal brain on T1-weighted images. The tumors are most commonly situated within the inferior vermis and can sometimes be seen originating from the inferior medullary velum, which is seen best on sagittal projections. T2-weighted images typically reveal a heterogeneous hypo- or isointense mass (compared to gray matter). The etiology of the hypo intensity or isointensity on T2-weighted images is most likely the highly cellular nature of the tumor and its relatively low interstitial water content. The heterogeneity probably results from the cysts and calcification. Enhancement may be uniform or patchy. The necrotic portions do not enhance.

Share This