Imaging description

A three-year-old girl presented with back pain, vomiting, lethargy, and ataxia over a 1-month period. The non-contrast head CT demonstrates a hyperdense midline posterior fossa mass with obstructive hydrocephalus (Fig. 6.1a). MRI examination demonstrates a heterogeneous mass predominantly isointense to gray matter on both T1- and T2-weighted images with moderate enhancement post contrast (Fig. 6.1b–d). Additional nodules of similar signal characteristics are seen within the lateral ventricle as well as along the spinal column consistent with cerebrospinal fluid (CSF) metastatic spread.

Importance

Medulloblastomas are the most common posterior fossa tumors in children (30–40%) and comprise 15–20% of intracranial neoplasms in children. They comprise up to 1% of adult brain tumors. The tumors can arise in the cerebellar vermis or hemispheres.

Medulloblastomas are highly malignant tumors and may be composed of classic undifferentiated primitive small round cells, although histologic variants such as desmoplastic, extensive nodular, and large cell histologies may be seen, the latter of which imparts a worse prognosis. The case presented here is of the large cell histology with extensive spread and rapid growth. On CT, medulloblastomas are hyperdense due to their high nuclear to cytoplasmic ratio. On MRI, they are variable in signal although usually isointense to gray matter on T1 and hypo- to isointense on T2-weighted images and associated with reduced diffusion due to the highly cellular nature of the tumor. Cysts and calcifications are possible within the tumor and enhancement is variable.