Specific Imaging Findings

Medulloblastomas typically arise in the midline of the posterior fossa, but may occur more laterally and sometimes extend through the fourth ventricle foramina. They are characteristically hyperdense on CT and with very low signal on ADC maps, typically darker than the normal brain. Cystic components are present in a majority of cases and the tumors are hypo- to iso-intense on T1WI. The appearance on post-contrast images is variable, ranging from marked and solid to only subtle marginal or linear enhancement. Calcification and hemorrhage may occasionally be observed, while surrounding edema is rarely prominent. Medulloblastomas have a high rate of early leptomeningeal disease and drop metastases, requiring MR imaging of the entire neural axis (head and spine). There are notable differences between the classic medulloblastoma (CMB) and some of the recently defined variants. CMB is T2 hyperintense, whereas desmoplastic/nodular (DMB) and medulloblastoma with extensive nodularity (MB-EN) are usually isointense; these two variants are also frequently located off-midline. MB-EN may show a characteristic gyriform pattern. In contrast to CMB, all medulloblastoma variants show marked contrast enhancement.

Pertinent Clinical Information

Patients with medulloblastoma are typically children, and increased intracranial pressure is responsible for common presentation with nausea, vomiting, and hydrocephalus. Due to the propensity for early leptomeningeal spread, the initial presentation may also be caused by metastatic disease, such as seizures or spinal cord compression. Extra-CNS spread may rarely occur, usually to the bone.