This 4 year-old male presented with vomiting. The image (axial T2W MRI) demonstrates a midline posterior fossa mass, which appears to arise from the cerebellar vermis and displace the 4th ventricle anteriorly. Post-gadolinium scans (not shown) demonstrated patchy enhancement. There was associated non-communicating hydrocephalus.

Medulloblastoma is a subtype of primitive neuroectodermal tumour (PNET) occurring in the posterior fossa of children. 75% are midline (vermis), and 75% are in patients less than 15 years. One third of posterior fossa tumours in children are medulloblastomas. There is frequently (50%) dissemination via the cerebrospinal fluid at the time of diagnosis.

Medulloblastomas are usually hyperdense on non-contrast CT, and may calcify (15%). Homogeneous contrast enhancement is usual. Typical MR findings are hypointense T1W signal, and variable T2W. Cysts may be seen in 75-80%.

Metastases occurs predominantly to other CNS locations. Extracranial metastases occur, predominantly to bone. Contrast-enhanced MRI is the modality of choice to demonstrate CSF spread, and the spine should always be included.

Reference: Osborn AG. Diagnostic Neuroradiology, Mosby, 1994.

Credit: Dr Laughlin Dawes