A rare case of medulloblastoma with a large cystic component is reported. A 13-year-old girl was admitted to Kanto Rosai Hospital presenting the symptoms of increased intracranial pressure (ICP) and truncal ataxia. Computed tomographic scan revealed a mid-cerebellar high density mass lesion with a cystic low density component rostral to the high density mass, resulting in obstructive hydrocephalus. There was no contrast enhancement around the cyst wall. The patient underwent emergent suboccipital decompression and the symptoms of increased ICP disappeared thereafter. Subsequent excision of the tumor was performed. The cystic component, which contained xanthochromic fluid, was observed as a cavity isolated from the fourth ventricle. The wall of the cyst was apparently normal and was not excised. The histological diagnosis was medulloblastoma.
Besides the hemorrhages and the necrotic changes in the tumor tissue, cerebrospinal fluid entrapment can play a major role in the cyst formation of medulloblastoma. The latter was considered to be the main mechanism of cyst formation in this case.