In a young adult, the differential for a cystic lesion in the posterior fossa is really between a pilocytic astrocytoma and a haemangioblastoma. The absence of flow voids favours the former. Both lesions can uncommonly haemorrhage.
The patient went on to have a resection.
MICROSCOPIC DESCRIPTION: The sections show cerebellar tissue containing tumour with greatly variable appearances. The cerebellar tissue itself is atrophic, with marked gliosis. The tumour contains areas of organising old haemorrhage and extensive areas of calcification, including a large calcified nodule as described macroscopically. Calcification appears to relate to proliferated, small, thick-walled blood vessels. There are areas of proliferation of piloid astrocytes, with fine elongated cytoplasm and fairly regular oval nuclei, with scattered granular bodies and Rosenthal fibres evident. In places there is a well developed fascicular architecture. Neither mitotic figures nor microvascular proliferation are apparent.
FINAL DIAGNOSIS: Pilocytic astrocytoma, WHO grade 1.