The differential is of a haemangioblastoma, high grade glioma, a metastasis or of an adult medulloblastoma. The hypderdensity, excetic location, and restricted diffusion are all consistent with the latter, confirmed at pathology.
Sections show a highly cellular tumour mass composed of cells with hyperchromatic oval nuclei and little visible cytoplasm. The nuclei have coarsely clumped chromatin and inconspicuous nucleoli. Mitotic activity and individual cell necrosis and apoptosis are frequent. In some areas, the nuclei show a tendency to mold to one another. The cells are arranged in solid disorganized sheets and in small lobules and columns separated by collagenous septa. Multiple small cystic spaces are present but there is no geographic necrosis. One small fragment of recognizable cerebellar cortex is present and shows diffuse infiltration by neoplastic cells. Immunohistochemistry shows diffuse cytoplasmic reactivity for NSE and synaptophysin and occasional cells are positive for GFAP and S-100. The tumour is negative for cytokeratin, LCA, HMB-45 and Mel A.
Final diagnosis: Medulloblastoma