The differential is of a hemangioblastoma, high grade glioma, a metastasis or an adult medulloblastoma. The hypderdensity, exccentric location, and restricted diffusion are all consistent with the latter, confirmed at pathology. 

Histology

Microscopic Description:

Sections show a highly cellular tumor 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 tumor is negative for cytokeratin, LCA, HMB-45 and Mel A.

Final diagnosis: Medulloblastoma