This case demonstrates the predilection primary CNS lymphoma has for the periventricular region, as well as the characteristic signal intensity and homogeneous contrast enhancement. 

Histology

Sections show a tumor comprised of round cells with hyperchromatic nuclei. There is individual cell karyorrhexis. Mitotic figures are seen. Immunohistochemistry shows that the tumor is negative for the neuronal markers synaptophysin and phosphorylated neurofilament. The Bielschowsky stain is also unremarkable. The tumor cells are negative for glial fibrillary acidic protein (GFAP). The tumor is intensely positive for leukocyte common antigen (CD45). It is also positive for CD20. There is slight expression of CD10. Bcl-2 protein is negative. Many cells are positive for MIB1.

Final diagnosis comment: [The] tumor is a lymphoma. It is best characterized as a diffuse large B-cell lymphoma. The immunophenotype suggested the possibility of Burkitt's lymphoma and suggested consideration of testing for HIV, but given the histological findings and the anatomic site, it is felt that diffuse large B-cell lymphoma is more likely.