Answer: Malignant lymphoma

Histology: The brain is essentially overrun by small cell malignant neoplasm. On closer inspection, the tumor cells have irregular nuclear contours, open chromatin, and prominent nucleoli. There is a suggestion of clustering around blood vessels. Tumor cells were diffusely immunoreactive for the B cell marker CD20, and were negative for Epstein-Barr Virus by in situ hybridization.

Discussion: Infiltrating gliomas, such as astrocytoma and oligodendroglioma, have the infiltrative growth pattern of lymphoma, but they do not show the perivascular clustering typical of lymphoma. The distinction is best made on touch preps where the absence of processes in malignant lymphoma is the best clue to the diagnosis. Carcinomas are more cohesive on touch preps and histologic sections.

Most primary CNS lymphomas are high-grade B cell lymphomas, such as the current case. In immunosuppressed patients (usually those with AIDS), these tumors are almost always EBV positive. In non-immunosuppressed patients, a subset of primary CNS lymphomas is EBV positive. The diagnosis is suggested radiographically if the lesion is deep and homogeneously enhances on CT scan. Diagnosis is often established on stereotactic biopsy rather than open resection.