Here’s another primary CNS tumor that arises in the brain parenchyma: primary CNS lymphoma.

This is a rare tumor, overall: it accounts for 2% of all extra-nodal lymphomas, and only 1% of intracranial tumors. However, in immunosuppressed patients (like patients with AIDS, or patients who have had a transplant), it is the most common tumor of the CNS.

Just to clarify the word “primary” here – these are lymphomas that arise right in the brain itself. You can also have a lymphoma that arises elsewhere in the body that secondarily spreads to the brain parenchyma. This is exceedingly rare (usually, if lymphoma spreads to the CNS, it just involves the CSF or very superficial areas of the brain). Weird:  lymphoma arising in brain parenchyma rarely spreads elsewhere, and lymphoma arising elsewhere rarely spreads to the brain parenchyma.

Most primary CNS lymphomas are B-cell in nature; diffuse large B-cell lymphoma is the most common type. In patients who are immunosuppressed, as you might expect, the tumor cells are usually are positive for Epstein-Barr virus. Grossly, primary CNS lymphoma is usually multifocal within the brain; it can involve both deep gray matter and white matter. There’s often a lot of necrosis. Here’s a weird thing: the tumor cells tend to accumulate around blood vessels (check out the section above, with little groups of tumor cells just hanging out around the vessels). Here’s another weird thing: if you stain a section of tumor with reticulin, you’ll often see silver staining material in between the tumor cells, a pattern called “hooping.”

This is not a nicely-behaving tumor. Compared to regular lymphoma, primary CNS lymphoma is more aggressive, with a worse response to chemotherapy.