B cell-rich non-neoplastic sentinel lesion preceding primary central nervous system lymphoma.

Primary central nervous system lymphoma (PCNSL) is an uncommon tumor in the brain. Although most PCNSL are readily diagnosed as diffuse large B cell lymphoma (DLBCL) on the first biopsy, very rare cases have been described in which the first detected intracerebral lesions are non-neoplastic, and are composed mostly of perivascular T cells, not B cells. This phenomenon is known as "sentinel lesions."