Diffuse large B-cell lymphoma AIDS-related, light micrograph. Primary central nervous system lymphoma (PCNSL) is defined as lymphoma arising in the brain, spinal cord, or leptomeninges without prior or concurrent tumour outside the central nervous system (CNS). The patients with PCNSL may be immunocompetent or have immunocompromised status. The incidence of CNS lymphoma has increased in recent decades due to the increased number of immunocompromised patients (organ transplant recipients, HIV infection, etc.). PCNSL develops in 2% to 12% of AIDS patients. However, the recent trends have been downwards due to the availability of highly effective anti-retroviral therapy. Primary central nervous system lymphoma (PCNSL) is characterized by perivascular growth of large atypical lymphoid cells. With continued proliferation, the distribution becomes more diffuse and sheet-like. Apoptosis and necrosis are prominent, especially if the patient received steroids before biopsy. The vast majority of the cases, especially in immunocompromised patients, are diffuse large B-cell lymphomas composed of centroblasts, immunoblasts, or a mixture of the two. This photomicrograph is from AIDS-related diffuse large B-cell lymphoma.