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 angiocentric growth of lymphoid cells is often accompanied by perivascular reticulin deposits in a concentric fashion. 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. The remainder consist of intravascular large B-cell lymphomas (tumor cells selectively present within small and medium-sized blood vessels), low-grade B-cell lymphomas, Burkitt lymphomas, peripheral T-cell lymphoma, and anaplastic large cell lymphomas.