Primary CNS lymphoma, like systemic lymphoma, arises from lymphocytes. The source of the pre-malignant lymphocytes is controversial as the CNS lacks lymphoid tissue and lymphatics. The perivascular distribution of cerebral tumor suggests that it may originate from systemic neoplastic lymphocytes that selectively infiltrate CNS tissue. This selectivity may depend on adhesion molecules within the CNS vasculature and parenchyma specific for the malignant cells.

The macroscopic appearance of deposits of PCNSL within the brain varies. They may be lep-tomeningeal, parenchymal, subependymal or a combination of these. Primary CNS lymphoma may be unifocal or multicentric. In either case, it can appear to be relatively well circumscribed or irregularly margined. In the leptomeningeal variant of PCNL, tumor exclusively infiltrates the leptomeninges, cranial nerves and spinal nerve roots. Primary parenchymal CNS lymphoma softens affected brain tissue and turns it yellow-brown. There may be areas of focal hemorrhage or necrosis.

Microscopically, diffuse infiltration of tumor into brain parenchyma well beyond the macroscopic borders of the tumor is common. Neoplastic lymphocytes aggregate perivascu-larly and infiltrate the walls of small blood vessels (Fig. 16.1). This perivascular cuffing is most prominent at the tumor margins. Tumor cells also commonly extend in the subpial plane or in the subarachnoid space. An astrocytic reaction can be prominent, especially in the peripheral parts of the tumor. An infiltrate of T lymphocytes is typically present at the tumor margins and occasionally within the tumor itself.

The tumor has histological features similar to those of systemic non-Hodgkin's lymphoma. Hodgkin's lymphoma is almost never seen as a primary CNS tumor. The great majority of PCNSLs are monoclonal B cell lymphomas. T cell lymphomas of the CNS are exceptionally rare and must be differentiated from B cell lymphomas with a prominent reactive T-cell

Table 16.5. Risk factors for primary CNS lymphoma.

Acquired immunodeficiency

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