Primary malignant lymphoma is a non-Hodgkin's lymphoma that occurs in the brain in the absence of systemic involvement. These tumors are highly cellular and grow rapidly. Favorite sites include the deeper parts of the frontal and parietal lobes, basal ganglia, and hypothalamus. Most occur in patients who are immunocompromised secondary to chemotherapy or acquired immunodeficiency syndrome (AIDS) or in organ transplant recipients who are on immunosuppressant drugs. Cerebral lymphomas are very radiosensitive and respond dramatically to steroid therapy.

            Lymphomas typically appear as homogeneous, slightly high signal to isointense masses deep within the brain on T2-weighted images. The observed mild T2 prolongation is probably related to dense cell packing within these tumors, leaving relatively little interstitial space for accumulation of water. They are frequently found in close proximity to the corpus callosum and have a propensity to extend across the corpus callosum into the opposite hemisphere, a feature that mimics glioblastoma. Multiple lesions are present in as many as 50%. Despite their rapid growth, central necrosis is uncommon. They are associated with only a mild or moderate amount of peritumoral edema. By time of presentation they can be quite large and yet produce relatively little mass effect, a feature that sets lymphoma apart from glioblastoma and metastases. Intratumoral cysts and hemorrhage are unusual. Most lymphomas show bright homogeneous contrast enhancement. Endnote

            The pattern is modified somewhat in AIDS patients. Multiplicity seems to be more common. Moreover, lymphomas exhibit more aggressive behavior and readily outgrow their blood supply. As a result, central necrosis and ring enhancement are often seen in lymphomatous masses in AIDS patients. Endnote On MR spectroscopy, lymphomas exhibit elevated choline little or no NAA.

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