Primary Central Nervous System Lymphomas

Primary CNS lymphomas (PCNSLs) account for only for 2% to 3% of all intracranial tumors but their incidence is rising. About 80% to 90% of PCNSLs are B-cell lymphomas, the majority of which are large B-cell variants with a high grade of malignancy. The rest are T-cell lymphomas.

PCNSLs affect both immunocompetent and immunocompromised individuals. The incidence is as high as 5% to 10% in patients with AIDS. It also affects patients with organ transplants, patients receiving long-term steroid therapy, and those with congenital immunodeficiency syndrome. The incidence of lymphomas in AIDS patients increases with the duration of antiretroviral therapy and the decline of CD4 counts. AIDS- and organ transplant-related lymphomas are strongly associated with a latent Epstein-Barr virus infection.

PCNSLs can occur at any age from childhood on. The peak incidence is in the fifth and sixth decades in immunocompetent patients, and in the third and fourth decades in immunosuppressed patients. The clinical history is short, only a few weeks or months. General manifestations are headaches, neuropsychiatric symptoms, cognitive decline, altered mentation, and seizures. Focal neurologic signs indicate the location of the tumor. A cytologic study of the CSF using immunohistochemi-cal markers and neuroimaging are the appropriate diagnostic tests.

Neuroimaging demonstrates the tumors around the lateral ventricles, in the corpus callosum, and in basal ganglia. On CT scan, in immunocompetent patients, lymphomas are hyperdense, occasionally hypo- to isodense relative to gray matter, and homogenously enhancing with contrast media (Fig. 11.44). In immu-nocompromised patients, they are hypodense and show ring-enhancement after contrast administration.

On Tl-weighted MR images, lymphomas are iso- to hypointense and show homogenous enhancement. On T2-weighted images they are isointense- to moderately hyperintense. Single photon emission tomography (SPECT) and positron emission tomography (PET) are useful in differentiating lymphomas from toxoplasmosis, a common complication in AIDS patients. Lymphomas are sensitive to steroid therapy; they regress and may become temporarily undetectable in neuroimaging.

In the general population, the mean survival of lymphoma patients is 1 to 2 years; and in immunocompro-mised patients, it is 3 to 6 months.

Grossly, PCNSLs are often multiple, preferentially situated in the supratentorial compartment, rarely in the cerebellum and brainstem. They are often deep-seated in the periventricular regions, corpus callosum, and basal ganglia. Their margins are indistinct; cut surfaces are gray or brown, firm or soft, friable or granular, and may contain hemorrhages and necrosis. Periventricular and meningeal spreads are usual.

Histologically, PCNSLs diffusely spread and extend beyond their macroscopic appearance. Typically, the cells are arranged around blood vessels and are encircled by reticulin fibers (Fig. 11.45). A panel of immunohis-tochemical stains using B- and T-cell markers identifies the cell of origin.

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