Imaging

CT reveals an isodense or moderately hyper-dense lesion that enhances strongly and homogeneously with contrast. Mild or moderate edema is typical. Tumors in immunocompro-mised patients in particular may show a ring enhancing pattern. The periventricular area is a common location; most tumors abut the ependyma.

Primary CNS lymphomas are usually isointense or slightly hypointense on T1-weighted MR images and isointense or slightly hyperin-tense on T2-weighted images (Fig. 16.2). The enhancement pattern in immunocompetent patients is usually dense and homogeneous, but can be heterogeneous (Figs 16.2 and 16.3).

TUMORS: CEREBRAL METASTASES AND LYMPHOMA

Fig. 16.2. Axial T2-weighted images (left) and T1-weighted images without (center) and with (right) gadolinium enhancement in an immunocompetent patient with primary CNS lymphoma. The right frontal mass appears heterogeneously isointense and hyperintense on T2-weighted images, with surrounding hyperintense edema . On T1-weighted images, the isointense mass enhances densely and homogeneously, following contrast administration.

Fig. 16.2. Axial T2-weighted images (left) and T1-weighted images without (center) and with (right) gadolinium enhancement in an immunocompetent patient with primary CNS lymphoma. The right frontal mass appears heterogeneously isointense and hyperintense on T2-weighted images, with surrounding hyperintense edema . On T1-weighted images, the isointense mass enhances densely and homogeneously, following contrast administration.

Fig. 16.3. Axial T1-weighted images without (left) and with (right) gadolinium enhancement in an immunocompetent patient with primary CNS lymphoma. The ring enhancing lesion could also be metastasis, malignant glioma or even abscess.

Fig. 16.3. Axial T1-weighted images without (left) and with (right) gadolinium enhancement in an immunocompetent patient with primary CNS lymphoma. The ring enhancing lesion could also be metastasis, malignant glioma or even abscess.

Non-enhancing tumors have been seen, but are rare. In patients with AIDS-associated PCNSL, enhancement may be heterogeneous and is frequently rim-enhancing (Fig. 16.4). Radiographic evidence of hemorrhage and necrosis may be seen.

In patients with AIDS, CNS lymphoma may be very difficult to distinguish radiographically from other common intracranial pathologies, such as toxoplasmosis. Positron emission tomography and thallium SPECT have been proposed as more sensitive imaging modalities for distinguishing lymphoma from infectious lesions. Some authors claim that the combination of increased uptake on thallium SPECT and EBV DNA in CSF has 100% sensitivity and specificity for CNS lymphoma and obviates the need for biopsy [19].

The typical CSF profile with primary CNS lymphoma is elevated protein, low glucose and pleocytosis. CSF cytology showing a monomor-phic population of abnormal lymphocytes is diagnostic, but only 10% of patients undergoing CSF analysis at the time of presentation have positive cytologic findings. Furthermore, lumbar puncture may be contraindicated by

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