Thallium-201 retention in focal intracranial lesions for differential diagnosis of primary lymphoma and nonmalignant lesions in AIDS patients.

UNLABELLED: The purpose of this study was to determine whether 201Tl retention in focal intracranial lesions can help distinguish central nervous system (CNS) lymphoma from toxoplasmosis and other nonmalignant CNS lesions in patients with acquired immunodeficiency syndrome.
METHODS: Forty-nine patients who presented with focal lesions on CT and/or MRI had 201Tl brain SPECT studies (early and delayed image sets) performed shortly after admission. Early and delayed 201Tl uptake ratios were obtained for the positive studies, and the retention index of thallium was calculated (delayed/early target-to-background mean count ratio).
RESULTS: Twenty-nine patients had foci of significantly increased 201Tl uptake on the early images in regions of corresponding CT/MRI lesions. Ten of these patients had biopsy-proven lymphomas. Another patient was found to have metastatic adenocarcinoma. Twelve additional patients had a response to radiation therapy or a clinical course consistent with lymphoma and six patients had a false-positive SPECT study. The early uptake ratio could not separate malignant from nonmalignant lesions. The 201Tl retention index in patients with lymphomas (1.18 +/- 0.16) was significantly higher than the retention index in adenocarcinoma (0.24) and in the six nonmalignant lesions (0.62 +/- 0.07). The lowest retention index in patients with lymphoma was 1.07, and the highest retention index in nonmalignant lesions was 0.70. Twenty patients showed no 201Tl uptake in the regions of CT/MRI lesions. Three of them had biopsies consistent with a benign etiology, and one patient was diagnosed with tuberculosis. Fifteen patients improved clinically on antitoxoplasmosis medications alone, and one patient had CNS lymphoma. The overall sensitivity of 201Tl brain SPECT was 96%. The specificity was 76% by counting all studies with abnormal 201Tl uptake, but it increased to 100% when the retention index was also considered.
CONCLUSION: The retention index increases the specificity of 201Tl brain SPECT in human immunodeficiency virus patients. In the presence of abnormal early 201Tl uptake, it is essential to perform delayed imaging and calculate the retention index to distinguish nonmalignant lesions from lymphoma. The absence of 201Tl uptake on early images at the site of a CT/MRI abnormality excludes the diagnosis of lymphoma with a high degree of confidence and delayed imaging is unnecessary.