RAPID DIFFERENTIAL-DIAGNOSIS OF CEREBRAL TOXOPLASMOSIS AND PRIMARY CENTRAL-NERVOUS-SYSTEM LYMPHOMA BY TL-201 SPECT

This study sought to assess whether Tl-201 brain SPECT can significant ly reduce the time required for the differential diagnosis of primary central nervous system (CNS) lymphoma and cerebral toxoplasmosis in pa tients with AIDS. Methods: Eighteen patients who presented with focal lesions on CT or MRI, or both, underwent Tl-201 brain SPECT shortly af ter admission and before a CT-guided stereotactic brain biopsy. Early and delayed Tl-201 uptake ratios were obtained for patients with posit ive Tl-201 study results, and the retention index of Tl-20l was calcul ated. Results: Ten patients had 11 foci of significantly increased Tl- 201 uptake in regions of corresponding CT/MRI lesions. Five of these p atients had biopsy-proven lymphomas, one of them in two separate foci. Another patient was found to have metastatic adenocarcinoma. Three pa tients had a clinical course and response to radiation therapy consist ent with lymphoma, and study results in another patient were considere d falsely positive. Of nine patients with no Tl-201 uptake in regions of CT/MRI lesions, two had biopsy findings consistent with a benign et iology, and the other seven improved clinically on antitoxoplasmosis m edications alone. The overall sensitivity of Tl-201 blain SPECT was 10 0%, and specificity was 90%. The Tl-201 retention index in patients wi th lymphomas was significantly higher than that in patients with adeno carcinomas and nonmalignant lesions (1.35 +/- 0.16 versus 0.24 and 0.5 6, respectively). Conclusion: Thallium-201 brain SPECT is a sensitive and specific method for rapid differential diagnosis of CNS lymphoma a nd toxoplasmosis in patients with AIDS. The Tl-201 retention index is useful in differentiating CNS lymphomas from other malignant and nonma lignant pathologies.