THE ROLE OF TL-201 UPTAKE AND RETENTION IN INTRACRANIAL TUMORS AFTER RADIOTHERAPY

This study prospectively assessed the diagnostic accuracy and prognost ic value of Tl-201 uptake and retention in primary and metastatic intr acranial tumors treated by conventional radiotherapy and/or radiosurge ry. Methods: An initial Tl-201 study (early and delayed images), was o btained in 60 postsurgical patients, 6-12 wk after radiotherapy or rad iosurgery, Repeat imaging was performed as clinically warranted. Tumor -to-background count ratios and a retention index (RI) were calculated for all lesions. Results: Abnormally increased Tl-201 uptake was obse rved in 40 of 60 patients. In all patients with positive results, the diagnosis of residual tumor was confirmed at biopsy or by clinical fol low-up. In 20 of 60 patients, no abnormal Tl-201 uptake was observed, despite findings on CT and/or MRI scans that were suspicious for tumor , Ten of the negative Tl-201 studies were confirmed as true-negatives by the clinical course and by resolution of CT/MRI abnormalities. The remaining 10 negative SPECT studies ultimately proved to be false-nega tives: six of these patients had lesions <1 cm in maximum diameter; on e patient had a large metastatic choriocarcinoma; and three patients h ad low-grade astrocytomas >2 cm in minimum diameter. Tumor-to-backgrou nd ratio of Tl-201 uptake did not distinguish between tumor type, or p redict clinical outcome. The RI of Tl-201 was significantly higher for metastatic melanoma than for other tumor metastases, it demonstrated reasonably good correlation with clinical outcome: 6/7 patients with e ventual tumor regression showed a decrease in Rt on follow-up examinat ion, and 4/5 patients with eventual tumor progression had an increase in RI. Conclusion: Thallium-201 brain SPECT appears to be a useful non invasive imaging technique in patients irradiated for intracranial tum ors. Thallium-201 scintigraphy has very high specificity (100% in this cohort) for detecting viable residual tumor. False-negative findings may occur. Quantitative analysis of Tl-201 uptake has limited diagnost ic and prognostic significance, but changes in Tl-201 retention after radiation therapy seems to have prognostic value.