The role of thallium-201 uptake and retention in intracranial tumors after radiotherapy

This study prospectively assessed the diagnostic accuracy and prognostic value of ²⁰¹Tl uptake and retention in primary and metastatic intracranial tumors treated by conventional radiotherapy and/or radiosurgery. Methods: An initial ²⁰¹Tl study (early and delayed images), was obtained in 60 postsurgical patients, 6-12 wk after radiotherapy or radiosurgery. 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 ²⁰¹Tl uptake was observed in 40 of 60 patients. In all patients with positive results, the diagnosis of residual tumor was confirmed at biopsy or by clinical follow-up. In 20 of 60 patients, no abnormal ²⁰¹Tl uptake was observed, despite findings on CT and/or MRI scans that were suspicious for tumor. Ten of the negative ²⁰¹Tl 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-negatives: six of these patients had lesions <1 cm in maximum diameter; one patient had a large metastatic choriocarcinoma; and three patients had low-grade astrocytomas >2 cm in minimum diameter. Tumor- to-background ratio of ²⁰¹Tl uptake did not distinguish between tumor type, or predict clinical outcome. The RI of ²⁰¹Tl was significantly higher for metastatic melanoma than for other tumor metastases. It demonstrated reasonably good correlation with clinical outcome: 6/7 patients with eventual tumor regression showed a decrease in RI on follow-up examination, and 4/5 patients with eventual tumor progression had an increase in RI. Conclusion: Thallium-201 brain SPECT appears to be a useful noninvasive imaging technique in patients irradiated for intracranial tumors. Thallium-201 scintigraphy has very high specificity (100% in this cohort) for detecting viable residual tumor. False-negative findings may occur. Quantitative analysis of ²⁰¹Tl uptake has limited diagnostic and prognostic significance, but changes in ²⁰¹Tl retention after radiation therapy seems to have prognostic value.