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Background. The optimal management of patients with brain tumors requires knowledge of the tumor characteristics upon presentation and the discovery of recurrence after therapy. Thallium-201 (Tl-201) chloride has shown varying uptake in tumors, depending on their viability and the type and degree of malignancy. This study explores the diagnostic potential of thallium imaging in patients with brain tumors. Methods. Forty-three Tl-201 single photon emission computed tomographic scintigrams were performed on 40 patients with intracranial neoplasms, nearly equally divided between patients with no prior treatment and patients who had prior treatment and were suspected to have recurrent tumor and/for radiation necrosis. A thallium tumor index was calculated as the ratio of counts for a region of interest drawn in the lesion area and its mirror image in normal brain tissue. A two-tailed Student's t test was performed to compare the thallium index and histopathologic findings. Results. A value of 1.5 for the thallium tumor index allowed for the best correlation between the prediction of malignancy and the histopathologic results. In the pretreatment group, a thallium tumor index greater than 1.5 correlated with high grade malignancy, and less than 1.5 correlated with either a well differentiated astrocytoma or benign cyst. In the posttreatment group, a thallium tumor index greater than 1.5 correlated with recurrent and/or residual malignant tumor. Conclusions. For those patients undergoing initial evaluation, the thallium study can help in the differential diagnosis of an intracranial mass lesion and offers confirmation of results of biopsy. For those patients who already have received treatment, the study can be used to detect recurrent or residual tumor.