Single-photon emission CT in the evaluation of recurrent brain tumor in patients treated with gamma knife radiosurgery or conventional radiation therapy.

PURPOSE To differentiate radiation necrosis from tumor recurrence using single-photon emission CT (SPECT). METHODS Forty-two thallium-201 SPECT scans were obtained in 33 adult patients. All patients had previously been treated with either conventional external-beam radiation therapy (XRT) (26 patients) or gamma knife radiosurgery (16 patients) and had subsequent contrast-enhanced MR examinations before nuclear medicine imaging. Patients were injected intravenously with 4 mCi of thallous chloride Tl 201 with rapid acquisition of SPECT scans thereafter. Findings on thallium-201 SPECT scans were categorized as either positive or negative on the basis of an abnormal focus of increased activity in the area of the known lesion. Evaluation of the accuracy of the thallium-201 SPECT scan was made by correlation with subsequent histologic or cytologic analysis (28 cases) or, in cases in which biopsy was not performed, by subsequent clinical course or MR findings (14 cases). RESULTS For all patients in this series, the sensitivity and specificity of thallium-201 SPECT scans in detecting tumor recurrence were 94% and 63%, respectively. The sensitivity and specificity for patients who had been treated with XRT were 95% and 60%, respectively. The sensitivity and specificity for patients who had been treated with gamma knife therapy were 92% and 67%, respectively. Tumor/scalp (T/S) ratios were calculated for all patients. High T/S ratios (> 2.0) had a positive predictive value for tumor recurrence of 92%. T/S ratios less than 0.5 had a negative predictive value for tumor recurrence of 83%. CONCLUSION Thallium-201 SPECT is a sensitive and accurate test to differentiate tumor recurrence from radiation necrosis in a population of patients with abnormal MR findings. This technique was equally efficacious in patients who had undergone gamma knife radiosurgery and in those who had received XRT.