Thallium brain SPECT and MRI correlation in the evaluation of tumour recurrence versus radiation necrosis

Full text: This study compares different methods of determining thallium tumour uptake indices. Correlation with MR was performed to evaluate features that may affect the thallium index (TI) and to improve specificity for differentiation of recurrent tumor from radiation necrosis. 23 patients who had received radiotherapy for a brain neoplasm were included. The TI was determined using three different methods including large and small regions-of-interest (ROI). The concordance between the thallium SPECT and MRI was assessed. The effect of central necrosis on the different thallium indices derived was evaluated. 18 patients were determined to have recurrent tumor and five had inactive disease. The optimal TI cut-off values was statistically delivered and sensitivity and specificity was 78-94% and 80% respectively for cut-off values between 2.0 and 2.6 depending on the method used to calculate the TI. When compared with MRI, the majority of SPECT abnormalities correlated well with location and degree of uptake and enhancement. Seven cases showed central necrosis and the degree of necrosis had less effect on the TI when a small ROI was used in these cases. In conclusion thallium brain SPECT is a sensitive technique for detecting recurrent tumour. When performing semi-quantitative assessment of thallium uptake, a smaller ROI over the most intense area of uptake will reduce the underestimation of the TI in the presence of necrosis and a Tl cut-off value of 2.6 gave optimal accuracy using this method. Correlation with MRI aids in localization, particularly where there is anatomic distortion and enables more accurate analysis of these lesions by avoiding areas of necrosis. Copyright (2000) The Australian and New Zealand Society of Nuclear Medicine Inc