Thallium-201 SPECT imaging of recurrent brain tumors. Analysis of 8 cases

With post-irradiated brain tumors, it is sometimes difficult to distinguish between tumor recurrence and late radiation necrosis, because both conditions are similar by conventional neuroimaging and the length of the latent period following initial treatment. Thallium-201 SPECT was performed in 8 cases. The mean time elapsed after post-operative radiation therapy was 3 years and one month. The patients were of 4 adult males and 4 adult females, ranging in age from 29 to 66 with a mean age of 47.5 years at the first surgery. All cases were histologically verified: 3 low grade gliomas; 3 malignant gliomas; 1 malignant lymphoma, and 1 giant invasive pituitary adenoma. Five minutes after the intravenous injection of approximately 111 MBq of thallium-201, a ring-typed head scanner, SET-050 (Shimadzu), was used to acquire SPECT images. To evaluate objectively, we set up an ROI (region of interest) of 16 pixels at the tumor site (T), the contralateral cerebrum (C), the contralateral cerebellum (C'), and the scalp (S), and then calculated the ratios of T/C, T/C' and T/S. All patients underwent second surgical interventions, such as a craniotomy or a stereotactic biopsy. In the end, seven cases were diagnosed as recurrent and the remaining one was determined to have radiation necrosis. Of the 7 recurrent tumors, 2 low grade gliomas had turned malignant. On the SPECT images, the 7 recurrent cases showed an abnormally strong accumulation at the tumor site, whereas the case of late radiation necrosis revealed no abnormal deposit, suggesting that thallium-201 SPECT might be useful in the differential diagnosis between recurrence and late radiation necrosis. Among the various quantitative methods of evaluating SPECT images, the T/S ratio seemed to be the most useful since it had the smallest standard deviation. (author)