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[Differential diagnosis between recurrence of brain tumor and radiation necrosis by 201Tl SPECT].

In order to differentiate recurrence of brain tumor from radiation necrosis, we performed 201Tl brain SPECT in 16 post-operative patients. Five of them had residual anaplastic astrocytoma. Final diagnosis of the other 11 patients was recurrent astrocytoma (n = 6), cerebral radiation necrosis (n = 5), based on re-operation (n = 9) and more than one year follow-up finding (n = 2). Early and delayed images were obtained at 10-15 min. and 4 hrs. after i.v. injection of 111 MBq of 201Tl chloride. Counts ratio of a lesion to the normal brain (L/N) was calculated by creating rectangular ROI over each area. All of the patients with recurrent or residual tumor showed L/N ratios of more than 2.5 on the early images (mean 4.70 +/- 2.65). On the other hand, L/N ratios on the early images were less than 2.5 in patients with radiation necrosis (mean 2.05 +/- 0.25, p less than 0.001). In conclusion, a quantitative assessment using 201Tl brain SPECT may be useful in differentiating recurrence of brain tumor from radiation necrosis.