Method of ¹³¹I-thyroid uptake calculation using gamma camera system

Dosimetry method is one of the methods used to decide radioiodine dose for Treatment of hyperthyroidism. Two approaches are generally used to calculate radioiodine uptake by gamma camera with and without subtraction of background counts. We evaluated the difference in determining thyroid uptake caused by these two methods of calculation. Fifty patients with hyperthyroidism referred for radioiodine treatment to our hospital were included in this study. We calculated radioiodine uptake at 2 and 24 h by the method with background counts subtraction (method 1: corrected neck counts/standard × 100; corrected neck counts: neck counts − background counts (neck pixel area/background pixel area)) and other one is non background counts subtraction (method 2: neck counts/standard × 100). Values were analyzed by Wilcoxon signed-rank test as these data are nonparametric non normal distribution data. We found statistically significant difference between uptake values of two methods (P < 0/001) with median value at 2 h by method (1) was 17.7% (minimum 2.21% and maximum 68.7%) and by method (2) was 28.25% (minimum 5.79% and maximum 86.84). Uptake median value at 24 h by method (1) was 47.53 (minimum 4.08 and maximum 81.02) and by method (2) was 68.80 (minimum 7.73, maximum 97.32). Although the non background correction is simple and convenient for calculation, it gives overestimated neck uptake. As in dosimetry method, the neck uptake value is an important factor so we conclude that background correction neck counts should take into consideration while calculating the uptake values. (author)