Objective To analyze and explore the possible mechanism for pelvic ¹³¹I uptake after ¹³¹I post treatment whole body scan(RX-WBS)in patients with differentiated thyroid cancer.

Methods ① Data were retrospectively reviewed from 168 female patients with differentiated thyroid cancer(everyone has a RX-WBS). ②46 patients were accepted by analyzing the characteristics of Rx-WBS and combing with some inclusion criteria, and then followed up.

Results Among the 46 patients(46 positions accumulated ¹³¹I)with significant pelvic ¹³¹I uptake, 6 patients had two reasons leading to pelvic ¹³¹I uptake, and 2 patients had no specific reason. Among the 50 reasons for pelvic ¹³¹I uptake, 41 reasons related with uterus, 3 reasons related to rectum, 5 related to bladder and 1 related to ovarian chocolate cyst. Among the 41 reasons related to uterus, by combining the examinations of SPECT/CT, ultrasound, CT and the follow-up results, 18 were uterine leiomyomas, 9 were intrauterine devices, 2 were endometrial thickening, 3 were uterine cavity effusion, 7 were menstrual periods, 1 were uterine adenomyosis, 1 were gestational sac.

Conclusions ① In the Rx-WBS of female, the significant pelvic ¹³¹I uptake is generally caused by uterus, but not bladder. And it usually means gynecological disease, especially uterine leiomyomas when excluding physiological factors. ②It is generally easy to differentiate bladder from rectum because they have different characteristic features of the pelvic ¹³¹I uptake. ③SPECT/CT plays a very important role in locating ¹³¹I uptake in uterus.