Biodistribution and dosimetry of 131I in thyroidectomy patients using semiquantitative γ-camera imaging

The aim of this study was to determine the biodistribution, effective retention half-time (Teff), and internal organ dosimetry of 131I administered to patients with papillary thyroid cancer after a total or near total thyroidectomy, and whole-body and organ activity was measured by γ-camera imaging at 16 time points over 5 days following the administration of 1.1 GBq of 131I in 5 patients. Whole-body scanning was performed by using a Siemens E-CAM™ γ-camera at a fixed distance of 5 cm from the patient. 131I activity in regions of interest (ROIs) corresponding to the bladder, heart wall, kidneys, liver, lower gastrointestinal tract, lung, stomach, and thyroid remnants were measured, with measurements in the thigh used as the background. Most activity was localized to the lower gastrointestinal tract and thyroid remnants on day 1. The longest Teff was in the lung and the shortest was in the bladder (34.0 and 16.8 hours, respectively). The three organs with the highest absorbed doses per unit of administered activity were the lower gastrointestinal tract (2.03 mSv/MBq), the stomach (1.32 mSv/MBq), and the heart wall (1.24 mSv/MBq). T eff for thyroid remnants and the whole body were also estimated. We evaluated these organ doses to determine the normal tissues most likely at risk for stochastic and deterministic effects associated with 131I therapy for thyroid cancer and benign disease. Our results represent preliminary findings, and the study will continue to accrue a greater number of study participants.