Abstract
BACKGROUND: Monitoring the external dose rate (EDR) attenuation serves as a key consideration in supporting discharge decisions for patients with differentiated thyroid cancer (DTC) who have undergone radioiodine therapy. We aimed to study the EDR attenuation and its related factors in DTC patients during I-131 therapy. METHODS: This study enrolled 886 DTC patients who first underwent I-131 therapy at the Third Bethune Hospital of Jilin University, China. We measured the EDR at approximately 2, 24, 48, and 72 h post-therapy. Two formulas were established to represent the EDR decay with time: 1) EDR =[Formula: see text] and EDR(%) = [Formula: see text], where EDR is the absolute external dose rate (µSv/h), EDR(%) is the percentage EDR relative to the initial EDR (100%), SI (speed index, μSv/h(2)) is the absolute decay rate of I-131 with the time, SI(%) (%/h) is the relative decay rate with the time, and b is a constant. RESULTS: The finally fitted SI and SI(%) from patients' data were -0.020 μSv/h(2) and -0.026%/h, respectively. EDR(%) exhibited a stronger correlation with administration time than EDR (R(2): 0.951 vs. 0.829). Body mass index (BMI), smoking, history of type 2 diabetes mellitus, Follicular Thyroid Carcinoma (FTC) subtype, increasing residual thyroid tissue grading, FT3 and Tg levels positively associated with SI. The factors negatively associated with SI were female sex, a higher N stage and a higher I-131 dose. SI(%) was positively associated with smoking history, history of type 2 diabetes mellitus, and FTC pathological subtype, and negatively with female sex and higher I-131 dose. CONCLUSIONS: EDR(%) had better correlation than EDR with I-131 administration time. The related factors for SI and SI(%) included I-131 dose, sex, BMI, thyroid cancer pathology, medical history and thyroid function. These findings provide a reference for radiation protection officers in evaluating radioactive activity during I-131 therapy.