Abstract： Objective:To explore the radiation dose and its variation patterns of diagnostic 131I SPECT/CT whole-body imaging in patients with differentiated thyroid cancer after iodine remnant ablation to provide a basis for radiation safety evaluation and occupational protection research. Methods:Using convenience sampling, retrospective analysis was conducted on 20 patients (including 5 males and 15 females, aged (45.7±11.7) years, ranging from 24.0 to 65.0 years old) who underwent diagnostic 131I SPECT/CT whole-body imaging at the 980th Hospital of the Joint Logistics Support Force of PLA from June to December 2021. An X-γ dose rate meter was used to measure the radiation dose rate on the chest surface of patients at different time periods (2, 24, 48, 72, and 96 h) and at different distances (1.0, 2.0, and 3.0 m) after taking 131I. The effective dose equivalent of patients was measured on the first to fourth day by using a thermoluminescent dosimeter after taking 131I. Results:After 2 h of taking 131I, the highest radiation dose rate was observed on the chest surface of patients (186.12 (135.52-201.01) μSv/h), and the radiation dose rate significantly decreases with the prolongation of time after taking 131I and with the increase of distance from the chest surface. The radiation dose rate on the chest surface above 96 h after taking 131I, as well as the radiation dose rate at any time 3.0 m away from the body surface, can reach the background radiation level. After taking 131I, the radiation equivalent dose on the chest surface of patients was (1.19±0.28) mSv (ranging from 0.90 to 1.66 mSv) on the first day, and no significant increase was found from the second day onwards compared with the previous day. Conclusions:The radiation dose rate of patients with differentiated thyroid cancer who underwent diagnostic 131I SPECT/CT whole-body imaging after iodine remnant ablation was low, and it decreased rapidly. Therefore, quarantine is not required for patients, but the radiation dose on the body surface is relatively high in the early stages of taking 131I, so abstaining the patients from contacting others is necessary.