Objective To evaluate how long after iodine-131 treatment of hospitalized patients with differentiated thyroid cancer (DTC), the radiation dose in patients fit the discharged requirements, as well as the radiation levels in wards and facilities in which the patients lived.

Methods A total of 143 patients with DTC were hospitalized and treated with iodine-131 from January 2015 to March 2017 in the department of nuclear medicine, third affiliated hospital of Sun Yat-sen university. The participants consisted of 46 males and 97 females aged 37.6±12.4 years. The patients were divided into three groups according to the radiation dose of iodine-131. Groups 1, 2, and 3 were administered with 3.70, 5.55, and 7.40 GBq, respectively. After the administration of iodine-131 for 0.5, 2, 4, 8, 12, 24, 36, 48, 72, and 80 h, the radiation dose rate was measured at 1 m in front of the patients by using a real-time radiation monitor, and the changes in radioactivity in the patients were estimated. On the day after the patients were discharged, the radiation doses on the patients' clothes, bed sheets, ward floor, and ground at the bathroom door were monitored using a surface radiation monitor. Data were subjected to variance analysis, and a P<0.05 was considered statistically significant.

Results Groups 1 (597.5±196.3) μsv/h, 2 (794.5±254.2) μsv/h, and 3 (1114.1±258.5) μsv/h had significantly different radiation dose rates (F=62.757, P<0.01) after 0.5 h of iodine-131 administration. Prolonged radiation exposure resulted in rapid exponential decrease in the radiation dose rate and gradual reduction in the differences among the data. The radiation dose rates of groups 1, 2, and 3 (13.3±10.7), (16.1±9.7), and (21.9±14.5) μsv/h, respectively after 72 h of iodine-131 administration were not significantly different (F=2.313, P>0.05). The dose rate in 95.8% of the patients (137 out of 143 patients) was less than 23.3 μSv/h after approximately 80 h of iodine-131 administration. After the patients were discharged from the hospital, the surface dose rates on the bed sheets, patients' clothes, ward floor, and ground at the bathroom door were (3.9±1.2), (4.1±1.9), (3.8±1.6), and (6.2±2.6) μsv/h, respectively. These values were lower than the corresponding radiation limit levels.

Conclusions After 80 h of treatment with no more than 7.40 GBq iodine-131, the residual dose of iodine-131 in most patients was lower than its restricted value in China. The radiation pollution level of the patients' ward grounds, bed sheets, and clothes were also lower than the radiation limit. Thus, these materials and facilities require no special treatment.