Abstract: Objective To investigate the shielding effect of distance in radioactive iodine treatment in patients with
differentiated thyroid cancer (DTC). Methods Eighty-seven DTC patients underwent postoperative radioactive iodine
treatment at the therapeutic doses ranging from 2.96 GBq to 7.4 GBq. The patients were divided into two groups to receive
high-dose therapy (≥3.7 GBq, 48 patients) and low-dose therapy (<3.7 GBq, 39 patients). The radiation doses at 0.05 m, 1 m,
and 3 m were recorded at different days; the doses at 1 m and 3 m on the third day, the dose of standard radioactivity source of
1.11GBq 131I, and the natural background radioactivity were also recorded. Results The radiation dose at a 1-meter distance
was significantly higher in the high-dose group than in the low-dose group (P<0.05). The radiation doses in different dose
groups at the other distances or at different time points showed no significant differences (P>0.05). On the third day after
therapy, the radiation dose at 1 m was significantly lower than the reference radioactivity source of 1.11 GBq 131I (P=0.000), but
still higher than the natural background radioactivity at 3 m (P=0.000). Conclusion In DTC patients who receive radioactive
iodine therapy, the radioactive radiation dose decreases rapidly after 3 days. The radioactive radiation dose on the third day is
significantly lower than the reference radioactive radiation dose, so that the patients can be discharged with safety for contact
at a distance beyond one meter.