[Shielding effect of distance in (131)I-131 therapy for differentiated thyroid cancer]

To investigate the shielding effect of distance in radioactive iodine treatment in patients with differentiated thyroid cancer (DTC).

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 (131)I, and the natural background radioactivity were also recorded.

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 (131)I (P=0.000), but still higher than the natural background radioactivity at 3 m (P=0.000).

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.