A retrospective study on the transition of radiation dose rate and iodine distribution in patients with I-131-treated well-differentiated thyroid cancer to improve bed control shorten isolation periods

Objective

To evaluate for how long patients should be isolated after I-131 treatment for thyroid cancer according to the guidelines issued by the Japanese Ministry of Welfare.

Methods

We reviewed 92 therapies performed in 76 patients who were administered I-131 at our hospital from July 2007 to September 2009. Fifty-six patients were given 2220 or 2960 MBq I-131 at the first therapy, and 29 patients underwent 36 repeated therapies using 2960, 3700, 5550 or 7400?MBq I-131. We surveyed radioactivity for a 1?cm dose equivalent rate at 1?m intervals at 30 and 48?h after administration of I-131, obtained planar scintigrams at 48?h, and surveyed radioactivity repeatedly until it fell to under 30???Sv/h.

Results

The radioactivity was under 30???Sv/h at 30?h in 51 out of 92 cases (55%). Among the remaining 41 (45%) cases, 27 (29%) and 32 (35%) cases showed decreased radioactivity under 30???Sv/h at 48 and 72?h, respectively, and it remained higher than 30???Sv/h at 72?h in another 9 cases (10%). In 5 (38%) of the 13 cases with bone metastasis, the radioactivity remained over 30???Sv/h after 72?h, and scintigrams showed strong accumulation in bone metastases. Among the 27 cases demonstrating below 30???Sv/h at 48?h, 26 showed radioactivity being below 50???Sv/h at 30?h, while it was above 50???Sv/h at 30?h in all 14 cases which demonstrated above 30???Sv/h at 48?h. We compared the radioactivity levels of 27 cases under 30???Sv/h at 48?h and 14 cases over 30???Sv/h at 48?h using a cutoff value of under 50???Sv/h at 30?h to release patient at 48?h, the positive predictive value and negative predictive value were 100 and 93%, respectively, and radioactivity was found to differ significantly (P?<?0.001).

Conclusions

To predict external radiation levels at 48?h, it is helpful to consider external radiation levels at 30?h after treatment. Consideration of intracellular uptake in thyroid cancer, especially in cases of bone metastases, digestive tract function, and renal function, is important for predicting isolated period.