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. 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 mu Sv/h. The radioactivity was under 30 mu 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 mu Sv/h at 48 and 72 h, respectively, and it remained higher than 30 mu 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 mu Sv/h after 72 h, and scintigrams showed strong accumulation in bone metastases. Among the 27 cases demonstrating below 30 mu Sv/h at 48 h, 26 showed radioactivity being below 50 mu Sv/h at 30 h, while it was above 50 mu Sv/h at 30 h in all 14 cases which demonstrated above 30 mu Sv/h at 48 h. We compared the radioactivity levels of 27 cases under 30 mu Sv/h at 48 h and 14 cases over 30 mu Sv/h at 48 h using a cutoff value of under 50 mu 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). 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.
