Isolation period prediction in patients with differentiated thyroid carcinoma treated after thyroidectomy by radioiodine-131.

In patients with differentiated thyroid carcinoma (DTC) who after thyroidectomy underwent radioiodine-131 ((131)I) treatment for the ablation of the thyroid remnant, isolation period is considered as the period of time needed for patients' radiation dose rates to be reduced below specific adopted dose rate release limits. The aim of our study was to determine mathematical equations for the prediction of the isolation period in these patients and of their discharge from special isolation rooms in nuclear medicine departments. In order, to predict the duration of the isolation period, we studied twenty-eight patients with DTC and total thyroidectomy having no metastases, who underwent (131)I ablation treatment for a minimal residual thyroid remnant. The administered (131)I activity was 5.22 +/- 0.68 GBq, ranging from 3.66 to 6.21 GBq. Dose rates, as mean +/- SD, at a distance of 1 m from the patients were 277 +/- 44 microSv/h, immediately after (131)I administration, 72 +/- 18 microSv/h at 24 h and 23 +/- 9 microSv/h at 48 h. Whole body (131)I retention was 0.261 +/- 0.05 (range 0.168-0.385) at 24 h and 0.082 +/- 0.03 (range 0.041-0.149) at 48 h, calculated as the ratio of dose rate at 24 h and 48 h versus the initial dose rate after (131)I administration. Isolation period was calculated by a mono-exponential fitting in dose rate decay data according specific dose rate release limits. For a dose rate release limit of 30 microSv/h at 1 m, isolation period was 42.7+/-7.2 h (range 31.2-56.6 h). Seventy-five percent of the patients satisfied this limit within 48 h and 25% between 48 h and 72 h. This isolation period was positively correlated with the whole body (131)I retention and the dose rates at 24 h and 48 h, but not always with the administered activity or the initial dose rate, measured immediately after (131)I administration. On the contrary, a strong negative correlation was found between patients' isolation period and dose rate release limits between 3 and 60 microSv/h at 1 m. This study indicates that isolation period is variable but can be predicted by multiple formulas, since it depends strongly on the adopted dose rate release limits, (131)I dose rates and whole body retention at 24 h and 48 h after (131)I treatment. For a dose rate release limit of 30 microSv/h at 1 m, isolation period is sufficient for 72 h for all of our patients, while 75% of them had dose rates below that limit within 48 h.