131I Effective Half-Life and Dosimetry in Thyroid Cancer Patients

131I treatment in thyroid cancer patients may induce side effects, including extrathyroidal cancer and leukemia. There are still some uncertainties concerning parameters that may influence the effective half-life of 131I and the absorbed doses by extrathy-roidal organs. Methods: Whole-body retention of radioiodine was measured in 254 patients, and repeated quantitative whole-body scans and measurements of the urinary excretion of 131I were performed on 30 of these patients. Results: The mean effective half-life (10.5 h) was shorter by 31%,with little dif-ference between patients, in the 36 patients who received re-combinant human thyroid-stimulating hormone than in the 218 patients who underwent thyroid hormone withdrawal (15.7 h). The residence times in the stomach and in the rest of the body were significantly shorter in patients who received recombinant human thyroid-stimulating hormone than in patients who under-went withdrawal, but the residence times were similar in the co-lon and bladder. Conclusion: In patients who undergo thyroid hormone withdrawal, the longer mean effective half-life is mainly due to delayed renal excretion of 131I and results in dose esti-mates higher than the data in report 53 of the International Com-mission on Radiological Protection, which were obtained from healthy, euthyroid subjects. Key Words: 131I; effective half-life; recombinant human TSH; thyroid cancer; dosimetr