en Medical Physics and Radiation Therapy Medical Physics and Radiation Therapy Original Research: Basic Science in Medicine Original Research: Basic Science in Medicine The treatment of thyroid cancer using an unsealed source of radioactive materials is usually associated with a large amount of 131I. A major problem for hospital admission of these patients is the waste disposal which requires protection of the public. 152 patients with thyroid cancer admitted to the nuclear medicine department of Said AI- Shohada Hospital for UI I treatment were studied. 1311 excreted from these patients during isolation was calculated. Exposure from patients were measured using a Victorian 190F survey dosimeter. More than 70 percent of the administered 131I was excreted after 24 hours, 90 percent after 48 hours and 96 percent after 72 hours of isolation. The mean biological half life of 1311 in patients with thyroid cancer was found to be 13.9±1.9 hours. There was no significant difference between the mean effective half life in the patients related for the first time and second time at 95 percent significance level. The results of this study showed that the difference in the discharge rate of 1311 from patients with thyroid cancer receiving first and second treatment was not significant. The mean discharge rate after the first 24 hours was more than 70%, and it was more than 96% after the third 24 hours of drug administration. The results can be used to design safe collecting and discharge methods of the wastes. Thyroid cancer, Radioactive waste discharge, Biological half-life, Radiation dose. 273 276 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-298-62&slc_lang=en&sid=1 MOHAMAD BAGHER TAVAKOLI mbtavakoli@mui.ac.ir 20031947532846003457 20031947532846003457 Yes From the Department of Medical Physics and Medical Engineering, School of Medicine, Isfahan UniversityOf Medical Sciences, Isfahan, Iran