Determination of organ doses in radioiodine therapy using medical internal radiation dosimetry (MIRD) method

Background: Radioiodine therapy has proven to be an effective method in the treatment of patients with differentiated thyroid carcinoma after thyroidectomy. The scope of this study is to describe a method to obtain the dose of organs using medical internal radiation dosimetry (MIRD) method. At the end, the results of MIRD calculations were compared with thermoluminescent dosimeter (TLD-100). Materials and Methods: The study was performed on 27 patients using TLD for thyroid, sternum and cervical vertebra. There were 5 TLDs for each organ which they were taken after 4, 8, 12, 20 and 24 hr. To calculate the amount of activity in the thyroid a head and neck phantom with a source of 10 mCi of 131I was used. A head and neck phantom was used to determine the absorbed dose. A source of 10 mCi of 131I was putted on phantom. Several TLDs were placed on the surface of thyroid on phantom for 24 hr and then compared with the dose of phantom and patients followed by calculation of the activity in patient's thyroid. Finally, MIRD formula was used to calculate absorbed dose in cervical vertebra and sternum. Results: The average of measurements of TLDs on phantom for 10 mCi of iodine was 33.3 cGy. The absorbed activity in thyroid in three groups for 100, 150 and 175 mCi administered 131I were 94.9, 104.6 and 108.8 mCi cumulated activity in 24 hrs. The absorbed dose obtained by MIRD calculations was found to be 419.9, 463.2, and 481.5 for thyroid, 288.9, 252.4 and 252.4 for sternum and 288.9, 252.4 and 252.4 for cervical vertebra. Conclusions: The results of MIRD method was similar to the results obtained experimentally. It was shown that 75% of absorbed dose calculated by the MIRD method is detectable by the TLD method. Iran. J. Radiat. Res., 2011 8(4): 249-252