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

*Corresponding author: Dr. Daryoush Shahbazi-Gahrouei, Dept. of Medical Physics and Medical Engineering, Isfahan University of Medical Sciences, Isfahan, Iran. Fax: +98 311 6688597 E-mail: [email protected] carcinoma after thyroidectomy (1-6). In this therapy method, the prescription of sodium iodine is done orally (7). Radioiodine is a beta emitting radionuclide with a physical half life of 8.1 day, a principal gamma ray of 364 keV, and principal beta particle with a maximum energy of 0.61 MeV and average of 0.192 MeV and a range in tissue of 0.8 mm (7). Usually 30-100 mCi of 131I is prescribed for the first radioiodine therapy and higher amounts are given in subsequent therapies or in the case of metastatic disease. Usually the activity is limited for safety aspects around 200 mCi. However, a higher administered activity is desired to achieve higher tumor doses (1). Treatment with 131I may result in abnormalities in other organs so it is important to estimate organ doses. The accuracy of absorbed dose of an internally distributed radio-nuclides estimated by different methods such as MIRD (Medical Internal Radiation Dosimetry). Dosimetry is required by the clinician for several reasons. First, treatment is often limited by the dose delivered to critical organs, for example bone marrow. Second, dosimetry is required to prescribe the correct activity of radioiodine. Indeed, internal radiation dosimetry of radiopharmaceuticals is an important aspect of nuclear medicine to weigh risk versus benefit considerations. In MIRD method, the dose absorbed in the target organs are estimated by the activities accumulated in 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): 249­252