Determination of absorbed dose of organs (thyroid, sternum, cervical vertebra) in thyroid cancer patients following radioiodine therapy

  Introduction: In patients with thyroid carcinoma, radiation absorbed doses of the thyroid and surrounding tissues is important to weigh risk and benefit considerations. In nuclear medicine, the accuracy of absorbed dose of internally distributed radionuclides is estimated by different methods such as MIRD and direct method using TLD. The aim of this study is using TLD and a phantom to determine the amount of cumulated activity in thyroid and surrounding tissues. Methods: Thermoluminescent dosimeter (TLD) measurements were performed on 27 patients on the skin over the 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. Several TLDs were placed putted on the surface of thyroid on phantom (similar to patients) for 24 hr and then compared the dose of phantom and patients followed by calculation of the activity in patient's thyroid. Results: TLD measurements showed cumulated radiation absorbed doses (cGy) of 315.6, 348.1 and 361.9 for thyroid with administration of 100, 150 and 175 mCiof 131I, respectively. For sternum the values found to be 201.5 cGy, 275.2 cGy and 242.6 cGy. For cervical vertebra results were 311.5 cGy, 184.1 cGy and 325.9 cGy. The average of measurements was 33.3 cGy using of TLDs on phantom and absorbed activity in thyroid were 94.9, 104.6 and 108.8 mCi in 24 hr for mentioned doses administration. Conclusion: In this work a method to obtain the absorbed activity in the thyroid and other surrounding tissues is described. By this method, the amount of 131I needed for each patient also could be determined.  The results of this work can be used in estimation of absorbed dose in thyroid and other organs using of MIRD method.