Comparison of Iodine-131 Absorbed Dose and Technetium-99m in Thyroid Scanning Using the MCNPX Simulation Code in the ORNL-MIRD Phantom

Thyroid scanning is performed to examine the thyroid gland disorders using 99mTc. Through the obtained images can evaluate the size of the thyroid gland, the amount of its removal compared with the salivary gland, examine the thyroid nodules and its performance impairments. According to the scan view, it is possible to differentiate between hot and cold thyroid nodules that need different therapies. The study of cervical mass with expansion to the chest is another major function of thyroid scanning with 131I. In this method, 24 hours after administration of little 131I, imaging of the neck and chest area is performed. The purpose of the present study was to simulate the emitted particles of 131I radiation and 99mTc, calculate and compare the absorbed dose and the S-value of these two radiopharmaceuticals in the thyroid and other organs around the thyroid, such as the lung, skin, and brain. Simulation and computations were performed using the Monte Carlo method by MCNPX software. ORNL MIRD phantom geometry was used for thyroid and other organs geometry. The absorbed dose in the thyroid and other organs increased with increasing activity, but the absorbed dose in the other organs was lower than the thyroid. The results showed that absorbed dose in 99mTc was lower than 131I. Although  131I has a diagnostic function in thyroid diseases, it has its own special disadvantages. 131 I in addition to 364kev high energy of gamma photons radiations, emits high-energy particles of 647 kev beta, too. Secondly, it has a lifetime of 8/05. Therefore, in studies conducted with 131I, the amount of radiation exposure of the thyroid gland and the total body  is very high (about 15,000 rades are the absorbed thyroid dose). On the other hand, the Tc pertechnetate with a lifetime of 6 hours and gamma energy of 140 kilos of electron volts, like 131I, is absorbed by the thyroid gland. Therefore, regarding the advantages of Tc pertechnetate to the 131I radiopharmaceutical , it is possible to use the absorbing of Tc  pertechnetate  as a measure to evaluate thyroid function.