Investigation of the effects of thyroid size on the iodine-131 S-values of the adult mesh-type reference computational phantoms (MRCPs)

This study investigated the effects of thyroid size reduction on the iodine-131 S-values of organs using the adult mesh-type reference computational phantoms (MRCPs) by developing adult male and female phantoms with 75 %, 50 %, 25 %, and 3.23 cm 3 thyroid sizes and then implementing them in Particle and Heavy Ion Transport code System version 3.341 (PHITS 3.341) using the thyroid as source. First, thyroid size reduction increased the number of electrons escaping from the thyroid which increased the S-values of its neighboring organs. Also, it decreased the thyroid mass which increased the S-value of the thyroid itself. Last, the thyroid modification process shifted the modified thyroid leftward and posteriorly relative to the position of the unmodified thyroid, potentially affecting the S-values of organs according to the inverse-square law. The added attenuation by the muscle tissue which was used to replace the resected thyroid tissue must have also decreased the S-values of other organs, especially those in the anterior side of the thyroid, but this effect requires further investigation. These findings are important when using the adult MRCPs for the assessment of the internal dose from radioactive iodine (RAI) therapy.