Body-size-dependent Iodine-131 S values

Abstract

In a recent epidemiologic risk assessment on late health effects of patients treated with radioactive iodine (RAI), organ/tissue doses of the patients were estimated based on iodine-131 S values derived from the reference computational phantoms of the International Commission on Radiological Protection (ICRP). However, the use of the S values based on the reference phantoms may lead to significant biases in the estimated doses of patients whose body sizes (height and weight) are significantly different from the reference body sizes. To fill this critical gap, we established a comprehensive dataset of body-size-dependent iodine-131 S values (rT ← thyroid) for 30 radiosensitive target organs/tissues by performing Monte Carlo dose calculations coupled with a total of 212 adult male and female computational phantoms in different heights and weights. We observed that the S values tend to decrease with increasing body height; for example, the S value (gonads ← thyroid) of the 160 cm male phantom is about 3 times higher than that of the 190 cm male phantom at the 70 kg weight. We also observed that the S values tend to decrease with increasing body weight for some organs/tissues; for example, the S value (skin ← thyroid) of the 45 kg female phantom is about two times higher than that of the 130 kg female phantom at the 160 cm height. For other organs/tissues, which are relatively far from the thyroid, in contrast, the S values tend to increase with increasing body weight; for example, the S value (bladder ← thyroid) of the 45 kg female phantom is about 2 times lower than that of the 130 kg female phantom. Overall, the majority of the body-size-dependent S values deviated to within 25% from those of the reference phantoms. We believe that the use of body-size-dependent S values in dose reconstructions should help quantify the dosimetric uncertainty in epidemiologic investigations of RAI-treated patients.