Evaluation of maternal and fetal S-values from iodine isotopes in nuclear medicine using the Monte Carlo method.

Internal dosimetry is critical in nuclear medicine, particularly for high-risk groups such as pregnant patients where minimizing radiation exposure to both the mother and fetus is essential. This study employs Monte Carlo simulations to calculate S-values for various target organs in a pregnant woman and her fetus, considering different maternal source organs and three iodine isotopes (131I, 125I, and 123I). Our findings indicate that 131I, with the highest emission energy, produces higher dose deposition in organs located farther from the source, while 125I results in more localized dose absorption in adjacent tissues. The highest S-values for the mother were observed in the thyroid (5.8E-5 mGy/MBq·s for 123I and 125I) and the urinary bladder (3.7E-4 mGy/MBq·s for 131I). In the fetus, the brain and skull received the highest radiation doses from 131I, with values of 6.3E-6 and 5.6E-6 mGy/MBq·s, respectively. These results provide key quantitative insights for optimizing dosimetric protocols in clinical practice, aiding in the development of strategies to reduce radiation exposure in pregnant patients and their fetuses.