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.