Updating internal dosimetry coefficients of I-131 based on new mesh-type computational phantoms of adult male and female using the DoseCalcs platform

The use of radionuclides in nuclear medicine, both for diagnostic and therapeutic purposes, represents a cornerstone of modern cancer treatment, particularly when utilizing iodine-131. When iodine-131 is administered, it is designed to specifically target the thyroid gland, delivering maximum radiation energy to the tumor. However, iodine-131 circulates through the bloodstream and reaches various regions in the body, exposing them to radiation. In this context, accurate dosimetry calculations are essential to ensure both the safety and efficacy of the treatment.

We used the latest mesh-type phantom generations to update existing S-values derived from stylized and voxel-based phantoms, leveraging the enhanced anatomical representation of organs/tissues in the latest phantom models.

To perform the simulations, we used the Monte Carlo-based DoseCalcs platform, in which we implemented anatomical characteristics such as shape data and chemical composition of regions provided by ICRP Publication 145 and radiation data of I-131 from ICRP Publication 107 to simulate the transport of radiation emitted by I-131 and calculate the corresponding S-values for several source-organ combinations.

We calculated I-131 S-values for nine source regions and twenty-eight target regions using mesh-type phantoms of adult male and female. When comparing these values with those calculated using voxel-based phantoms, we observed that the two datasets were generally similar for most target$\leftarrow$source combinations. However, notable differences were identified, particularly for distant combinations (e.g., brain $\leftarrow$ urinary bladder contents) and adjacent regions such as content and wall combinations. In some cases, the ratio reached up to 4 (e.g., stomach wall $\leftarrow$ stomach contents, urinary bladder wall $\leftarrow$ urinary bladder contents).

These results underscore the impact of geometric modifications in mesh-type phantoms, particularly for wall regions, where anatomical representations differ significantly from voxel-based phantoms.