Organ Dosimetry in Radionuclide Therapy: Biokinetics of 131 I and Dose with a Healthy Thyroid and after Thyroidectomy Using MCNP6.

An accurate internal dosimetry is important in 131 I radionuclide therapy. In patients who have undergone a thyroidectomy, organs other than the thyroid absorb higher doses as the radionuclide is transported through the body because most of the thyroid gland is removed, and it cannot retain the 131 I. The primary objective of this study is to compare the absorbed doses in at-risk organs due to intake of 131 I after thyroidectomy vs. a healthy thyroid scenario. The biokinetic model of internal dosimetry for healthy thyroid as proposed by the International Commission on Radiological Protection (ICRP) in Publication 137 and a radiation transport simulation in Monte Carlo N-Particle 6 (MCNP6) were used. From the biokinetic model, average activities of 131 I over a 30 h period were obtained, and these activities were used as source terms in MCNP6 simulations transporting gamma rays and beta particles to estimate absorbed doses in at-risk organs. The use of average activities is a new approach proposed in this study. Once the internal dosimetry model for a healthy thyroid was complete and functional, it was adapted for the scenario following thyroidectomy. This involved changing the transfer coefficients connected with the thyroid in the biokinetic model to constants proposed by Taprogge et al. in 2021 . With this adjusted model, average activities of 131 I over a 30-h period were obtained again and used as source terms in MCNP6 simulations of a patient following thyroidectomy, and from this simulation, absorbed doses in at-risk organs were estimated. These coefficients had not been used to estimate organ doses using MCNP. The 131 I activity in the thyroid gland was found to be 20.4% of the initial administered dose in the scenario with a healthy thyroid vs. 0.64% following thyroidectomy. After thyroidectomy, all organs other than the thyroid absorbed higher radiation doses compared to the scenario with a healthy thyroid. The results of this study allow comparison of the absorbed doses between the scenarios of a healthy thyroid and after thyroidectomy, showing substantial differences between the modeled scenarios, which underscores the need to consider the larger context when assessing radiological impact on at-risk organs during 131 I therapy.