A hybrid biokinetic-Monte Carlo framework for internal dosimetry of I-131 using a modified MIRD phantom

Iodine-131 is widely used in nuclear medicine, and accurate assessment of its internal dosimetry requires appropriate biokinetic and radiation transport models. Integrating physiological iodine kinetics with Monte Carlo simulation provides a methodological framework for estimating organ doses. Nevertheless, during systemic distribution, several non-target organs such as the salivary glands, stomach, and urinary bladder are exposed to ionizing radiation, leading to potential side effects including sialadenitis and xerostomia. A hybrid dosimetric framework was developed by integrating a physiological biokinetic model of iodine metabolism with Monte Carlo photon and beta transport simulation. The Leggett iodine model was implemented in MATLAB to compute organ-specific time-activity curves and cumulated activities. Since the Leggett model is based on normal thyroid function, the comparison with ICRP data was performed for validation of the calculation method and not for direct application to thyroidectomized differentiated thyroid cancer patients. The classical MIRD phantom was modified by incorporating salivary glands as additional anatomical structures to enable comparative assessment of absorbed dose in non-target tissues. The modified phantom was simulated by MCNP code to estimate organ absorbed doses following oral administration of I-131. The calculated results showed acceptable agreement with ICRP Publication 128 reference data, with a median deviation of 3.7% and a worst case deviation of 40% for the heart wall, attributable to known geometric limitations of the stylized cardiac model. The total absorbed dose to the salivary glands was found to be 0.23 mGy MBq^-1, highlighting their significant exposure during therapeutic procedures.