Accuracy of Iodine-131 dosimetry using planar imaging in patients with hyperthyroidism: a SIMIND Monte Carlo study

Abstract

Radioiodine treatment is one of the therapeutic strategies for the management of hyperthyroidism. In clinical practice, the therapy is based on administering the same amount of [131I] sodium iodide (around 370 MBq) to all patients, with the potential risk of a suboptimal therapeutic response in some patients or unnecessary radiation exposure in others. Dosimetry calculations, which require accurately quantifying the activity uptake by the thyroid gland, are essential for personalizing the therapy. This study assessed the accuracy of absorbed dose calculations for radioiodine treatment planning for hyperthyroidism, based on a computational approach. The simulation model was grounded on two existing tools: the SIMIND Monte Carlo code, which models a clinical scintillation camera, and a voxelized anthropomorphic computational Zubal phantom as an anatomical model of the patient. The clinical simulator was used to obtain anterior planar thyroid images from computational models of hyperthyroid patients under various acquisition scenarios, to determine the accuracy of a pre-therapeutic planar dosimetry procedure using [131I] sodium iodide; a study that, to date, has not been addressed systematically. A schedule involving three- early uptake assessments, i.e., thyroid images obtained up to 48 hours after radiopharmaceutical administration, was modeled as a clinically representative scenario. The simulated images were analyzed to quantify the 131I localized in the thyroid gland, and dosimetry calculations were performed using these data. The results showed that the activity quantified from the simulated anterior planar thyroid images overestimated both the actual activity and the resulting absorbed doses in the thyroid (error less than 7 %). No meaningful differences were observed in the quantified activity if medium- or high-energy collimators were used. The study suggests that it is possible to perform pre-therapeutic dosimetry calculations based on early anterior planar images in hyperthyroid patients without concomitant conditions that prolong radioiodine uptake, with an error one order of magnitude smaller than the spread of thyroid absorbed dose values observed in patients with hyperthyroidism.