Analysis of I-131 uptake in various organs at risk in papillary thyroid cancer patients undergoing pre-therapeutic radioactive iodine whole-body scans at a tertiary government hospital

Purpose: Radioactive iodine (RAI) therapy is an adjuvant treatment for papillary thyroid carcinoma (PTC) following thyroidectomy. Despite resection of the thyroid, remnants may persist and contribute to disease recurrence or metastasis. Inter-patient variability in I-131 pharmacokinetics also affects organ uptake and residence times. This exploratory retrospective study aimed to identify organs at risk (OARs) with the highest I-131 uptake and evaluate iodine biodistribution in five PTC patients undergoing pre-therapeutic RAI scans at the University of the Philippines–Philippine General Hospital (UP-PGH). Methods: Each patient received 3 mCi of oral I-131, with whole-body planar imaging acquired at 24 and 48 hours post-administration. OARs were contoured on anterior and posterior projections, and geometric mean counts were calculated. OARs selection was based on solid organs identified in established biokinetic models of I-131 biodistribution. Scatter plot analysis was used to compare the temporal behavior of normalized counts for each OAR per patient. Results: Results showed that among the evaluated organs, the liver, stomach, and kidneys presented relatively higher uptake. The liver recorded consistent normalized intensities of approximately 0.02 at 24 hours and 0.02–0.03 at 48 hours relative to the whole-body counts. The stomach exhibited normalized intensities ranging from 0.01 to 0.03, while the kidneys showed values between 0.01 and 0.02 at both time points. Physiologic uptake in these organs is presumably associated with sodium iodide symporter expression and hepatic metabolism of I-131. In contrast, the thyroid region showed the lowest uptake, likely due to minimal thyroid remnants and pronounced partial volume effects (PVEs), which are more evident in the absence of computed tomography (CT) and may lead to underestimation of I-131 uptake in small structures. Conclusion: These preliminary findings demonstrate consistent physiologic I-131 distribution across key OARs, providing a baseline for future patient-specific dosimetric modeling and improved interpretation of pre-therapeutic RAI imaging.