Combined Maximum Tolerated Activity and Lesion-based Dosimetry for Personalized Radioiodine Therapy in Metastatic Differentiated Thyroid Carcinoma: An Early Institutional Experience

Objectives: Empirical fixed-activity radioiodine (RAI) therapy remains widely used in differentiated thyroid carcinoma (DTC), despite substantial inter-patient variability in iodine kinetics. Pre-therapeutic dosimetry offers a personalised approach by estimating radiation exposure to organs at risk as well as to tumour lesions. To describe our institutional experience with combined maximum tolerated activity (MTA) and lesion-based dosimetry in patients with metastatic or recurrent DTC, and to evaluate its impact on clinical decision-making. Methods: A retrospective case series of six DTC patients undergoing pre-therapeutic Iodine-131dosimetry over two years was recorded. Whole-body and blood-based dosimetry were used to calculate MTA, while lesion-based dosimetry was calculated via estimated absorbed lesion-based dose per administered activity (LDpA). Treatment decisions were made following multidisciplinary discussion. Results: Five patients completed the dosimetric evaluation. In one patient with high tumour burden, MTA and LDpA provided complementary safety and efficacy data, although facility constraints had limited the final administered activity. In three patients with small-volume inoperable cervical nodal disease, LDpA was <5 Gy/GBq, predicting a limited therapeutic benefit thus preventing further ineffective RAI therapy administration. In another patient with osseous metastases, prior high cumulative RAI exposure as well as low LDpA indicated limited benefit in further high dose RAI therapy, highlighting the importance of early dosimetric assessment. Conclusion: Personalised dosimetry with combined safety-based and lesion-based approach may enhance the therapeutic precision in patients with advanced DTC. However, careful patient selection is essential in maximising clinical utility.