Dose-Activity Relationship in [131I]NaI Therapy for Differentiated Thyroid Carcinoma

Background : Current clinical practice guidelines often advocate for fixed [ ¹³¹ I]NaI activity prescriptions based on patient diagnosis and risk stratification, frequently disregarding individual patient-specific radioiodine kinetics. This approach may result in substantial variability in the absorbed dose to target tissues and organs at risk. Purpose : This study aimed to analyze the relationship between the absorbed dose in target organs (thyroid remnants) and organs at risk (red bone marrow) with potential explanatory variables, particularly the administered [ ¹³¹ I]NaI activity. Methods : A cross-sectional pilot study was conducted involving DTC patients treated with [ ¹³¹ I]NaI at the Hospital Clínico Universitario Virgen de la Arrixaca (HCUVA) in Murcia between September 2024 and May 2025. Baseline patient data and other treatment-specific data available in their clinical records were collected to calculate the absorbed dose according to the existing protocol for this purpose at the center. Results : No significant correlation was observed between the absorbed dose in thyroid remnants and the administered activity. Conversely, a significant association was found between the absorbed dose in the red bone marrow and the administered activity. Conclusion : Administering [ ¹³¹ I]NaI without individualizing treatment based on radiopharmaceutical kinetics and patient-specific tumor burden does not result in proportionally higher absorbed doses in thyroid remnants or enhanced therapeutic efficacy. Instead, it significantly increases the absorbed dose to organs at risk, such as the red bone marrow.