Optimal Cumulative I-131 Activity in Metastatic Differentiated Thyroid Cancer: Balancing Efficacy and Adverse Events

Abstract Background The optimal cumulative activity (CA) of I-131 therapy for patients with metastatic differentiated thyroid cancer (mDTC) remains contentious. This study aimed to determine the maximum CA of I-131 that could be administered without a significant increase in adverse events (AEs) by analyzing a long-term cohort of patients. Methods Data from patients with mDTC treated with I-131 therapy and followed for at least 2 years from 1967 to 2019 were reviewed. Patients were categorized into 3 groups based on the received CA: group A (≤600 mCi), group B (>600-1000 mCi), and group C (>1000 mCi). The study assessed long-term AEs and survival outcomes. Results The study included 671 adult patients with mDTC (mean age, 48 years; range, 19-81) with a median follow-up of 122 months (interquartile range: 82-180). Group A, group B, and group C comprised 269 (40.0%), 212 (31.6%), and 190 (28.4%) patients, respectively. Ten-year survival rates were 72%, 42.7%, and 29% in groups A, B, and C, respectively. A total of 40/671 (6%) AEs were observed in 38 patients: 3 (1.1%), 12 (5.7%), and 25 (13.2%) in groups A, B, and C, respectively. Five patients developed second primary malignancy: 3 in group A and 1 each in group B and C. However, CA >1000 mCi of I-131 was associated with significant increase in bone marrow suppression, decreased pulmonary function, and xerostomia (P < .001). Conclusion The study suggests that a maximum CA of up to 1000 mCi strikes a favorable balance between keeping AEs low and benefiting a subset of patients with extensive metastases showing intense I-131 concentration.