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

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 mDTC patients treated with I-131 therapy and followed for at least two years from 1967 to 2019 were reviewed. Patients were categorized into three groups based on the received CA: Group-A (≤600mCi), Group-B (>600-1000mCi), and Group-C (>1000mCi). The study assessed long-term AEs and survival outcomes.

RESULTS: The study included 671 adult mDTC patients (mean age 48 years, range: 19-81) with a median follow-up of 122 months (IQR: 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 (SPM): 3 in Group A and one each in Group B and C. However, CA >1000mCi of I-131 was associated with significant increase in bone marrow suppression, decreased pulmonary function, and xerostomia (p < 0.001).

CONCLUSIONS: The study suggests that a maximum CA of up to 1000mCi strikes a favorable balance between keeping AEs low and benefiting a subset of patients with extensive metastases showing intense I-131 concentration.