Avidity and Outcomes of Radioiodine Therapy for Distant Metastasis of Distinct Types of Differentiated Thyroid Cancer

Context: The recommendations for radioactive-iodine treatment (RAIT) in metastatic differentiated thyroid cancer (DTC) are mostly based in the experience with papillary histotype and do not consider the differences within the distinct types of DTC, in terms of RAIT uptake and response. Objective: This work aims to investigate the association between histology and RAIT avidity and response, and to evaluate whether histotype was an independent prognostic factor in progression-free survival (PFS) and disease-specific survival (DSS) after RAIT for distant metastatic disease. Methods: A retrospective analysis was conducted of all DTC patients who underwent RAIT for distant metastatic disease, from 2001 to 2018, at a thyroid cancer referral center. We included 126 patients: 42 (33.3%) classical variant papillary thyroid cancer (cvPTC), 45 (35.7%) follicular variant PTC (fvPTC), 17 (13.5%) follicular thyroid cancer (FTC) and 22 (17.5%) Hürthle cell carcinoma. Main outcome measures included RAIT avidity and response. Results: RAIT avidity was independently associated with histology (P<.001) and stimulated thyroglobulin (Tg) at first RAIT for distant lesions (P=.007). Avidity was lowest in HCC (13.6%), intermediate in cvPTC (21.4%), and highest in fvPTC (75.6%) and FTC (76.5%). Regarding RAIT response, HCC and FTC were not different; both showed significantly more often progression after RAIT than fvPTC and cvPTC. Histology influenced PFS (P=.014), but tumor type was not a significant prognostic factor in DSS. Instead, age at diagnosis, resection status, and stimulated Tg at the first RAIT were significantly associated with DSS. Conclusion: DTC histotype influenced RAIT avidity and PFS. It is crucial to better detect the metastatic patients that may benefit the most from RAIT.