Avidity and outcomes of radioiodine therapy for distant metastasis of distinct types of differentiated thyroid cancer
Abstract Context The recommendations for radioiodine therapy (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 To investigate the association between histology and RAIT avidity and response; 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. Design Retrospective analysis of all DTC patients submitted to RAIT due to distant metastatic disease, between 2001-2018. Setting Thyroid cancer referral centre. Patients 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 RAIT avidity and response. Results RAIT avidity was independently associated with histology (p<0.001) and stimulated thyroglobulin at first RAIT for distant lesions (p=0.007). 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=0.014), but tumour type was not a significant prognostic factor in DSS. Instead, age at diagnosis, resection status, and stimulated thyroglobulin 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.