Clinical factors related to the efficacy of tyrosine kinase inhibitor therapy in radioactive iodine refractory recurrent differentiated thyroid cancer patients.

New insights in thyroid cancer biology propelled the development of targeted therapies as salvage treatment for radioiodine-refractory differentiated thyroid cancer (RR-DTC), and the tyrosine kinase inhibitor (TKI) lenvatinib has recently become available as a new line of therapy for RR-DTC. The aim of this study is to investigate clinical factors related to the efficacy of TKI therapy in recurrent RR-DTC patients and identify the optimal timing for the start of TKI therapy. The subjects consisted of 29 patients with progressive RR-DTC, 9 males and 20 females, median age 66 years. A univariate analysis was conducted in relation to progression free survival (PFS) and overall survival (OS) by the Kaplan-Meier method for the following variables: age, sex, histology of the primary tumor, thyroglobulin doubling time before the start of lenvatinib therapy, site of the target lesions, presence of a tumor-mediated symptom at the start of lenvatinib therapy, and baseline tumor size of the target lesions. Median duration of lenvatinib therapy was 14.7 months and median drug intensity was 9.5 mg. At the time of the data cut-off for the analysis, 9 patients (31.0%) have died of their disease (DOD), and a PR (partial response), SD (stable disease), and PD (progressive disease) were observed in 20 patients (69%), 6 patients (20.7%), 3 patients (10.3%), respectively. Univariate analyses showed that the presence of a symptom was the only factor significantly related to poorer PFS and OS. Clinical benefit of TKI therapy will be possibly limited when the therapy starts after tumor-mediated symptoms appear.