Tumor doubling time predicts response to sorafenib in radioactive iodine-refractory differentiated thyroid cancer

Sorafenib has emerged as an effective therapeutic option for radioactive iodine (RAI)-refractory, locally advanced or metastatic differentiated thyroid cancer (DTC). We investigated the efficacy and safety of sorafenib treatment in a real-world setting and unveil predictive markers of responsiveness to sorafenib. The treatment response, progression-free survival (PFS), overall survival, and adverse events (AEs) of sorafenib-treated RAI-refractory, locally advanced or metastatic DTC patients at three institutes were retrospectively reviewed, and their tumor doubling time was calculated by three investigators. Total eighty-five patients were treated with sorafenib, and seven patients discontinued sorafenib due to AEs before the first tumor assessment. The median PFS was 14.4 months, and the objective response rate was 10.3% in 78 patients who were able to evaluate the tumor response. Age, sex, histologic type, tumor location, RAI avidity, or the presence of FDG-PET uptake did not affect PFS. However, smaller tumor size (≤1.5 cm) of the target lesions in lung showed better PFS (hazard ratio [HR] 0.39, p = 0.01), and tumors with the shortest doubling time (≤6 months) had worse outcome (HR 2.70, p < 0.01). Because of AEs, dose reductions or drug interruptions were required in 64% of patients, and eventually, 23% of patients discontinued sorafenib permanently. The most common AE was hand-foot skin reaction (HFSR). Patients with severe HFSR showed better PFS, but there were no statistical significance (HR 0.65, p = 0.05). In conclusion, small tumor size and long doubling time of each target lesion can be a prognostic marker to predict the responsiveness to sorafenib in RAI-refractory DTC patients.