6070 Background: Lenvatinib is now available for unresectable radioactive iodine (RAI)-refractory differentiated thyroid cancer (DTC). However, toxicities are considerable and require frequent dose interruption and modification. Recently, planned drug holidays, which are dose interruptions in accordance with the timing of severe or intolerable adverse events, have been proposed to avoid severe adverse events due to lenvatinib (Tahara M.ESMO Open 2018). Our retrospective study demonstrated that progression-free survival (PFS) and overall survival (OS) were significantly longer in patients who used planned drug holidays than those who did not (Matsuyama C et.al, 2020 Annual Meeting of the Japan Association of Endocrine Surgeons). Methods: In this prospective observational study, patients with curatively unresectable and progressive RAI-refractory DTC were treated with lenvatinib in a real-world clinical setting. Lenvatinib was administered orally at a dose of 24 mg daily. Dose modification for toxicities were permitted. Primary endpoint was OS, and secondary endpoints were time to treatment failure (TTF), time to failure of strategy (TFS), PFS with clinical progressive disease, response rate, quality of life, safety, and patient reports. This study was registered with UMIN Clinical Trials Registry (UMIN000022243). Results: 262 patients were accrued. Of 255 evaluable, 153 were female; median age was 70 (range 27.0-88.0); histology was papillary thyroid carcinoma/follicular thyroid carcinoma/poorly DTC in 204/45/4; previous therapy was surgery/RAI/molecular targeted drug in 246/164/14; reason for initiation of lenvatinib was disease progression/unsuitable for RAI in 241/4. 1-year OS was 85.6% (95%CI: 80.6-89.4%); 1-year TTF rate was 74.9% (95%CI: 69.1-79.8%); 1-year TFS rate was 80.8% (95%CI: 75.4-85.2%); and 1-year PFS rate was 84.4% (95%CI: 79.3-88.4%). Overall response by RECIST was 3 (1.2%) in CR and 151 (61.9%) in PR. Most common grade 3 or 4 toxicities were hypertension (61.4%), hand foot syndrome (10.2%), fatigue (9.1%), anorexia (8.3%) and diarrhea (4.7%). Grade 5 toxicities occurred in 4 patients (fistula, hypoxia, respiratory failure, trachea stenosis). Of 253 patients evaluable for efficacy, 73 used planned drug holidays. TTF, TFS and PFS were significantly longer in patients who used planned drug holiday than those who did not (Table). Conclusions: Planned drug holiday for lenvatinib demonstrated significantly better clinical outcomes, including TTF, TFS and PFS, than daily oral administration. These data further support use of a planned drug holiday in RAI-refractory DTC patients receiving lenvatinib. Clinical trial information: 000022243. [Table: see text]