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Backgrounds:Concurrent chemoradiotherapy has been recommended as the standard treatment for limited-stage small cell lung cancer(LS-SCLC).Although the preponderance of current evidence supports the delivery of early-initiated thoracic radiotherapy(TRT),controversy remains regarding the optimal timing and duration of thoracic radiotherapy(TRT)and their effects on survival.This study sought to elucidate the prognostic values of TRT timing and duration on progression-free survival for LS-SCLC patients.Methods:In this retrospective study,we included 197 eligible patients diagnosed with LS-SCLC and received concurrent chemoradiotherapy to evaluate the effect of TRT timing and duration on progression-free survival(PFS).Patients were divided into early TRT and late TRT group,short TRT and long TRT group respectively,based on the optimal cutoff points of TRT timing and TRT duration generated by Cutoff Finder.Univariate and multivariate Cox analysis were performed to assess the correlation between TRT timing,TRT duration and PFS.Results:Univariate analysis revealed that early TRT(P=2.54*10^-4)and short TRT(P=0.001)significantly predicted better PFS respectively.When stratified by RT fractionation,the PFS benefit of early TRT(P=0.003)and short TRT(P=0.006)remained significant in patients received hyperfractionated twice-daily TRT,but insignificant in patients received once-daily TRT.Multivariate analysis further identified early TRT(P=0.004)and short TRT(P=0.017)as independent prognostic factors for better PFS.Their benefits were more prominent when combining with standard etoposide-platinum-based chemotherapy and hyperfractionation TRT but were not confined by specific patient characteristics.Conclusion:Results of this study support the delivery of early initiated hyperfractionated TRT with a short duration to patients with LS-SCLC and suggest that both TRT timing and duration are significant for the survival of LS-SCLC patients.Hence,they should be equally valued during the design of a CRT plan in the context of apposite systemic management.