Abstract. The aim of the present study was to evaluate the effect of 3-dimensional conformal thoracic radiotherapy (TRT) on extensive-stage small-cell lung cancer (ES-SCLC). A total of 165 patients with ES-SCLC were enrolled in the present study, including 82 patients receiving chemotherapy combined with TRT (the ChT/TRT group) and 83 patients receiving chemotherapy alone (the ChT group). The overall survival (OS) and progression-free survival (PFS) rates were compared between the ChT/TRT and ChT groups, and the prognostic factors for OS rate were identified. It was found that the patients had a median OS time of 15 months, and 2-and 5-year OS rates of 31.5 and 2.4%, respectively. The 2-and 5-year OS rates were 35.3 and 2.4% in the ChT/TRT group, and 14.5 and 2.4% in the ChT group, respectively (P<0.05). The 1-and 2-year PFS rates were 35.4 and 6.0% in the ChT/TRT group, and 20.5 and 6.0% in the ChT group, respectively (P<0.05). The median PFS was 11 months in the 20 patients receiving TRT at 45 Gy/30 fractions twice daily, and 9 months in the 22 patients receiving TRT at 60 Gy/30 fractions daily (P=0.043). Multivariate analysis revealed that receiving ≥4 cycles of chemotherapy (P=0.001) and TRT (P=0.008) were favorable prognostic factors for OS. It was concluded that the addition of TRT improves the OS and PFS rates of patients with ES-SCLC, and TRT administration at 45 Gy/30 fractions twice daily is feasible and tolerable for the treatment of ES-SCLC. Thus, TRT and receiving ≥4 cycles of chemotherapy are independent, favorable prognostic factors for OS in patients with ES-SCLC. IntroductionLung cancer is a leading cause of cancer-associated mortality worldwide, with >1 million mortalities annually (1). Small-cell lung cancer (SCLC) accounts for 10-15% of lung cancer cases (2). It is estimated that 70% of patients with SCLC have extensive-stage SCLC (ES-SCLC) at the time of diagnosis (3).Currently, the first-line therapy for ES-SCLC remains dependent on platinum-based chemotherapy with cisplatin-etoposide (EP) (4). The addition of thoracic radiotherapy (TRT) is suggested in certain patients due to its confirmed value in the improvement of the overall survival (OS) rate (5,6). Although SCLC is sensitive to chemotherapy and radiotherapy, with a 60-80% response rate, the median survival rate of patients with ES-SCLC is estimated to be only 7-12 months following 4-6 cycles of standard chemotherapy (4).Systemic chemotherapy in combination with concurrent TRT have been demonstrated to be effective in the treatment of limited-stage SCLC (LS-SCLC) by improving the progression-free survival (PFS) and OS rates (7-9). The National Comprehensive Cancer Network (NCCN) guidelines recommend that TRT should be initiated during the first or second cycles of chemotherapy in cases of LS-SCLC at 45 Gy administered twice per day in 1.5 Gy fractions, or 60-70 Gy in 2 Gy daily fractions (10-12). However, the effect of TRT on ES-SCLC remains unknown. The present retrospective study evaluated the effect of 3-dimensional (3D) conformal ...