BackgroundThe CREST study showed that the addition of thoracic radiotherapy (TRT) could improve the survival rate in patients with extensive stage small cell lung cancer (ES-SCLC), but whether TRT can bring survival benefit in the era of immunotherapy remains controversial. This study aimed to explore the efficacy and safety of adding TRT to the combination of PD-L1 inhibitors and chemotherapy.MethodsThe patients who received durvalumab or atezolizumab combined with chemotherapy as the first-line treatment of ES-SCLC from January 2019 to December 2021 were enrolled. They were divided into two groups, based on whether they received TRT or not. Propensity score matching (PSM) with a 1:1 ratio was performed. The primary endpoints were progression-free survival (PFS), overall survival (OS) and safety.ResultsA total of 211 patients with ES-SCLC were enrolled, of whom 70 (33.2%) patients received standard therapy plus TRT as first-line treatment, and 141 (66.8%) patients in the control group received PD-L1 inhibitors plus chemotherapy. After PSM, a total of 57 pairs of patients were enrolled in the analysis. In all patients, the median PFS (mPFS) in the TRT and non-TRT group was 9.5 and 7.2 months, respectively, with HR = 0.59 (95%CI 0.39–0.88,p= 0.009). The median OS (mOS) in the TRT group was also significantly longer than that in the non-TRT group (24.1 months vs. 18.5 months, HR = 0.53, 95%CI 0.31–0.89,p= 0.016). Multivariable analysis showed that baseline liver metastasis and the number of metastases ≥ 3 were independent prognostic factors for OS. Addition of TRT increased the incidence of treatment-related pneumonia (p= 0.018), most of which were grade 1–2.ConclusionsAddition of TRT to durvalumab or atezolizumab plus chemotherapy significantly improves survival in ES-SCLC. Although it may leads to increased incidence of treatment-related pneumonia, a majority of the cases can be relieved after symptomatic treatment.