2000P Efficacy and safety of thoracic radiotherapy after first-line immunotherapy in extensive stage small cell lung cancer: A multi-center retrospective study

Immunotherapy combined with chemotherapy have become the standard first-line options in extensive stage small-cell lung cancer (ES-SCLC) in the past years. However, whether patients can benefit from consolidative thoracic radiotherapy (TRT) after first-line immunotherapy is still undetermined. ES-SCLC patients who received standard first-line immunotherapy combined with chemotherapy with/without subsequent TRT were included from 3 cancer centers in China between February 2020 to December 2022. The clinical outcomes and safety were evaluated. 220 ES-SCLC patients were eligible for our analysis, in which 136 received TRT and maintenance immunotherapy after chemotherapy, and 84 received maintenance immunotherapy only. TRT significantly improved the progression-free survival (PFS) (median 10.7 VS 8.5months, HR=0.662 [95% CI: 0.476-0.919], p=0.013), and decreased the intrathoracic lesion progression rate (13.23% VS 40.48%). Further analysis revealed that TRT with different biological effective dose (>60Gy or ≤60Gy), or different dose fractionation mode (conventional fractionation, hypofractionation or hyperfractionation) didn’t impact patient survival. However, addition of TRT in 3-6 weeks after chemotherapy generated significantly superior PFS compared with adding TRT after tumor progression on first-line treatment (median PFS 10.7m VS 8.3m, HR=0.414 [95% CI: 0.207-0.825], p=0.013). In addition, patients without liver metastases or with bone metastases were more likely to benefit from TRT (both p<0.05). TRT slightly increased ≥grade 3 adverse effect (30.88% VS 25%). Notably, the incidence of pneumonia was higher in the TRT group than in the maintenance immunotherapy group (19.9% VS 6%). The addition of TRT after first-line immunotherapy improves PFS and local control in ES-SCLC patients. TRT should be preferential performed in 3-6 weeks after chemotherapy. Patients without liver metastases or with bone metastases benefit more from TRT. The application of TRT is generally safe excepted for increased pneumonia occurrence.