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OBJECTIVE To evaluate the efficacy of thoracic radiotherapy to primary site in patients with extensive stage small cell lung cancer (SCLC) who had responded completely to systemic chemotherapy. STUDY DESIGN Observational study. PLACE AND DURATION OF STUDY Departments of Radiation and Medical Oncology, Baskent University and Dr. Ersin Arslan Research and Training Hospital in Turkey, between the years of 2011 and 2020. METHODOLOGY The study included 125 patients with extensive stage SCLC. Demographic data and outcomes of chemotherapy and radiotherapy were collected. The efficacy of thoracic radiotherapy to primary site was evaluated in patients who had responded completely to systemic chemotherapy, in terms of progression-free survival and overall survival (OS). RESULTS The median follow-up time was 12 months and 98 (78.4%) patients died during follow-up. Seventy-three (58.4%) patients had complete response. Progression-free survival (PFS) for complete responder patients was 8 months, and OS for the whole group was found 13 months. Twenty (16%) patients received thoracic radiotherapy to primary site after complete response to platinum etoposide combination treatment. Patients receiving thoracic radiotherapy had better OS than those who did not (19 versus 12 months respectively and p=0.002). Patients receiving thoracic radiotherapy had better PFS than those who did not (11 versus 8 months, respectively, and p=0.01). CONCLUSION Thoracic radiotherapy to primary site may improve the survival outcomes in extensive stage SCLC patients who had complete response to initial systemic chemotherapy. KEY WORDS Small cell lung cancer, Thoracic radiotherapy, Complete response, Outcomes, Lung cancer.