We report the case of a 72-year-old woman who underwent partial mastectomy due to left breast cancer(invasive ductal carcinoma)in March 20XX-4. This was followed by radiotherapy(50 Gy/25 Fr)and hormone therapy. In July 20XX, she was referred to our department because a chest computed tomography(CT)scan performed at the postoperative follow-up revealed a band-like consolidation adjacent to the pleura in the lingular segment, with enlarged ipsilateral hilar and mediastinal lymph nodes. CT-guided lung tumor biopsy was performed, and she was diagnosed with limited-stage small cell lung cancer. Chemotherapy with carboplatin, etoposide, and atezolizumab was initiated. Radiotherapy was not performed due to the overlap between the distribution of the lung tumor and the postoperative irradiation field of breast cancer. Due to the difference in the histopathological findings of the first and second primary tumors, and the location of the tumor in the postoperative irradiation field, the second cancer was considered to be radiation-induced cancer despite the short latency period.