A case of resected synchronous multiple primary lung cancer comprising adenocarcinoma and carcinoid (cancer-in-cancer).

We performed surgical resection of synchronous multiple primary lung cancer, comprising adenocarcinoma and carcinoid components. The latter tumor was located within the central scar tissue formed by the adenocarcinoma (cancer-in-cancer). The patient was a 67-year-old Japanese woman who had a nodular lesion in the right S1 on chest computed tomography. Lung cancer was diagnosed intraoperatively by rapid needle aspiration cytology, and right upper lobectomy with mediastinal lymph node dissection was performed. Histological examination of the resected specimen revealed cancer-in-cancer, consisting of adenocarcinoma and a carcinoid tumor. The carcinoid component was localized within the central fibrous scar of the dominant adenocarcinoma component.