The management of synchronous multiple primary lung cancer is a challenge. In this report, we describe our experience in a patient with three synchronous multiple cancers. The first lesion was completely surgically removed, the second lesion received postoperative irradiation, and the third lesion was treated with radiotherapy alone. Radiation therapies were performed using a combination of external irradiation and endobronchial brachytherapy. Endobronchial brachytherapy is an effective radiation therapy for endobronchial tumors owing to its advantage of high‐dose concentration. However, adverse events (AEs) such as hemoptysis or severe bronchitis are a problem. Thus, we have developed an applicator to keep the radioactive source in the center of the bronchial lumen. A total of 28 months after treatment, the patient had not experienced any relapses or AEs. Endobronchial brachytherapy using an applicator can be an alternative treatment for cases in which surgery is expected to lead to pulmonary dysfunction.

Radiation‐induced lung damage (RILD) is a critical problem in lung cancer radiotherapy, and it is difficult to predict its severity. Although no biomarkers for RILD have been established, tenascin C (TNC) is an extracellular matrix glycoprotein involved in the remodeling of damaged tissues and has been implicated in inflammation and fibrosis. We report the unique case of a 36‐year‐old man with adenocarcinoma of the lung, Union for International Cancer Control stage IIIB, who was treated with radiotherapy before lung surgery. The surgical specimen showed histopathological expression of TNC in the region where radiation pneumonitis was observed radiographically. Serum TNC levels were elevated after radiotherapy. In this case, TNC is suggested to be implicated in RILD and may be a potential candidate as a biomarker for the onset and severity of the condition.