An 81-year-old man was referred to our center due to an abnormal shadow finding on a chest plain radiograph with chief complaints of cough and hip pain. The patient had no smoking history. His Eastern Cooperative Oncology Group performance status was 1. He had chronic renal failure; hence, we did not use any contrast agents for computed tomography (CT) or magnetic resonance imaging (MRI). CT revealed a lung nodule in the right upper lobe, mediastinal lymph node enlargement (Fig. 1a), an osteolytic lesion of the left iliac bone, and an anterior mediastinal cyst measuring 5 cm in size. The former three lesions showed significant fluorodeoxyglucose (FDG) accumulation on FDG-positron emission tomography (PET), and advanced lung cancer with clinical stage T1bN2M1b was suspected. Endobronchial ultrasound-guided transbronchial needle aspiration indicated that the lung adenocarcinoma was ALK-negative, with wild-type EGFR and a high PD-L1 tumor proportion score (95%). The latter mediastinal cyst showed no abnormal accumulation on PET-CT, and MRI demonstrated high intensity on T2-weighted images (Fig. 1b). Magnetic resonance cholangiopancreatography performed 1 year before the appearance of the mediastinal tumor indicated that the tumor occurred within the year. We determined that the patient had double neoplasms with advanced NSCLC and a mediastinal tumor. It was unknown whether the mediastinal tumor was benign or malignant. The patient’s prognosis seemed to depend on the treatment of the NSCLC rather than the mediastinal tumor. Therefore, we prioritized lung cancer treatment. Blood screening before immune checkpoint inhibitor (ICI) therapy (such as autoimmune and endocrine function, including thyroid function) showed normal data with the exception of renal failure. Radiation therapy with 39 Gy to the left iliac bone was performed, and pembrolizumab was administered as the first-line treatment; we observed the mediastinal tumor. Treatment with pembrolizumab yielded a partial response from the NSCLC in 3 months (Fig. 1c), but the mediastinal cyst enlarged extremely rapidly to 15 cm in diameter (Fig. 1d) and was suspected to be a sarcoma. Owing to the NSCLC treatment, the size of the mediastinal tumor rather than the NSCLC seemed to be more directly associated with prognosis. We decided to resect the mediastinal tumor under immunotherapy. Two weeks after the fourth administration of pembrolizumab, we performed mediastinal tumor excision by posterolateral thoracotomy, combined with resection of the right phrenic nerve and pericardium, which was reconstructed with an expanded polytetrafluoroethylene patch. We achieved a pathologically curative resection, but the tumor tended to infiltrate the surrounding structures and bled easily even with careful dissection. The blood loss was 1400 ml, and the patient received blood transfusion. The tumor was filled with a jelly-like substance and weighed 920 g. Pathological findings revealed spindle cells in a rich myxoid stroma with a high rate of mitosis and multinucleated pleomorphic nuclei (Fig. 2). Immunohistochemical staining was negative for CD68, bcl2, ckit, ALK, SMA, desmin, S100, AE1/3, EMA, STAT6, and MUC4 and positive for CD99, CD34, MDM2, and CDK4. The pathological diagnosis was myxofibrosarcoma with no expression of PD-L1 (tumor proportion score, 0%). The postoperative course was uneventful. Immunotherapy was resumed 3 weeks after the operation, and a total of 13 courses were administered without any adverse effects. No adverse events were noted on the blood screens performed. No recurrence of mediastinal sarcoma or progression of NSCLC was found until the patient died in an accident 8 months after surgery. Patient consent was obtained for publication of this case report.
