━━ Background. Polymyositis rarely occurs after a diagnosis of lung cancer. Case. A 63-year-old man had been diagnosed with squamous cell lung carcinoma (T4N2M1b, c-stage IV) in February 2015. He had been treated with chemotherapy consisting of carboplatin (CBDCA) and tegafur/gimeracil/oteracil (S-1) and had shown stable disease with this treatment. However, his creatinine kinase (CK) levels began to increase during the chemotherapy. Rhabdomyolysis by S-1 was initially suspected, so the

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... rapy was changed to CBDCA and nanoparticle-albumin-bound-paclitaxel (nab-PTX). However, his CK levels continued to increase, and he presented with a fever and weakness of the muscles. Myositis was suspected, and muscle biopsy was performed. The pathological findings confirmed polymyositis. The polymyositis improved with prednisolone (PSL) and immunosuppressant treatment. Conclusions. We should consider the possibility of polymyositis when the CK levels increase in patients with lung cancer.