A Case of Pleomorphic Carcinoma of the Lung Complicated by Anti-HMGCR Antibody-Positive Necrotizing Myositis.

A 72-year-old man presented with a four-month history of muscle weakness and pain. Serum creatine kinase level was markedly elevated at 22,720 IU/mL, and magnetic resonance imaging revealed high signal intensity in the thigh and upper arm muscles. The presence of an autoantibody against 3-hydroxy-3-methylglutaryl-coA reductase (HMGCR) in the serum confirmed the diagnosis of immune-mediated necrotizing myositis. Subsequent systemic examination revealed primary lung carcinoma in the right lower lobe. Despite the initial corticosteroid therapy, there was no improvement in muscle symptoms. Surgical resection of the tumor, followed by additional intravenous immunoglobulin therapy, controlled necrotizing myositis. The postoperative pathological diagnosis was pleomorphic carcinoma. Despite adjuvant chemotherapy, lung cancer recurred within six months, leading to the patient's death. This case suggests that while treatment of malignancy can be effective for tumor-associated syndromes, such as anti-HMGCR antibody-positive necrotizing myositis, immunosuppressive therapy used for myositis may be a ssociated with an increased r isk of cancer recurrence.