Atezolizumab-induced myositis in a patient with small-cell lung cancer.

OBJECTIVE: Myositis, an inflammatory disease affecting muscles, is a rare and potentially fatal immune-related adverse event associated with immune checkpoint inhibitors. There are limited data on its clinical features and management. CASE PRESENTATION: Atezolizumab, in combination with etoposide and carboplatin, was initiated in the patient diagnosed with metastatic small-cell lung cancer. After four cycles, maintenance atezolizumab was initiated. At the third visit of the maintenance therapy, the patient reported weakness, edema, and tightness in the muscles that had progressed over the course of a week. Mild solid-food dysphagia was also observed. Neutrophilic leukocytosis with elevated creatine phosphokinase (9234 U/L), erythrocyte sedimentation rate (111 mm/h), and transaminase levels were observed. A diagnosis of myositis was considered based on clinical findings. Atezolizumab was omitted and an oral 0.5 mg/kg/day dose of methylprednisolone was administered. The myositis resolved within 10 days. During the treatment of myositis, the patient underwent prophylactic cranial irradiation. The steroid dose was tapered off within 35 days and then atezolizumab was restarted. CONCLUSION: The literature contains only a few case reports about atezolizumab-induced myositis, highlighting the challenges in defining its clinical features and management. Prompt diagnosis and treatment are crucial to prevent severe complications, such as myocarditis or respiratory muscle paralysis.