A 79-Year-Old Woman With Stage IIIB Lung Squamous Cell Carcinoma Presenting With Late-Onset Durvalumab‑Associated Myocarditis Requiring Differentiation From Pericardial Invasion.

BACKGROUND Durvalumab is a humanized monoclonal antibody and immune checkpoint inhibitor used for the treatment of advanced non-small cell lung cancer. Myocarditis, particularly in late-onset cases, is a rare but serious adverse event associated with durvalumab. This report describes a 79-year-old woman with non-small cell lung cancer who developed late‑onset myocarditis requiring differentiation from direct pericardial invasion. CASE REPORT A 79-year-old woman with stage IIIB lung squamous cell carcinoma developed myocarditis after 11 cycles of durvalumab maintenance therapy (5 months after initiation) following chemoradiation. She presented with reduced cardiac function, pericardial effusion, and elevated high-sensitivity troponin T, initially raising concern for pericardial invasion because the primary tumor was in direct contact with the myocardium. Urgent myocardial biopsy confirmed immune checkpoint inhibitor-related myocarditis, showing mononuclear lymphocyte and macrophage infiltration. Her condition improved after discontinuation of durvalumab and initiation of high-dose corticosteroids and intravenous immunoglobulin. Cardiac function recovered within 3 weeks, and biomarkers normalized. Notably, she has maintained a complete oncologic response for more than four years without further cancer treatment or recurrence of myocarditis, suggesting a durable antitumor effect despite early discontinuation of immunotherapy. CONCLUSIONS This case highlights the diagnostic challenge of distinguishing immune checkpoint inhibitor‑induced myocarditis from direct cardiac invasion in patients with lung cancer, particularly in late‑onset presentations. Early myocardial biopsy enabled prompt diagnosis and treatment, leading to a favorable long‑term outcome. Careful cardiac monitoring is essential, and immune-related adverse events should be considered even when tumor invasion is suspected.