Severe probable immune checkpoint inhibitor-related pneumonitis after thoracic radiotherapy complicated by acute pulmonary embolism in lung squamous cell carcinoma: a case report.

Immune checkpoint inhibitor-related pneumonitis (CIP) is a potentially life-threatening adverse event in lung cancer. We report a case of severe probable CIP complicated by acute pulmonary embolism (PE) in a 76-year-old man with stage IIIA lung squamous cell carcinoma after chemoimmunotherapy and thoracic radiotherapy. He developed progressive dyspnea and severe hypoxemia. Serial chest computed tomography (CT) showed bilateral interstitial and ground-glass abnormalities beyond the radiation field, favoring probable CIP, although superimposed infection and obstructive pneumonia could not be fully excluded. During hospitalization, CT pulmonary angiography confirmed acute PE, and lower-extremity ultrasound suggested calf venous thrombosis. The patient required high-flow nasal cannula and received methylprednisolone, tocilizumab, intravenous immunoglobulin, mycophenolate mofetil, antimicrobial therapy, and therapeutic anticoagulation. Despite partial clinical improvement, serial chest CT demonstrated continued radiographic progression. This case highlights the need for dynamic differential diagnosis, multidisciplinary management, and close bedside monitoring in complex post-radiotherapy CIP.