Pulmonary Lymphangitis Carcinomatosa Mimicking Immunotherapy-Related Interstitial Pneumonitis: A Case Report.

While immunotherapy with immune checkpoint inhibitors has achieved promising effects in advanced lung cancer treatment, it can induce some unique adverse events, known as immunotherapy-related adverse events (irAEs). Immunotherapy-related interstitial pneumonitis is one of the irAEs, and its incidence is reported as 3.5-8.3% in phase III trials of nivolumab with or without ipilimumab for advanced non-small cell lung cancer patients. However, in the real-world setting, pathology is not routinely used in the diagnostic process of interstitial pneumonitis because diagnosis is usually made using chest computed tomography (CT). Here, we report an educational case of pathologically diagnosed pulmonary lymphangitis carcinomatosa mimicking immunotherapy-related interstitial pneumonitis. The patient was diagnosed with advanced adenocarcinoma of the right lung (stage IVA) and received immunochemotherapy for 6 months. He manifested acute respiratory failure, and a chest CT scan revealed the emergence of diffuse grand-grass opacity predominantly in the left lung. Immunotherapy-induced interstitial pneumonitis was clinically suspected because the primary lesion was stable, and the level of the serum carcinoembryonic antigen decreased. However, the detection of adenocarcinoma cells in the bronchoalveolar lavage sample from the left lung confirmed the diagnosis of pulmonary lymphangitis carcinomatosa. Clinicians' assumptions can sometimes mislead treatment methods; hence, this case draws attention to the perils of misdiagnoses.