Immunotherapy related organizing pneumonia mimicking metastatic progression in a patient with endometrial cancer: A case report

Background Immune checkpoint inhibitor (ICI)–related pneumonitis is a well-recognized immune-related adverse event; however, its manifestation as organizing pneumonia (OP) mimicking metastatic disease remains underrecognized and presents a significant diagnostic challenge. Distinguishing true tumor progression from inflammatory processes is critical to avoid inappropriate treatment decisions. Case presentation A 60-year-old woman with high-grade serous endometrial carcinoma (pT2N2mi, pMMR) treated with adjuvant carboplatin, paclitaxel, and dostarlimab followed by maintenance dostarlimab. Nineteen months after diagnosis, routine surveillance CT and FDG-PET/CT demonstrated new bilateral FDG-avid pleural-based masses and pulmonary consolidations (SUV up to 8.9), raising strong concern for metastatic progression. Despite these findings, the patient remained asymptomatic, and serial circulating tumor DNA (ctDNA) testing was persistently negative. Initial bronchoscopy was non-diagnostic. A subsequent CT-guided core needle biopsy of a pleural lesion revealed organizing pneumonia with non-necrotizing granulomatous inflammation and no evidence of malignancy, establishing a diagnosis of ICI-related OP. Immunotherapy was discontinued, and corticosteroid therapy was initiated, resulting in complete radiographic resolution within two months. Conclusion ICI-related OP may present with mass-like, FDG-avid lesions that closely resemble metastatic disease. This case underscores the essential role of tissue biopsy in confirming the diagnosis and preventing misclassification of disease progression. Additionally, persistently negative ctDNA may serve as a supportive, noninvasive adjunct in distinguishing inflammatory processes from true malignancy.