Abstract Background Organizing pneumonia (OP) is an inflammatory pattern of lung injury that can occur as an immune-related adverse event from immune checkpoint inhibitors such as pembrolizumab. It often mimics infection or tumor progression, posing diagnostic challenges. Case report A Pulmonology consult was requested for a 68-year-old woman hospitalized with extensive bilateral pneumonia. Assessment of her chest CT showed bilateral parenchymal consolidations with air bronchogram, raising the possibility of an OP pattern. She had, indeed, right-sided invasive breast carcinoma and had completed neoadjuvant chemotherapy three months earlier (four cycles of carboplatin and paclitaxel, followed by one of adriamycin and cyclophosphamide combined with pembrolizumab); tumorectomy had been performed one week before hospitalization. An immune-mediated pneumonitis (IP) secondary to pembrolizumab was suspected. Intravenous corticosteroids were initiated, leading to clinical and radiologic improvement. She was discharged on a rapid tapering regimen and referred to our consultation. At three-month follow-up, after completing the regimen, the patient was asymptomatic and CT revealed marked, though incomplete, improvement. She had begun adjuvant radiotherapy and pembrolizumab was not resumed. One month later, she presented with fever, fatigue and cough. Laboratory testing was unremarkable, but imaging again demonstrated bilateral consolidations, this time predominantly in the right lower lobe. A bronchoscopy was performed, presenting no endobronchial lesions. Cytology from the bronchoalveolar lavage revealed cells with carcinoma-like morphology. Repeat CT now showed new left lower lobe consolidation. A second bronchoscopy with transbronchial biopsy identified histologic features consistent with organizing pneumonia, while cytologic atypia was attributed to reactive hyperplasia rather than malignancy. A final diagnosis of pembrolizumab-induced IP was established. The patient remains asymptomatic under regular follow-up. Discussion OP is typically identified by migratory lung consolidations and a corticosteroid-responsive course, as seen here. A biopsy was pursued in this case to exclude malignancy. Our patient experienced grade 3 IP, which most often manifests radiologically as OP. Pembrolizumab is a recognized cause, even after a single dose, with lesions typically arising around three months after initiation and possibly after discontinuation, as in this case. Relapse occurs in 10-20% of cases, particularly following severe initial pneumonitis. Early recognition of pembrolizumab-induced IP is essential, as timely corticosteroid therapy generally results in rapid clinical and radiologic improvement while preventing further pulmonary toxicity. This abstract is funded by: None
Pimentel et al. (Fri,) studied this question.