Abstract Introduction Radiation recall pneumonitis (RRP) is a rare but increasingly recognized inflammatory reaction in previously irradiated lung tissue following systemic agents, now frequently associated with immune checkpoint inhibitors (ICIs). Distinguishing RRP from immune-related pneumonitis, infection, or tumor progression is challenging and requires correlation of radiographic distribution, treatment timeline, and exclusion of alternative causes 1,2. Case Description We present a 60-year-old woman with bulky subcarinal metastatic adenopathy, biopsy proven poorly differentiated non-small cell lung cancer with unknown primary consistent with clinical stage IIIA (cT1-2, N2, M0). Previously treated with four cycles of definitive chemo immunotherapy consisting of carboplatin, pemetrexed and pembrolizumab followed by maintenance therapy with pemetrexed and pembrolizumab. She presented with a two-month history of progressive cough, exertional dyspnea, and intermittent hemoptysis. Examination revealed hypoxemia (SpO2 88% on room air), requiring 5L supplemental oxygen. Labs showed normocytic anemia 7.8 g/dl, WBC 6.8 × 10³/µL, and negative infectious workup including procalcitonin, blood cultures, and respiratory viral panel. Chest CT with contrast demonstrated new, bilaterally symmetric, medially distributed interstitial infiltrates confined to prior radiation fields1. Despite 7 days of empiric treatment with cefepime and azithromycin, symptoms persisted. Bronchoscopy revealed mild bronchial hyperemia without endobronchial lesions; bronchoalveolar lavage from right middle lobe showed neutrophil predominance and sterile cultures. Radiographic pattern, temporal proximity to immunotherapy, and infection exclusion supported RRP precipitated by ICIs. Intravenous methylprednisolone 1mg/kg/day was given in two divided doses for 5 days which led to rapid clinical and radiographic improvement, and she was discharged on a corticosteroid taper with ongoing pulmonary and oncologic follow-up. Discussion This case illustrates the diagnostic complexity of RRP in NSCLC patients receiving ICIs. Unlike radiation pneumonitis which arises weeks to months post-radiation, our patient developed localized inflammation seven months after thoracic irradiation, highlighting delayed immune-mediated recall. Radiographic pattern, negative infectious workup, and rapid response to corticosteroids were key in distinguishing RRP from diffuse immune-related pneumonitis or lymphangitic carcinomatosis. Clinical Significance: This case underscores the importance of a high degree of suspicion for RPP to ensure early diagnosis in patients on ICIs. Recognition of temporal and radiographic hallmarks enables timely intervention and multidisciplinary management. This abstract is funded by: None
Saeed et al. (2026) studied this question.