Abstract Introduction Organizing pneumonia (OP) is a recognized but uncommon complication of thoracic radiotherapy. Although radiation-induced OP typically develops within or adjacent to the irradiated field, contralateral involvement is rare. The pathophysiology includes both classic in-field radiation injury and a sporadic/hypersensitivity-type immunologic response. Radiation triggers pneumocyte injury, oxidative stress, and cytokine release (notably TGF-β), leading to alveolar damage and fibroblast recruitment. However, BAL studies have shown CD4-predominant lymphocytic inflammation in both lungs, supporting a systemic inflammatory mechanism that may explain out-of-field or contralateral disease. Case Description A 49-year-old woman with triple-negative left breast carcinoma, diagnosed in December 2023, underwent bilateral mastectomy with reconstruction followed by chemotherapy and left-sided adjuvant radiotherapy. By January 2025, she developed persistent nonproductive cough, intermittent fever, and weight loss. PET-CT revealed FDG-avid areas of consolidation in the right lung, radiation pneumonitis in the left lung and mediastinal lymphadenopathy. Given her history, metastatic disease was initially suspected. A subsequent CT scan performed 8 months later showed organizing Pneumonia with Atoll sign in the right lobe (Image 1). During hospitalization, she developed a pneumothorax requiring chest tube drainage. Empiric antibiotics were discontinued when infectious evaluation remained negative, and imaging features were deemed consistent with radiation-induced organizing pneumonia. She improved significantly on systemic corticosteroids and was discharged with a taper. Pathology demonstrated acute and chronic inflammation without infection or malignancy, consistent with a reactive process supporting radiation-induced pneumonitis. By June 2025, she reported minimal dyspnea and no cough while on a tapering dexamethasone regimen. Following steroid taper completion, she had mild recurrence of cough and dyspnea. She declined repeat steroids due to hyperglycemia but remained clinically stable. Discussion Contralateral Radiation induced organizing pneumonia is extensively rare. Unlike classic in field pneumonitis caused by direct epithelial injury and dose dependent oxidative damage, Contralateral organizing pneumonia is likely driven by a systemic process. Radiation to the lung releases cytokines and generates free oxygen radicals that enhance the immune response. BAL lymphocytosis composed predominantly of CD4+ T cells provides evidence of a hypersensitivity-type process capable of affecting remote lung regions. Recognition is essential, as symptoms and radiographic findings may mimic infection or metastatic progression. Organizing Pneumonia typically responds to corticosteroids but may relapse after taper. This case highlights an uncommon presentation of radiation-induced organizing pneumonia developing in the contralateral lung after localized breast irradiation. Understanding its immune-mediated mechanism can prevent misdiagnosis and unnecessary invasive procedures, while promoting timely steroid treatment and surveillance This abstract is funded by: None
Sateesh et al. (Fri,) studied this question.