Trastuzumab-Induced Pneumonitis: A Rare but Potentially Severe Pulmonary Complication.

Trastuzumab is a monoclonal antibody targeting the human epidermal growth factor receptor 2 (HER2) and is widely used in the treatment of HER2-positive breast cancer. Although generally well tolerated, trastuzumab has been rarely associated with pulmonary toxicity, including interstitial pneumonitis, a potentially life-threatening complication that may progress to acute respiratory distress syndrome (ARDS). We report the case of a 75-year-old woman with HER2-positive pleomorphic lobular breast carcinoma treated with docetaxel, carboplatin, trastuzumab, and pertuzumab. Shortly after her fifth chemotherapy cycle, she developed acute respiratory failure characterized by severe hypoxemia. Chest computed tomography revealed bilateral ground-glass opacities with interstitial thickening and pleural effusions. Extensive infectious workup, including respiratory viral panels and bronchial cultures, was negative. Given the temporal relationship with trastuzumab exposure and exclusion of alternative diagnoses, trastuzumab-induced pneumonitis was suspected. The patient progressed to severe ARDS requiring invasive mechanical ventilation and prone positioning. High-dose systemic corticosteroid therapy resulted in significant initial clinical and gasometric improvement. However, her course was complicated by ventilator-associated pneumonia due to Pseudomonas aeruginosa, leading to a fatal outcome. This case highlights the importance of early recognition of trastuzumab-induced pneumonitis, prompt exclusion of infectious and cardiogenic causes, and timely initiation of corticosteroid therapy to improve outcomes. Increased awareness of this rare but serious adverse event is essential as the use of anti-HER2 therapies continues to expand.