RATIONALE: Pembrolizumab, a monoclonal antibody against the programmed cell death 1 (PD-1) protein, can induce a stable regression of some malignancies refractory to conventional chemotherapy. Despite such therapeutic benefits, pembrolizumab can induce immune-related adverse events, with pneumonitis being the most critical problem. PATIENT CONCERNS: All 3 patients complained of fever, cough, and dyspnea after a variable time interval (1-21 days) from pembrolizumab treatment. DIAGNOSES: Chest computed tomography invariably showed ground glass opacity. All tests for possible infectious agents were negative. Based on high procalcitonin level, one of 3 patients was diagnosed to have accompanying bacterial pneumonia. INTERVENTIONS: All patients received antibiotics and steroid treatments (methylprednisolone, 1 mg/kg). OUTCOMES: The 3 patients showed different clinical courses ranging from mild pneumonitis to rapidly progressing respiratory failure. Among the 3 patients, 2 fully recovered with steroid treatment; 1 died from superimposed bacterial pneumonia. LESSONS: The prognosis of pembrolizumab-induced pneumonitis with a superimposed bacterial pneumonia would be poor. It is important to distinguish pure pneumonitis from that with a superimposed bacterial pneumonia.
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