Difficulties in differentiating between checkpoint inhibitor pneumonitis and lung metastasis in a patient with melanoma

The use of immune checkpoint inhibitors is associated with significant toxicities such as pneumonitis; the clinical presentation of the latter can be misleading and may mimic metastasis. We report the case of a melanoma patient who developed late-onset pneumonitis after discontinuation of treatment with anti-programmed cell death protein 1 (PD1) and anti-cytotoxic T lymphocyte antigen 4 (CTLA4) (patient had a complete response). The patient was asymptomatic, however, surveillance computed tomography (CT) scan showed a growing lung nodule and several new-onset, small lung lesions highly suspicious for recurrence. A biopsy of the lesions revealed organizing pneumonia with absence of malignant cells. The lung lesions completely resolved after 6 months without any intervention. The patient is still in complete remission 2 years after the initial diagnosis of melanoma.