Recurrent and atypical immune checkpoint inhibitor-induced pneumonitis

Introduction Pembrolizumab (Keytruda) is a monoclonal antibody against the programmed cell death-1 (PD-1) receptor on lymphocytes, which is one of the immune checkpoint inhibitors (ICIs) approved for multiple solid and hematologic malignancies. Although ICIs have proven to be more effective and less toxic compared to chemotherapy, there are reports of adverse side effects with ICIs. For example, pneumonitis is a potentially lethal side effect occurring in 1%–5% of patients who received ICIs in clinical trials, and there are case reports with clinical and radiological features of checkpoint inhibitor-pneumonitis (CIP). Case report We report an unusual case of pneumonitis with atypical imaging in a patient who received pembrolizumab for metastatic p16-positive squamous cell carcinoma of the base of the tongue. We discuss the approach to the recognition and management of atypical CIP in patients on pembrolizumab with the intent to standardize workup and increase awareness among healthcare providers in the new era of immunotherapy. Management and Outcome Serologic workup including laboratory studies for complete blood count (CBC), lactate, procalcitonin, SARS-CoV-2 (COVID-19), Legionella, Cytomegalovirus (CMV), Coccidioides, Coxiella, and viral respiratory panel were negative for infectious processes. Since CIP was suspected, the patient was started on steroid therapy. Interval computed tomography (CT) of the chest without contrast showed a resolution of pneumonitis. Discussion In this case report, we discuss our workup of CIP and initial testing to rule out other possible causes of the patient's symptoms and radiographic findings, and management of the patient's diagnosis of atypical CIP which led to complete clinical recovery from CIP.