Late-Onset Acute Pneumonitis Caused by Pembrolizumab Used to Treat Postoperative Recurrence of Squamous Cell Carcinoma-A Case Report

We present an unusual case of late-onset acute pneumonitis developing 21 months after pembrolizumab monotherapy. An 80-year-old male with primary, pulmonary, squamous cell carcinoma underwent right lower lobectomy and lymph node dissection(ND2a-2); the postoperative pathological stage was ⅢA(pT2bN2M0)and the PD-L1 tumor proportion score 70%. Six months after surgery, he developed mediastinal lymph node(#2R), bilateral pulmonary, and hepatic metastases; pembrolizumab was administered every 3 weeks as a first-line treatment. A partial response was evident after 3 courses; we thus continued the monotherapy. However, after 28 courses(21 months)of pembrolizumab, we discontinued the regimen because acute pneumonitis(Grade 3)developed; we prescribed prednisolone at 50 mg/day. The acute pneumonitis shadow improved and prednisolone was tapered over 2 months. The patient exhibited no new lesion and no progressive disease 6 months after pembrolizumab was discontinued.