Non‐bacterial cystitis with increased expression of programmed death‐ligand 1 in the urothelium: An unusual immune‐related adverse event during treatment with pembrolizumab for lung adenocarcinoma

INTRODUCTION: Immune checkpoint inhibitors are now a standard therapeutic option for lung adenocarcinoma. However, Immune checkpoint inhibitors often induce various immune‐related adverse events. CASE PRESENTATION: The patient was a 78‐year‐old woman with lung adenocarcinoma who had a partial response to pembrolizumab. During treatment, she complained of pollakiuria and nocturia with painful micturition. Histological analysis revealed infiltration of CD8‐positive and/or TIA‐1 cytotoxic granule‐associated RNA binding protein‐positive lymphocytes and programmed death‐ligand 1 expression in the urothelium. A diagnosis of immune‐related adverse event cystitis was made based on these clinical and pathological findings. The patient’s subjective symptoms and findings on cystoscopy improved dramatically after treatment with prednisolone. CONCLUSION: Immune checkpoint inhibitors‐induced cystitis is extremely rare. This report is the first to include an immunohistochemical analysis of the urothelial epithelium in immune‐related adverse event cystitis and describes an instructive case.