Dramatic response to pembrolizumab after pseudoprogression in a patient with advanced metastatic castration‐resistant prostate cancer

INTRODUCTION: Prostate cancer with a microsatellite instability‐high or mismatch repair‐deficient status is not common. Few reports of the response to pembrolizumab in metastatic castration‐resistant prostate cancer in a real‐world setting have been reported. This case report describes a dramatic response to pembrolizumab after initial pseudoprogression in a patient with microsatellite instability‐high metastatic castration‐resistant prostate cancer. CASE PRESENTATION: A 70‐year‐old man was administered pembrolizumab for metastatic castration‐resistant prostate cancer after the genetic evaluation of lymphadenectomy revealed a microsatellite instability‐high status. His general condition dramatically improved after pseudoprogression. His favorable condition has been maintained for 1 year since the final dose. CONCLUSION: We experienced a case of dramatic response to pembrolizumab after pseudoprogression in a patient with advanced metastatic castration‐resistant prostate cancer. In patients with metastatic castration‐resistant prostate cancer and the microsatellite instability‐high/mismatch repair‐deficient phenotype, a few months follow‐up is necessary to evaluate the efficacy of pembrolizumab.