A case of metastatic treatment‐emergent small cell/neuroendocrine prostate cancer with BRCA2 mutation diagnosed by liver biopsy

INTRODUCTION: Treatment‐emergent small cell/neuroendocrine prostate cancer occurs predominantly in advanced or metastatic castration‐resistant prostate cancer that arises when prostate adenocarcinoma is transformed after androgen deprivation therapy. The clinical course for the pathogenesis involved or associated genetic information have not been clearly elucidated. CASE PRESENTATION: A Japanese male, 63‐year‐old, underwent a para‐aortic lymph biopsy due to sudden severe bilateral leg edema, with a final diagnosis of stage IV prostate adenocarcinoma. He was initially responsive to upfront abiraterone with androgen deprivation therapy; however, relapse occurred in the liver and bone 10 months after initial treatment, with serum neuron‐specific enolase elevation and without prostate‐specific antigen elevation. Pathological findings of liver tumor revealed treatment‐emergent small cell/neuroendocrine prostate cancer. FoundationOne® CDx was used for cancer‐related gene profiling of liver tumor specimen; a BRCA2 mutation was identified. CONCLUSION: Early detection of this transformation and pathological diagnosis can improve patient survival when genetic mutations, including BRCA 1/2.