Disseminated carcinomatosis of the bone marrow caused by prostate cancer diagnosed with only bone marrow biopsy

INTRODUCTION: Disseminated carcinomatosis of the bone marrow caused by prostate cancer is a rare condition with poor prognosis. Diagnosis has mostly been by primary prostate biopsy. CASE PRESENTATION: A 60‐year‐old man had malaise, low platelet count (9000/μL), and high prostate‐specific antigen (1382 ng/mL). Bone marrow biopsy showed strongly positive immunostaining NKX3.1, leading to diagnosis of prostate cancer bone marrow metastasis, cT3aN1M1b. Definitive diagnosis by prostate biopsy was difficult because of the sparsity of atypical glands. He had progression to castration‐resistant prostate cancer after 3 months of hormonal therapy, and received 27 courses of docetaxel and six courses of cabazitaxel as chemotherapy, but finally died of respiratory failure 33 months after the start of treatment. CONCLUSION: Aggressive biopsy of the metastatic sites should be considered if a prostate biopsy at the primary site cannot be diagnosed definitively.