Definitive Histologic Diagnosis of Bone Marrow Carcinomatosis Followed by Dose-Adjusted Chemotherapy in Metastatic Urothelial Carcinoma: A Case Report.

INTRODUCTION: Bone marrow carcinomatosis (BMC) is a rare manifestation of metastatic urothelial carcinoma (UC) that may mimic hematologic disease and cause severe cytopenia, diagnostic difficulty, and rapid clinical deterioration. CASE PRESENTATION: A 65-year-old man presented with back pain, thrombocytopenia, and leukoerythroblastosis. Bone marrow aspiration resulted in a dry tap, but biopsy revealed diffuse carcinoma-cell infiltration consistent with UC, establishing BMC as the cause of marrow failure. Because of profound marrow dysfunction, reduced-dose gemcitabine and cisplatin chemotherapy was initiated, leading to hematologic recovery and radiologic improvement. The patient subsequently received standard-dose chemotherapy and avelumab maintenance and survived for 12.4 months after diagnosis. CONCLUSION: Histologic confirmation is essential to distinguish UC-related BMC from primary hematologic disease and may support individualized systemic treatment in carefully selected patients.