Multiple myeloma in a man with breast cancer: A case report and literature review.

RATIONALE: The development of multiple myeloma (MM) following male breast cancer is extremely rare and can often be mistaken for bone metastases in the early stages, potentially leading to delays in diagnosis and treatment. PATIENT CONCERNS: In this case report, we describe a 68-year-old male patient who presented with multiple osteolytic lesions on imaging following breast cancer surgery, and was ultimately diagnosed with MM via bone marrow biopsy. DIAGNOSES: MM. INTERVENTIONS: After being diagnosed with MM, the patient was treated with various chemotherapy regimens, with the subsequent emergence of rare extramedullary disease necessitating an adjustment in therapy. OUTCOMES: The patient successfully underwent autologous hematopoietic stem cell transplantation but later succumbed to a novel coronavirus infection. LESSONS: This case highlights the diagnostic challenge of misattributing osteolytic lesions to breast cancer and underscores the importance of biopsy in patients with metastatic disease.