Extramedullary Involvement of the Breast by Systemic Multiple Myeloma as the Initial Presentation of Plasma Cell Myeloma: A Rare Case Breast Plasmacytoma in MM

Abstract

Background: Extramedullary involvement by systemic multiple myeloma is a rare manifestation of plasma cell myeloma, most commonly arising in the upper aerodigestive tract. Breast involvement is exceptionally uncommon and can mimic primary or recurrent breast carcinoma, creating diagnostic challenges.
Case presentation: A 64-year-old woman with a history of invasive ductal carcinoma presented with a new breast lesion on screening mammography. Imaging demonstrated a small, circumscribed nodule, and core biopsy revealed monoclonal plasma cell infiltration. Magnetic resonance imaging (MRI) showed multiple breast lesions, while bone marrow biopsy confirmed more than 90% plasma cell involvement with high-risk cytogenetics, including 1q21 amplification. These findings established a diagnosis of extramedullary involvement of the breast by systemic multiple myeloma (MM). The patient was treated with daratumumab-based systemic therapy, achieving remission with eventual minimal residual disease negativity on follow-up testing. She remains without clinical or radiographic evidence of breast disease.
Conclusion: Breast involvement by systemic MM is a rare manifestation of plasma cell myeloma and should be considered in the differential diagnosis of new breast lesions. Early biopsy and comprehensive pathologic evaluation are essential to avoid misdiagnosis and guide appropriate systemic therapy.