Breast Plasmacytoma as the Initial Manifestation of Multiple Myeloma in a 36-Year-Old Woman.

Extramedullary plasmacytomas (EMPs) of the breast are extremely rare and may present as the initial manifestation of multiple myeloma (MM). We report the case of a 36-year-old woman who presented with a rapidly growing right breast mass. Mammography and ultrasound revealed an oval, circumscribed, heterogeneous, vascular mass, measuring up to 4.2 cm, categorized as Breast Imaging Reporting and Data System (BI-RADS) 4. Core needle biopsy revealed a plasmablastic/plasmacytic neoplasm with strong CD138 expression, lambda light chain restriction, high Ki-67 index, and negative Epstein-Barr virus early RNA in situ hybridization (EBER-ISH), consistent with plasmablastic plasmacytoma. Systemic staging confirmed the diagnosis of MM, and the patient underwent chemotherapy followed by autologous stem cell transplantation. This case illustrates how breast plasmacytoma (BP) can mimic primary breast malignancies on imaging, particularly triple-negative invasive ductal carcinoma, lymphoma, or malignant phyllodes tumor. Although rare, it should be considered in younger patients with atypical breast masses. Diagnosis relies on biopsy with histopathological and immunophenotypic confirmation. Early recognition and biopsy are key, as BP may mimic aggressive breast malignancies and carry prognostic significance in MM.