Multiple Myeloma Presenting as Breast Metastasis: Diagnostic Challenges and Case Analysis.

BACKGROUND Extramedullary disease represents an aggressive form of multiple myeloma (MM), with breast secondary involvement being very rare. This localization makes differential diagnosis from primary breast cancer difficult, as clinical and radiological patterns are similar between these entities. CASE REPORT An 83-year-old patient, treated for immunoglobulin A (IgA) kappa, (1q21+) MM, diagnosed in April 2016, presented to the Emergency Department for a fall. Clinically, she presented with multiple, bilateral breast lesions, confirmed by mammography and magnetic resonance imaging (MRI). Histology analysis documented a tumor with a solid and discreetly trabecular architecture, without calcifications or ductal/lobular in situ lesions, nor peritumoral lymphatic or vascular invasion. The tumor cells were negative for hormonal receptors and human epidermal growth factor receptor 2 (HER2). This histological pattern was first mistaken for an aggressive, infiltrating, pleomorphic lobular primary breast carcinoma. However, because of the history of a concomitant MM and the discordance between the histological diagnosis showing a tumor with a triple negative phenotype and the immunohistochemical profile of the pleomorphic lobular breast cancer, which usually overexpresses hormonal receptors, a histological revision was undertaken, and this confirmed the diagnosis of a breast localization of MM as the tumor cells expressed IgA kappa and multiple myeloma oncogene 1 (MUM1), and were negative for AE1/AE3 cytokeratin. The patient started fifth-line chemotherapy, but her clinical condition quickly worsened, leading to her death in June 2023. CONCLUSIONS The particularity of this case relies on the rarity of extramedullary disease from MM. Its difficult diagnosis represents a clinical challenge, as does the complexity of its management, due to a lack of evidence-based medical data.