Primary Myeloid Sarcoma of the Breast Mimicking Carcinoma: Aberrant Expression of PAX5 and KMT2A::MLLT10 Fusion.

Myeloid sarcoma (MS) is a rare extramedullary tumor characterized by the proliferation of immature myeloid precursor cells. It most commonly occurs in association with acute myeloid leukemia, either at initial diagnosis or during disease progression. Breast involvement is exceedingly rare and represents a significant diagnostic challenge, as it can closely mimic more common breast malignancies, particularly carcinoma or lymphoma, frequently leading to misdiagnosis. We report the case of a 39-year-old woman who presented with two bilateral breast masses, initially suspected to be malignant based on clinical and radiological findings. Histopathological examination revealed a poorly differentiated malignant neoplasm composed of immature cells with eosinophilic cytoplasm and prominent nucleoli, diffusely infiltrating the mammary parenchyma. Immunohistochemical analysis demonstrated strong positivity for leukocyte common antigen (LCA), myeloperoxidase (MPO), and CD68, with focal expression of CD34 and CD117, supporting the diagnosis of primary bilateral breast MS; notably, aberrant expression of PAX5, a marker usually associated with B-cell lineage, constituted a potential diagnostic pitfall. Molecular analysis further identified a KMT2A::MLLT10 fusion, a rearrangement associated with monocytic differentiation and extramedullary involvement. Bone marrow biopsy showed no evidence of neoplastic infiltration. The patient was treated with combined chemotherapy and radiotherapy. This case highlights the diagnostic challenges posed by bilateral breast MS and underscores the critical role of histopathology, immunohistochemistry, and molecular studies in establishing an accurate diagnosis and guiding appropriate management.