Relapse of Biphenotypic Acute Leukemia as a Breast Mass

In acute leukemia, leukemic infiltration of the breast is extremely rare. We report a case of biphenotypic acute leukemia (BAL) that presented as a breast mass. A 30-year-old woman presented with a 4-month history of a right breast mass with nipple discharge and easy fatigue. She had received chemotherapy and peripheral blood stem cell transplantation for BAL and had been in complete remission for the last 2 years. Core needle biopsy of the breast mass revealed monomorphous infiltrates of blast cells with round nuclei and fine chromatin, consistent with leukemic infiltration. Subsequent bone marrow biopsy showed diffuse infiltration of immature cells. However, bone marrow karyotyping showed 46, XY, suggesting complete engraftment of transplanted donor cells. This is the report of BAL recurring as a breast mass. In the differential diagnosis of a breast mass, extramedullary relapse should be considered when the patient has a history of leukemia.