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A 40-year-old woman underwent her routine mammography, which revealed a hypoechoic, heterogeneous mass measuring 3.3 × 2.9 cm (panel A, red circle). A core biopsy of the lesion showed periductal tumor cell infiltration, stromal tangles, and diffuse infiltration (panel B; hematoxylin and eosin stain; 200× objective). Immunohistochemical testing revealed negativity for estrogen receptor, progesterone receptor, androgen receptor, pancytokeratin, and CD45. Further analysis showed that tumor cells are positive for CD20 (panel C; 200× objective), BRAF (panel D; 200× objective), cyclin D1 (panel E; 200× objective), and tartrate-resistant acid phosphatase (panel F; 400× objective), and negative for CD5 (panel G; 200× objective). Molecular study for BRAF V600E mutation analysis was positive. Flow cytometry evaluation confirmed the diagnosis of hairy cell leukemia (HCL; panels H-J). Importantly, no tumor involvement was found in the peripheral blood or bone marrow, and no clonal rearrangements of immunoglobulin heavy chains were detected in the peripheral blood sample.

This case presents an exceptionally rare instance of HCL manifesting as a breast mass. It highlights the importance of considering this diagnosis, as it can mimic invasive lobular breast carcinoma. A misdiagnosis could result in inappropriate treatment and significant patient harm, emphasizing the need for accurate diagnosis and careful consideration of differential diagnoses.

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