Beyond the Usual: An Uncommon Cause of Breast Mass

Primary breast lymphoma (PBL) is a rare extranodal manifestation of non-Hodgkin lymphoma (NHL), accounting for <0.5% of all breast malignancies. The most common histopathological type is diffuse large B-cell lymphoma (DLBCL); however, other subtypes, like extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT), may also occur. Accurate diagnosis is crucial, as clinical presentation can mimic breast carcinoma, yet management is entirely different. A 44-year-old female presented with a left breast lump of 1year duration with palpable axillary nodes. Clinical examination revealed a 5 × 6 cm left breast mass with multiple axillary lymphadenopathies. Positron emission tomography/computed tomography (PET-CT) showed a hypermetabolic breast lesion, mediastinal lymphadenopathy, pleural deposits, and a pelvic lesion. Fine needle aspiration cytology (FNAC) suggested NHL. Histopathological examination of the breast biopsy revealed sheets of small to intermediate lymphoid cells with cleaved nuclei and vacuolated cytoplasm, infiltrating the adjacent adipose tissue. No residual breast tissue was identified. These features were consistent with extranodal marginal zone lymphoma (MALT type), a low-grade primary extranodal B-cell lymphoma. Immunohistochemistry showed positivity for CD20 and BCL2, confirming B-cell lineage. CD3 highlighted reactive T cells, while CD10, BCL6, C-MYC, Cyclin D1, and MUM1 were negative. The Ki-67 index was ∼20%, indicating a low proliferative rate. This case represents an unusual cause of a breast mass, extranodal marginal zone lymphoma of MALT type, highlighting the importance of considering lymphoma in the differential diagnosis of breast lesions. Histopathology combined with Immunohistochemistry remains indispensable for accurate diagnosis and guiding systemic therapy.