CD10-Negative Primary Breast Follicular Lymphoma: A Rare Case of Primary Breast Lymphoma With an Atypical Immunophenotype Mimicking Marginal Zone Lymphoma

In this article, we report the case of a 78-year-old woman who consulted our hospital for a right breast mass detected on mammography during her cancer screening. Biopsy specimens showed atypical lymphocytic infiltration with a follicle-like growth pattern, suggesting a follicular lymphoma (FL). Immunohistochemically, the atypical lymphoid cells were diffusely and strongly positive for CD20, BCL2, and BCL6, but negative for CD10. IGH-BCL2 translocation was confirmed by fluorescence in situ hybridization analysis, leading to the diagnosis of primary breast FL. The most important differential diagnosis of this case was marginal zone lymphoma (MZL), which usually shows a CD10⁻/BCL2⁺ immunophenotype and is one of the common histological types in primary breast lymphomas. FLs with an atypical immunophenotype exist in a certain percentage of patients. Therefore, FL is considered to be a heterogeneous entity. It is important to distinguish FL from MZL in primary breast lymphomas because FLs may have a worse prognosis than MZLs.