CD20 negative primary diffuse large B cell lymphoma of breast: Role of Pax-5.

Pax-5 is a B cell marker, the expression of which is detectable in as early as the pro B stage, and subsequently, in all further stages of B cell development except the plasma cells. Malignant lymphomas of breast are uncommon and occur as either primary or secondary lesions. Primary lymphoma is a rare disorder of breast and constitutes less than 0.6% of all breast malignancies and 2.2% of extranodal lymphomas. We report an unusual case of CD20 negative Pax-5 positive primary diffuse large B cell lymphoma (DLBCL) of breast. The case highlights the diagnostic challenge posed by extranodal CD20 negative DLBCL. Pax-5 immunohistochemistry has diagnostic benefit as a B-cell marker in the work-up of undifferentiated malignant neoplasms. Although it is available for nearly a decade now, it is not widely used. Pax-5 is a valuable addition to the armamentarium of markers currently available for lymphoma subtyping.