Beyond The Usual Suspects: Primary Breast Lymphoma Incidentally Detected With Routine Mammographic Screening.

Primary breast lymphoma (PBL) is a rare type of non-Hodgkin's lymphoma, comprising less than 1% of all lymphomas and 0.5% of breast malignancies. The majority of PBL cases are of B-cell origin, with diffuse large B-cell lymphoma (DLBCL) being the most common subtype, though follicular lymphoma (FL) also occurs. We present a case of a 74-year-old woman whose routine mammogram revealed a 1.5 cm mass in the right breast. Ultrasound and biopsy confirmed the presence of classic follicular lymphoma, with B-cell markers CD20, CD10, Bcl-6, and Bcl-2. A PET scan and biopsy of the left periaortic lymph node identified additional disease, confirming stage III FL. The patient was asymptomatic and exhibited favorable prognostic factors, including normal beta-2 microglobulin and LDH levels. Due to the indolent nature of her disease and low tumor burden, the decision was made to pursue active surveillance rather than immediate treatment. This case highlights the diagnostic challenges of PBL and the importance of personalized management, particularly in asymptomatic patients with low disease burden.