Consider infection early on in the differential
The differential diagnosis for unilateral breast swelling and lymphadenopathy includes infection and malignancies. Potential infectious etiologies include acute bacterial mastitis, cat scratch disease, and breast abcess. Malignant causes include inflammatory breast cancer and primary malignant lymphoma. Primary malignant lymphoma of the breast is rare, with an estimated prevalence of 0.04% to 1.1% of all breast tumors and 1.7% to 2.2% of all extranodal non-Hodgkin lymphomas.7
Initial evaluation should be directed toward the most likely etiologies of the symptoms and should include breast ultrasound, blood cultures, and serologies—such as Bartonella—as history dictates.
Once more common causes of unilateral breast swelling are excluded, less common etiologies, such as duct ectasia, mammary adenosis or fat necrosis, and blunt force trauma, should be considered.
Due to the age of our patient, mastitis was initially high on the differential; however, when she did not respond to antibiotic therapy, other causes for her symptoms had to be considered. An ultrasound (noted earlier) ruled out a breast abscess. Blood cultures and serologies were negative. Punch skin biopsies of the patient’s breast showed no evidence of carcinoma, making inflammatory breast cancer less likely.