Secondary breast involvement by peripheral T-cell lymphoma, not otherwise specified.

Peripheral T-cell lymphoma not otherwise specified (PTCL-NOS) is an aggressive nodal lymphoma that rarely manifests as breast involvement and may mimic mastitis or inflammatory breast cancer. We describe a 74-year-old woman with 1 month of progressive left breast pain, erythema, swelling, and fever, initially treated as refractory mastitis, who was later found to have secondary breast involvement by PTCL-NOS. Mammography revealed diffuse left breast skin/trabecular thickening, and ultrasound demonstrated a breast fluid collection with axillary lymphadenopathy. Cross-sectional imaging demonstrated extensive cervical, axillary, thoracic, and abdominal lymphadenopathy with secondary venous and lymphatic congestion of the breast. Axillary and inguinal lymph node biopsies confirmed the diagnosis of PTCL-NOS. The case highlights the diagnostic challenge of lymphoma-related breast manifestation, emphasizes the importance of early biopsy, and underscores the role of multidisciplinary collaboration among breast imaging, dermatopathology, and hematology-oncology.