Accessory breast plasma cell mastitis managed with traditional Chinese medicine

Rationale: Plasma cell mastitis occurring in accessory breast tissue is a rare and diagnostically challenging disease. This case report aims to raise clinicians’ awareness of this condition when they encounter axillary masses, and avoid unnecessary interventions on the normal breast tissue. It also demonstrates the feasibility of traditional Chinese medicine (TCM) as a conservative treatment for patients unwilling to undergo hormone therapy or surgery. Patient concerns: A 30-year-old nulliparous woman presented with a 1-month history of a 4 × 5 cm mass in the left axilla. Empirical antibiotic therapy failed to relieve symptoms, and serological workup excluded autoimmune or infectious etiologies. Diagnoses: Ultrasound revealed hypoechoic lesions in the left accessory breast. Diagnosis was confirmed by core needle biopsy and CD138 (syndecan-1) immunohistochemistry, which showed plasma cell-predominant infiltration and ruled out granulomatous mastitis. Interventions: Due to the patient’s refusal of long-term corticosteroids, she underwent oral TCM decoction along with topical Qingbao powder. Incision and drainage were performed on pus-formed areas, followed by Jiuyi Dan powder application to promote pus discharge. Outcomes: After 2 months of treatment, the mass significantly regressed with complete symptom relief. The patient remained recurrence-free at 6-month follow-up, with only residual pigmentation, and was satisfied with the scar preservation effect. Lessons: Enhancing the recognition of early axillary masses, combined with imaging examinations and pathological biopsy to determine their nature, serves as the foundation for individualized treatment. Accessory breast plasma cell mastitis should be included in the differential diagnosis of axillary masses. TCM offers a safe, effective nonsurgical option for eligible patients, while elective surgical excision of residual accessory breast tissue is recommended to prevent recurrence.