A Case of HER2-positive Metaplastic Breast Cancer whose Subtype Changed to Triple-negative after Anti-HER2 Therapy

Abstract

Metaplastic breast cancer (MpBC) is a rare histological subtype that is usually triple negative (TN). Herein, we report a case of HER2-positive MpBC. An 80-year-old female noticed a 3-cm palpable mass in her right breast. Mammography revealed a high-density mass with unclear boundaries and microcalcifications. Ultrasound revealed an irregular hypoechoic mass. The pathological findings were MBC negative for ER/PgR and slightly positive for HER2. Mastectomy with axillary lymph node dissection was performed. The final diagnosis was adenosquamous carcinoma, pT2pN2aM0 stage III A, and amplification of the HER FISH test was observed. On the 4th month after surgery, chemotherapy (Cap+HER+PER), which was very effective, began because of local recurrence in the left breast. On the 14th month, metastasis to the left subclavian lymph nodes emerged, and chemotherapy was changed to T-DM1. Subsequently, a local excision was performed because the skin lesions had deteriorated. However, skin recurrences occurred early. The breast cancer subtype was changed to TN. We encountered a case of HER2-positive MpBC that showed a certain response to anti-HER2 therapy. When the therapeutic effect is attenuated, treatment strategies that consider tumor heterogeneity are important.