Supplementary Material for: HER2-Positive Metaplastic Breast Cancer with Resistance to Neoadjuvant Chemotherapy: Case Report

Introduction Metaplastic breast carcinoma (MBC) is a rare histologic subtype of breast carcinoma, which is usually negative for estrogen receptor, progesterone receptor and HER2. HER2-positive MBC is therefore extremely rare. Most MBCs have poor response to chemotherapy. HER2-targeted neoadjuvant chemotherapy (NAC) is widely performed and has high efficacy in treating HER2-positive breast cancer. We report an atypical case of HER2-positive breast cancer that had poor response to NAC and was diagnosed with MBC after the surgery. Case presentation A 73-year-old woman noticed a mass in her right breast and visited our hospital. The mass was diagnosed as hormone receptor-negative, HER2-positive invasive ductal carcinoma, T2N0M0 stage IIA. She received HER2-targeted NAC comprising trastuzumab + pertuzumab + docetaxel. Despite three courses, we observed disease progression. The next NAC regimen comprised of two courses of epirubicin + cyclophosphamide, but the cancer continued to grow. She stopped receiving NAC and underwent a unilateral mastectomy and sentinel lymph node biopsy. Although the preoperative pathological result of core needle biopsy specimen showed invasive ductal carcinoma, the postoperative pathological result of the surgical specimen was MBC. Conclusion In this case, when the patient had undergone three courses of trastuzumab + pertuzumab + docetaxel, it would have been appropriate to review the result of the core needle biopsy with pathologists or to perform vacuum-assisted breast biopsy. This case suggests the importance of considering the possibility of special histologic subtypes such as MBC when a tumor with the diagnosis of invasive ductal carcinoma is resistant to NAC.