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 was composed 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.

中文翻译：

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对新辅助化疗耐药的 HER2 阳性化生性乳腺癌：病例报告。

简介 化生性乳腺癌 (MBC) 是一种罕见的乳腺癌组织学亚型，通常雌激素受体、孕激素受体和 HER2 呈阴性。因此，HER2 阳性 MBC 极为罕见。大多数 MBC 对化疗的反应较差。HER2靶向新辅助化疗（NAC）在治疗HER2阳性乳腺癌中得到广泛应用且疗效显着。我们报告了一例 HER2 阳性乳腺癌的非典型病例，该病例对 NAC 反应不佳，术后被诊断为 MBC。病例介绍 一名 73 岁女性发现右乳有肿块，来我院就诊。该肿块被诊断为激素受体阴性、HER2 阳性浸润性导管癌，T2N0M0 IIA 期。她接受了包含曲妥珠单抗 + 帕妥珠单抗 + 多西他赛的 HER2 靶向 NAC。尽管进行了三个疗程，我们还是观察到了疾病进展。接下来的NAC方案由两个疗程的表阿霉素+环磷酰胺组成，但癌症继续生长。她停止接受 NAC 治疗，并接受了单侧乳房切除术和前哨淋巴结活检。虽然术前空心针活检标本病理结果显示为浸润性导管癌，但手术标本术后病理结果为MBC。结论 本例患者在接受曲妥珠单抗+帕妥珠单抗+多西他赛三个疗程后，应与病理科医生复查空心针活检结果或进行真空辅助乳腺活检。该病例表明，当诊断为浸润性导管癌的肿瘤对 NAC 耐药时，必须考虑特殊组织学亚型（例如 MBC）的可能性。