The Japanese Breast Cancer Society Clinical Practice Guidelines are published as timely guidance on clinical issues in breast cancer treatment in Japan. In the recent edition of these guidelines, we addressed a new clinical question 34 (CQ 34, systemic treatment part) “Is trastuzumab deruxtecan recommended for patients with unresectable or metastatic HER2-low breast cancer?” and a new future research question 7 (FRQ 7, pathological diagnosis part) “How is HER2-low breast cancer diagnosed for the indication of trastuzumab deruxtecan?”. These questions address use of trastuzumab deruxtecan in patients with unresectable or metastatic HER2-low breast cancer who have previously received chemotherapy for metastatic disease. The strengths of evidence and recommendation were determined through a quantitative and qualitative systematic review using multiple outcomes, including efficacy and safety. We conclude that trastuzumab deruxtecan is recommended for this patient population (strength of recommendation: 1; strength of evidence: moderate; CQ34) and that HER2-low expression for the indication of trastuzumab deruxtecan should be diagnosed using companion diagnostics based on appropriate criteria (FRQ7).

日本乳腺癌协会临床实践指南的发布是对日本乳腺癌治疗临床问题的及时指导。在这些指南的最新版本中，我们解决了一个新的临床问题 34（CQ 34，全身治疗部分）“曲妥珠单抗 deruxtecan 是否推荐用于不可切除或转移性 HER2 低乳腺癌患者？” 以及未来新的研究问题7（FRQ 7，病理诊断部分）“如何诊断曲妥珠单抗deruxtecan适应症的HER2低乳腺癌？”。这些问题涉及曲妥珠单抗 deruxtecan 在既往接受过转移性疾病化疗的不可切除或转移性 HER2 低乳腺癌患者中的使用。证据和建议的强度是通过使用多种结果（包括功效和安全性）进行定量和定性系统评价来确定的。我们的结论是，曲妥珠单抗 deruxtecan 推荐用于该患者群体（推荐强度：1；证据强度：中等；CQ34），并且曲妥珠单抗 deruxtecan 适应症的 HER2 低表达应根据适当的标准使用伴随诊断进行诊断（FRQ7） ）。