[A Case of Breast Cancer Recurrence Responding to Trastuzumab Deruxtecan for Increased Cervical Lymph Node Metastasis...

A 65-year-old woman underwent breast-conserving surgery for right breast cancer 12 years ago. The primary lesion was ER-positive, PgR-negative, presenting no amplification of the HER2 gene, and endocrine therapy was continued. After 10 years postoperation, duodenal stenosis due to peritoneal metastasis was noted, and the positive conversion of the HER2 expression was confirmed in peritoneal metastasis. Peritoneal lesions could be reduced by chemotherapies combined with trastuzumab and pertuzumab; however, hoarseness due to cervical lymph node metastasis appeared. Administration of T- DXd was initiated. After 4 cycles of T-DXd, her cervical lymph nodes shrank, and hoarseness improved. Because of its high drug-to-antibody ratio and the bystander effect, T-DXd was considered effective even in metastatic lesions presenting tumor heterogeneity or low HER2 expression.