HER2-positive gastric cancer with paraaortic nodal metastasis successfully resected after chemotherapy with trastuzumab: a case report.

We report on a case of human epidermal growth factor receptor-2 (HER2)-positive gastric cancer with paraaortic lymph node metastasis. The patient (a 49-year-old female) received chemotherapy (capecitabine and cisplatin) plus molecular-targeted therapy (trastuzumab), followed by curative resection. Interestingly, the resected residual cancer was HER2-negative. Intra-tumor heterogeneity hinders molecular-targeted therapy for gastric cancer. In our case, continued trastuzumab administration presented few benefits since the residual cancer cells were HER2-negative. No consensus exists regarding the appropriate therapy for unresectable gastric cancers whose non-curative factors disappear following trastuzumab chemotherapy. The principal options are treatment with surgery or continued chemotherapy with trastuzumab. In our case, resection treated the HER2-negative residual cancer effectively, resulting in curative therapy. This is the first case of positive-to-negative change in the HER2 expression of residual tumor cells following trastuzumab therapy. It suggests that, due to intra-tumor heterogeneity, the risks presented by remnant HER2-negative cancer cells persist despite trastuzumab therapy.