A case of advanced gastric cancer showing HER2 positivity after chemotherapy

A 63-year-old man had advanced poorly differentiated gastric adenocarcinoma with para-aortic lymph node metastases. No HER2 expression was observed in four endoscopic biopsies from the primary tumor. Tumors shrunk after S-1 with cisplatin treatment, and he underwent simple gastrectomy due to stenosis. Interestingly, HER2 diffusely overexpressed in the resected surgical specimen. His disease was stable with trastuzumab-containing therapy for 6.4 months. This case may suggest a selection of HER2-positive cells that were insensitive to the chemotherapy, and further study is needed for the change of intratumoral HER2 heterogeneity after chemotherapy.