[Remarkable Response to Trastuzumab Observed in a Case of Gastric Cancer with HER2-Negative Conversion]

A 64-year-old woman was diagnosed with advanced gastric cancer with solitary liver metastasis. Although the HER2 status of the tumor was IHC2+, no further examination for HER2 status using FISH was performed. Four courses of S-1 and cisplatin chemotherapy were administered. The primary lesion and metastatic lesion were confirmed to have partially regressed. After 4 courses of chemotherapy, an open total gastrectomy, D2 dissection, pancreatosplenectomy, and posterior segmental hepatectomy were performed. Her postoperative course was uneventful. On histopathology, cancer cells were found in the resected stomach and resected liver. However, the HER2 statuses of the resected specimens were negative. After discharge, she received S-1 therapy for 1 year. Upon evaluation 1 year and 1 month postoperatively, the tumor marker CA19-9 was elevated. An enhanced CT scan showed multiple lung metastases and lymph node metastases in the pancreatic tail. Three courses of XELOX therapy(capecitabine and oxaliplatin)were administered. However, tumor marker levels continued to increase, and the metastasis continued to enlarge. Although the HER2 status of the resected site was negative, the HER2 status of the biopsy specimen before chemotherapy was positive on FISH. Therefore, weekly paclitaxel and trastuzumab therapy was initiated and repeated for 6 courses, after which an enhanced CT showed significant reduction(nearly CR)of multiple lung metastases and lymph node metastases. This suggests that HER2-negative conversion had occurred at the resected site. However, the HER2 status of the metastatic sites was considered positive.