Post-operative liver metastasis from gastric cancer with HER2-positive areas newly identified upon re-evaluation, leading to a favorable response to trastuzumab-based therapy: A case report.

Recent advances in molecular targeted agents and immune checkpoint inhibitors have markedly improved outcomes in patients with unresectable or recurrent advanced-stage gastric cancer. As treatment options continue to expand, biomarker testing has become essential for determining first-line treatment strategies. Among these, human epidermal growth factor receptor 2 (HER2) plays a key role in treatment selection; however, the intratumoral heterogeneity of HER2 expression in gastric cancer has long been recognized and may influence therapeutic decision-making. The present study describes the case of a patient who was initially diagnosed as having a HER2 score of 0 on the surgical specimen, but was later found to have a HER2 score of 2+ with FISH positivity upon reassessment, subsequently demonstrating a favorable response to trastuzumab-based therapy. The patient was a 79-year-old man with a type 3 lesion at the cardia, and enlarged lymph nodes adjacent to the primary tumor. The pre-operative diagnosis was cT4aN1M0, stage III. He underwent total gastrectomy. At 27 months post-surgery, MRI revealed a 21-mm metastatic lesion in liver segment 8. Given the initial HER2 score of 0, nivolumab plus the oxaliplatin + leucovorin + 5-fluorouracil regimen (mFOLFOX6) was initiated as first-line systemic therapy. However, as tumor markers continued to increase after two cycles, HER2 was re-evaluated using a different section from the resected specimen, which revealed a score of 2+ with FISH positivity. Therefore, the treatment was switched to trastuzumab plus mFOLFOX6. Following the regimen change, tumor markers decreased, and a CT scan demonstrated the shrinkage of the liver metastasis to 13 mm. Chemotherapy remains ongoing 7 months following the initiation trastuzumab treatment. On the whole, the present case report suggests that the reassessment of HER2 status may help identify patients who may benefit from HER2-targeted therapy, and may ultimately improve the therapeutic efficacy of chemotherapy.