Clinicopathological and prognostic significance of HER2-low expression in advanced gastric cancer: a retrospective observational study

Human epidermal growth factor receptor 2 (HER2) status is a critical biomarker in advanced gastric cancer (AGC). While the role of HER2-positive tumors in guiding targeted therapies is well-established, the clinical implications of HER2-low expression, defined as immunohistochemistry (IHC) 1+ or IHC 2+/in situ hybridization-negative (ISH-negative), remain undetermined. The aim of this study was to investigate the prognostic significance and clinicopathological features of HER2-low AGC. This retrospective analysis involved patients with AGC treated with first-line fluoropyrimidine and platinum-based chemotherapy from 2011 to 2020. Patients were categorized into HER2-zero (HER2 IHC 0), HER2-low (IHC 1+ or 2+/ISH-negative), and HER2-positive (IHC 2+/ISH-positive or 3+) groups. Among 548 patients analyzed, 33.0%, 45.1%, and 21.8% were classified as HER2-zero, HER2-low, and HER2-positive, respectively. The proportions of male patients, intestinal-type histology, esophagogastric junction/cardia involvement, metastatic disease status, ≥2 metastatic sites, liver metastasis, lymph node metastasis, and high serum carcinoembryonic antigen levels were gradually elevated in the HER2-zero, HER2-low, and HER2-positive groups. Overall survival (median) was 13.8, 13.6, and 23.0 months, respectively, with a non-significant trend favoring HER2-positive over HER2-low (adjusted hazard ratio: 0.80; P = .0672). A delayed separation of Kaplan-Meier curves for overall survival between the HER2-zero and HER2-low groups was observed, without reaching statistical significance (adjusted hazard ratio: 1.12; P = .2568). Patients with HER2-low status exhibited intermediate and specific clinicopathological features within the HER2-negative category. In terms of prognosis, HER2-low patients showed a worsening trend compared with HER2-positive patients. This evidence implies that HER2-low status represents a distinct clinical subset, bridging the gap between the HER2-zero and HER2-positive profiles.