Background: Trastuzumab in association with systemic cytotoxic chemotherapy is the standard of care for patients with advanced HER2-positive gastric carcinoma (GC). However, HER2 as a prognostic factor in GC remains controversial.
Methods: HER2 overexpression and amplification was evaluated by immunohistochemistry (IHC) and silver in situ hybridization (SISH) in 2,798 GCs obtained from 2,727 gastrectomy and 71 open/laparoscopic biopsy specimens from patients with peritoneal seeding. Regional heterogeneity was defined as the proportion of tumor cells showing membranous staining in 10-70 % of tumor cells. Genetic heterogeneity was determined by the existence of HER2/CEP17 ratio higher than 2.0 in >5 to <50 % of tumor cells.
Results: In IHC, 184 cases (6.6 %) were 3+ and 44 cases (1.6 %) were 2+. Of 44 HER2 2+ cases, SISH showed HER2 gene amplification in 21 cases (47.7 %), chromosome 17 polysomy in six cases (13.6 %), and genetic heterogeneity in five cases (11.4 %). HER2 positivity found in 7.3 % of GCs was significantly associated with older age, male gender, intestinal histology, upper third in location, higher lymph node stage (p < .002), and advanced AJCC stage (p = .033). Regional heterogeneity of HER2 was closely associated with 2+ (70.5 vs 42.9 % in 3+, p = .001) and diffuse or mixed histologic type (p = .005).
Conclusions: Regional heterogeneity of HER2 expression was closely associated with weak HER2 overexpression (2+) and with diffuse or mixed histology. Polysomy of chromosome 17 would be an important cause of HER2 2+ in IHC. Frequent HER2 positivity observed in GCs with advanced stages suggests that HER2 may be involved in tumor progression and poor prognosis.