HER2 Testing in Gastric Cancer: Are We Doing Enough?
"A comprehensive look at an Australia-wide study reveals the critical factors in HER2 testing for advanced gastric and gastro-oesophageal cancer, highlighting the importance of accurate assessment for effective treatment."
Gastric cancer, a formidable adversary, ranks as the fifth most prevalent cancer globally and stands as the third leading cause of cancer-related deaths. However, advancements in understanding the disease have paved the way for targeted therapies, offering a beacon of hope for patients. Among these, the human epidermal growth factor receptor 2 (HER2) has emerged as a crucial target.
HER2, when amplified and overexpressed, is associated with aggressive disease progression and poorer outcomes in gastric and gastro-oesophageal junction (GOJ) cancers. Yet, this knowledge has also opened a therapeutic avenue. Trastuzumab, a monoclonal antibody that targets HER2, has shown remarkable success in improving survival rates when combined with chemotherapy, transforming the treatment landscape for HER2-positive advanced gastric cancer.
But the path to effective HER2-targeted therapy hinges on precise and reliable HER2 status determination. This is where the GaTHER (Gastric HER2 Testing) program steps in, aiming to refine HER2 testing algorithms and establish robust criteria for assessment. By analyzing a vast pool of cases, the program seeks to address practical challenges in testing, such as tissue fixation, sample selection, and the impact of tumor heterogeneity, ultimately ensuring that the right patients receive the right treatment.
HER2 Positivity Rates and Testing Gaps
HER2 overexpression in gastric cancer varies widely across studies, with estimates ranging from approximately 7% to 34% depending on the cohort and methodology used. A large multicenter analysis of 2,168 patients reported an overall HER2 positivity rate of 22%, with markedly higher rates in gastroesophageal junction (GEJ) tumors at 34% compared to 20% in gastric cancer proper. Another study using gastric cancer-specific scoring criteria found 4% IHC 2+ and 13% IHC 3+ rates, suggesting a fourfold increase in detected positivity when standardized protocols are applied. Despite these figures, real-world testing rates show dramatic interhospital variation, from 29% to 100%, with low-volume hospitals significantly less likely to perform HER2 assessment.
Standardized Testing Protocols and Practical Constraints
Current guidelines mandate gastric cancer-specific HER2 testing protocols that differ substantially from breast cancer approaches, with immunohistochemistry (IHC) as the initial screening method followed by fluorescence in situ hybridization (FISH) or other in situ hybridization for equivocal (IHC 2+) results. Bright-field microscopy is preferred over fluorescent methods for identifying heterogeneous staining patterns characteristic of gastric tumors. Adequate sampling is critical, with recommendations for six to eight viable biopsy specimens or representative surgical samples; when limited biopsies are available, all viable specimens should be tested. Specific pathologist training is required before undertaking HER2 testing in gastric cancer, and concordance between biopsy and resection specimens, while fair, is not absolute.
From Histopathology to Molecular Biomarkers
Gastric cancer diagnosis historically relied on histopathologic classifications such as the Lauren system and conventional serum markers including CEA and CA19-9. The landscape shifted dramatically in the 2010s with the emergence of molecular biomarkers and targeted therapies, positioning HER2 as the first actionable biomarker for guided therapy in gastric cancer. This transition marked a fundamental change from purely morphologic classification to molecularly informed treatment selection, setting the stage for subsequent biomarker-driven approaches including immune checkpoint blockade.
Unpacking the HER2 Testing Criteria: What's the Standard?
Defining HER2 positivity isn't as straightforward as a simple yes or no. Various criteria have been proposed, leading to potential inconsistencies in assessment. Some definitions rely solely on protein expression levels, using immunohistochemistry (IHC) scores of 3+ as the gold standard. Others incorporate gene amplification, measured by in situ hybridization (ISH), focusing on the HER2:chromosome enumeration probe 17 (CEP17) ratio or the absolute HER2 copy number (CN). Still others combine both protein expression and gene amplification for a more comprehensive evaluation.
- IHC Score: Assesses the level of HER2 protein expression in tumor cells.
- HER2 Gene Copy Number (CN): Measures the number of copies of the HER2 gene present in tumor cells.
- HER2:CEP17 Ratio: Compares the HER2 gene copy number to the copy number of chromosome 17, which is used as a reference.
Emerging Biomarkers and Novel Therapeutic Insights
Recent research has identified circulating tumor cells (CTCs) as a promising non-invasive alternative for documenting HER2 gene amplification, with CTC-detected amplification showing high consistency with tissue-based results. The DESTINY-Gastric01 trial's exploratory biomarker analyses revealed that baseline HER2-associated biomarkers in circulating tumor DNA and tissue samples correlate with therapeutic response to trastuzumab deruxtecan in HER2-positive advanced gastric cancer. Prospective multicenter data from the VARIANZ study documented HER2 loss in previously HER2-positive tumors following trastuzumab therapy, raising important questions about retesting at progression. Updated reviews continue to emphasize the prognostic relevance of HER2 and highlight key interpretive differences between gastric and breast cancer scoring systems.
Testing Discordance and Sampling Pitfalls
The prospective multicenter VARIANZ study revealed HER2 test deviations in 22.3% of cases when comparing central versus local laboratory results, with these deviations associated with negatively impacted survival in trastuzumab-treated patients. A German expert meeting highlighted that unlike breast cancer, HER2 testing in gastric cancer is not routinely performed and faces significant implementation barriers. Sampling error represents a fundamental challenge due to the heterogeneous and often incomplete HER2 overexpression in gastric tumors, particularly in small biopsy specimens where areas of amplification may be missed entirely. These factors collectively undermine the reliability of HER2 status determination and subsequent treatment eligibility.
Gastric vs. Breast Cancer HER2 Testing Paradigms
HER2 testing in gastric cancer follows distinct criteria from breast cancer, established by the ToGA trial which defined eligibility as IHC 3+ or IHC 2+ with confirmatory FISH positivity using a validated assay. Gastric cancer scoring algorithms differentiate between surgical and biopsy specimens, accounting for the characteristic basolateral and incomplete membrane staining patterns not typically seen in breast cancer. Specimen selection also differs, with pathology guidelines recommending testing areas of lowest-grade morphology and accepting FNA cell blocks as alternatives when surgical or biopsy specimens are unavailable. These disease-specific adaptations reflect fundamental biologic differences in HER2 expression patterns between the two cancer types.
Looking Ahead: The Future of HER2 Testing
The findings of the Australian HER2 testing program offer valuable insights for refining HER2 testing algorithms and improving the accuracy of HER2 status assessment in gastric and GOJ cancers. By emphasizing the importance of HER2 CN and addressing the challenges of tumor heterogeneity, the study contributes to ensuring that patients receive the most appropriate and effective treatment. As research continues and new technologies emerge, the future of HER2 testing holds the promise of even more precise and personalized approaches to cancer therapy.
Guideline-Consistent Scoring and Reporting
ASCO-ASCP-CAP guidelines provide a standardized scoring algorithm for gastric HER2 assessment that defines separate criteria for surgical resection and biopsy specimens, addressing the unique heterogeneous staining patterns observed in gastric adenocarcinoma. The recommended approach employs IHC as the primary test with reflex to in situ hybridization (ISH) for equivocal (IHC 2+) results, ensuring confirmatory testing before therapeutic decisions. CAP has published a structured reporting template for gastric HER2 biomarker testing to promote consistency and completeness in pathologic reporting across laboratories.
Overcoming Resistance and Expanding Therapeutic Options
HER2-positive gastric cancer represents a biologically distinct subset where conventional chemotherapy shows limited efficacy, driving the need for novel HER2-directed strategies. Resistance mechanisms—including HER2 heterogeneity, activation of bypass signaling pathways, and histologic transformation—limit the durability of current anti-HER2 therapies such as trastuzumab, which remains the standard first-line treatment combined with chemotherapy. Emerging approaches include antibody-drug conjugates like trastuzumab deruxtecan, bispecific antibodies, and combination regimens targeting resistance pathways. The global HER2-positive gastric cancer market is projected to grow significantly through 2031, reflecting both increasing incidence and expanding therapeutic options.
Heterogeneity, Discordance, and Biological Complexity
HER2 evaluation in gastric cancer remains complex due to intrinsic tumor heterogeneity and methodological variability, resulting in discordant results between biopsies, resections, and metastatic sites. Unlike breast cancer, HER2 expression in gastric tumors is frequently incomplete and heterogeneous, complicating accurate assessment and contributing to inter-observer variability. Beyond its role as a therapeutic target, HER2 amplification drives metastatic potential, therapy resistance, and tumor recurrence through mechanisms including upregulation of DNA repair enzymes, drug efflux proteins, and modulation of cancer stem cell transcription factors. These biologic functions extend HER2's relevance beyond patient selection for targeted therapy.
Real-World Testing Patterns and Patient Outcomes
The multicenter REHEARSAL study in China assessed real-world HER2 testing practices and treatment patterns in HER2-positive locally advanced or metastatic gastric/gastroesophageal junction cancer, revealing gaps between guideline recommendations and clinical implementation. As the first biomarker for guided therapy in gastric cancer, HER2 testing pioneered the molecular stratification that now expands to include mismatch repair deficiency, Epstein-Barr virus status, and PD-L1 combined positive score for immune checkpoint inhibitor eligibility. These evolving biomarker requirements underscore the direct impact of testing quality and accessibility on individual patient treatment options and outcomes.