Foundation of HER2 Gastric Cancer ASCO/CAP Guidelines—Overview of the Differences Between ASCO-CAP Breast and Gastric Cancer Scoring-January 28, 2026

Why It Matters

Accurate HER2 testing in gastric cancer directly influences eligibility for targeted therapies, shaping patient outcomes and standardizing pathology practice worldwide.

Key Takeaways

• HER2 drives targeted therapy in gastric tumors
• Gastric IHC scoring differs from breast criteria
• ASCO/CAP recommends 10% membrane staining cutoff
• ISH testing required for equivocal IHC results
• Accurate reporting improves patient eligibility for trastuzumab

Pulse Analysis

HER2 amplification, long recognized as a driver in breast cancer, has emerged as a pivotal biomarker in gastric adenocarcinoma, affecting roughly 10‑15% of cases. Patients whose tumors overexpress HER2 benefit from trastuzumab‑based regimens, which have demonstrated survival advantages in pivotal trials. Consequently, pathologists must reliably identify HER2‑positive gastric cancers to ensure that eligible patients receive these life‑extending therapies, making the biomarker’s clinical relevance a top priority for oncology teams.

Scoring HER2 in gastric tissue diverges markedly from the breast paradigm. Unlike the uniform membranous staining required in breast specimens, gastric cancers often exhibit heterogeneous, basolateral, or incomplete membrane positivity, prompting ASCO/CAP to adopt a 10% tumor cell cut‑off for IHC 3+ classification. Equivocal (2+) cases now mandate reflex ISH testing, with a HER2/CEP17 ratio ≥2.0 confirming amplification. These nuanced criteria address the unique histologic architecture of gastric tumors and reduce false‑negative rates that previously limited therapeutic access.

The updated ASCO/CAP guidelines provide a clear, reproducible framework that laboratories can integrate into existing workflows, enhancing diagnostic consistency across institutions. By standardizing reporting language and establishing rigorous quality‑control measures, the recommendations help clinicians make evidence‑based treatment decisions, potentially expanding the pool of patients eligible for HER2‑targeted therapy. As more data emerge on combination regimens and novel agents, adherence to these guidelines will remain essential for aligning pathology practice with evolving therapeutic landscapes.