Decoding HER2: A Guide to Advanced Gastric Cancer Testing

HER2 testing is crucial because it identifies patients who are most likely to benefit from trastuzumab treatment. The ToGA trial demonstrated that patients with high HER2 protein expression (IHC 3+ or IHC 2+ with ISH positivity) show improved survival when treated with trastuzumab combined with chemotherapy. Accurate HER2 assessment ensures that appropriate patients receive targeted therapy, maximizing its effectiveness and avoiding unnecessary exposure to toxic treatments. However, other factors such as PD-L1 expression and microsatellite instability (MSI) are also important biomarkers in gastric cancer but were not discussed.

The primary methods for HER2 testing include Immunohistochemistry (IHC) and In Situ Hybridization (ISH). IHC measures the level of HER2 protein expression in tumor tissue, with scores ranging from 0 to 3+, where 3+ indicates high overexpression. ISH determines the number of HER2 gene copies in tumor cells, using techniques like fluorescence ISH (FISH) or chromogenic ISH (CISH). The HER2:CEP17 ratio, calculated from ISH results, is also a key factor, with a ratio ≥2.0 often used as a cutoff for HER2 positivity. Other methods such as next-generation sequencing (NGS) can also detect HER2 amplification or mutations but were not discussed.

Accurately determining HER2 status faces several challenges. There isn't a universally accepted definition of HER2 positivity; instead, it relies on a combination of protein expression levels from IHC and gene amplification measured by ISH. Different criteria from the ToGA trial, FDA, EMA, and Asia-Pacific task force add complexity. Factors such as tissue fixation, sample type (biopsy, resection, or metastatic site), and tumor heterogeneity can also impact HER2 assessment. Addressing these challenges through standardized testing algorithms and robust definitions is crucial for improving the precision of HER2 testing.

Trastuzumab is a targeted therapy that specifically binds to the HER2 protein, which is overexpressed in certain aggressive cancers. By targeting HER2, trastuzumab inhibits the growth and spread of cancer cells. The ToGA trial demonstrated that combining trastuzumab with chemotherapy significantly improved overall survival in patients with HER2-positive tumors. This has established trastuzumab as a standard of care for these patients, highlighting the importance of identifying HER2-positive individuals through accurate testing.

To standardize HER2 testing, ongoing research focuses on refining testing algorithms, addressing heterogeneity challenges, and establishing standardized protocols for tissue handling and test interpretation. Collaborative initiatives, like the GaTHER program in Australia, evaluate inter-laboratory agreement and promote best practices in HER2 testing. The goal is to enhance the precision of HER2 assessment, ensuring that all patients with advanced gastric and GOJ cancers receive the most appropriate and effective treatment strategies. Further research into novel biomarkers and testing methodologies is also essential for continued improvement in patient outcomes.