The College of American Pathologists, American Society for Clinical Pathology, and American Society of Clinical Oncology have published a guideline on HER2 testing and clinical decision making in gastroesophageal adenocarcinoma (GEA). The guideline addresses 2 overarching questions:

• What is the optimal testing algorithm for the assessment of HER2 status in patients with gastroesophageal adenocarcinoma?
• What strategies can help ensure optimal performance, interpretation, and reporting of established assays in patients with GEA?

Among the recommendations:

• Request HER2 testing on tumor tissue on potential candidates.
• Request HER2 testing on tumor tissue in the biopsy or resection specimens.
• Offer combination chemotherapy and HER2-targeted therapy as initial treatment in those who have advanced GEA with HER2-positive tumors.
• Select the tissue block with the areas of lowest grade tumor morphology in biopsy and resection specimens.
• Identify areas of invasive adenocarcinoma and also mark areas with strongest intensity of HER2 expression by IHC in GEA specimens for subsequent ISH scoring.
• There is insufficient evidence to recommend for or against genomic testing.