Association of ERBB2 Copy Number and Gene Coalterations With Trastuzumab Efficacy and Resistance in Human Epidermal Growth Factor Receptor 2–Positive Esophagogastric and Gastric Cancer

ERBB2 CN and gene coamplification in the HER2 signaling pathway were positive and negative predictors of PFS in trastuzumab-treated HER2-positive AGC patients, respectively. HER2-positive AGC patients with a high VAF of ERBB2 showed poor outcomes and may need HER2 tyrosine kinase inhibitors and trastuzumab deruxtecan.