Assessing the New American Society of Clinical Oncology/College of American Pathologists Guidelines for HER2 Testing by Fluorescence In Situ Hybridization: Experience of an Academic Consultation Practice

Context.— Evaluation of HER2 gene amplification by fluorescence in situ hybridization (FISH) was changed by recent American Society of Clinical Oncology/College of American Pathologists (ASCO-CAP) guidelines. Objective.— To determine frequencies and assess patterns of HER2 protein expression for each ASCO-CAP guideline FISH category among 7526 breast cancers accrued to our consultation practice. Design.— We retrospectively reevaluated the HER2 FISH status of breast cancers in our consultation practice according to ASCO-CAP FISH guidelines, and documented HER2 protein levels in each category. Results.— —According to new guidelines, 17.7% of our consultation breast cancers were “ISH-positive” with HER2:CEP17 FISH ratios ≥2.0 and average HER2 gene copies per cell ≥4.0 (group 1); 0.4% were “ISH-positive” with ratios ≥2.0 and average copies <4.0 (group 2); 0.6% were “ISH-positive” with ratios <2.0 and average copies ≥6.0 (group 3); 4.6% were “ISH-equivocal” with ratios <2.0 and average copies ≥4.0 and <6.0 (gr...