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Objectives: Human epidermal growth factor receptor 2 (HER2; ERBB2 gene) is of prognostic and predictive significance in breast carcinoma. Both fluorescence in situ hybridization (FISH) and dual-color in situ hybridization (DISH) methods are available. DISH and FISH are highly concordant in validation studies, but differences may be more prevalent in the equivocal range. Our goal was to compare FISH and DISH on a cohort enriched for equivocal cases, with respect to HER2 determination.Methods: The cohort was enriched for equivocal (2+) cases. DISH and FISH were evaluated using standard protocols and the results compared with respect to HER2 status, HER2 copy number, and HER2/chromosome 17 (Chr17) ratio.Results: In total, 109 cases were identified. The agreement rate of DISH with FISH was 74%. The mean ± SD HER2/Chr17 ratio by DISH was 1.63 ± 0.08 vs 1.59 ± 0.26 by FISH (P = .45). The mean ± SD HER2 copy number by DISH was 4.56 ± 0.45 vs 4.75 ± 1.08 by FISH (P = .004). Individual signals were more easily resolved using FISH in cases with higher copy numbers.Conclusions: In our cohort enriched for equivocal cases, the numerical values of HER2 copy number were significantly lower using DISH, resulting in discordances. Although DISH is a valid method, variations with FISH may be expected in high-equivocal cases and in quality assurance activities. [ABSTRACT FROM AUTHOR]

Objectives: To determine the diagnostic yield of testing multiple blocks for HER2 in cases of multifocal breast carcinoma. Methods: We identified 246 consecutive cases of multifocal invasive breast carcinoma in which HER2 was tested on more than 1 tumor focus. We performed an audit of all cases with respect to tumor size, grade, and histologic type. Results: HER2 status was concordant between multiple foci in 230 (93.5%) of 246 cases, with the largest focus having the most positive HER2 result in 242 (98.4%) of 246 cases. We did not find a single case in which a smaller focus demonstrated a more positive HER2 status unless this focus was either higher grade or different histologically. Conclusions: Our findings support the evaluation of HER2 on the largest focus, with additional testing on smaller foci having a different histologic type or higher grade. [ABSTRACT FROM AUTHOR]