This study aimed to determine the incidence and characteristics of HER-2 gene heterogeneity in invasive breast cancer in a single institution. Included were 971 cases of primary invasive breast cancer diagnosed between 2008 and 2010. Fluorescence in situ hybridization (FISH) image files were retrospectively reviewed and HER-2 gene heterogeneity was defined as more than 5% but less than 50% of analyzed invasive tumor cells with a HER-2/Chr17 ratio higher than 2.2, according to the College of American Pathologists guidelines. HER-2 gene heterogeneity was identified in 24 (2.5%) cases. The mean proportion of invasive tumor cells with a HER-2/chromosome 17 ratio higher than 2.2 was 11.6% (range: 5%-25%). Of 24 cases, HER-2 gene status was not amplified in 8, showed borderline amplification in 2, and amplification in 14. All HER-2 amplification cases were low-grade. In conclusion, HER-2 gene heterogeneity of invasive breast cancer is identified in routine FISH examination. This may affect the results of HER-2 gene amplification status in FISH studies.
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