OBJECTIVETo evaluate the application of dual-probe chromogenic in situ hybridization (dual-probe CISH) in analysis of HER2 gene status of breast cancer patients by comparison with fluorescence in situ hybridization (FISH). The potential impact of chromosome 17 polysomy in the determination of HER2 status was also studied.

METHODSOne hundred and forty-six cases of paraffin-embedded breast cancer tissues were retrieved. Analysis of HER2 gene and chromosome 17 copy numbers using CE-approved commercial kits of dual-probe FISH (for 146 cases) and dual-probe CISH (for 73 cases) were carried out. The results were interpreted according to ASCO/CAP, 2007 either HER2 gene copy number or the ratio of HER2/centromere 17 (CEN17).

RESULTSOf the 73 cases analyzed by both FISH and dual-probe CISH, the concordance rates for negative and positive results was 91.7% (33/36) and 97.4% (37/38) respectively, while the overall concordance rate between the two methods was 95.9% (70/73). Of the 146 cases analyzed by FISH, 13 cases were interpreted as equivocal if only HER2 copies were counted, compared with 8 equivocal cases by calculating the ratio of HER2/CEN 17. Moreover, 3 cases (4.8%) of the 63 HER2-positive cases determined by HER2 copies turned out to be HER2-negative when determined by the ratio of HER2/CEN 17; while using dual-probe CISH, 1 (3.0%) of the 33 positive cases turned out to be negative. In addition, when using FISH, there were more chromosome 17 polysomy cases (63.5%, 40/63) in the HER2-positive subgroup than HER2-negative subgroup (37.3%, 28/75) (P = 0.002).

CONCLUSIONSDual-probe CISH can achieve similar results as compared to FISH, indicating that this technology is a reliable alternative to FISH in HER2 testing. The accuracy can further be improved when HER2 and chromosome 17 are simultaneously tested and counted.