The ISH and IHC assays are recommended by the American
Society of Clinical Oncologists and the College of American
Pathologists.2,5 These methods are highly concordant because
gene amplification is the most common mechanism of HER2
overexpression; to date, no strong evidence for other mechanisms
of HER2 overexpression in breast cancer is presented.8,14
However, the efficacy of trastuzumab therapy is linked to FISH
HER2 positivity rather than IHC positivity,15 and discrepancies
between HER2 test results obtained with FISH and IHC are
observed. Between 5% and 22% of all IHC 3þ breast cancers
do not reportedly exhibit HER2 amplification, whereas between
2% and 11.5% of all IHC 0/1þ breast cancers show HER2
amplification.