The relation between percentage of immunostained cells and amplification status in breast cancers with equivocal result for Her2 immunohistochemistry

Her2 amplification was more frequent in cancers with >50% moderately immunostained cells in IHC equivocal group. However, most of Her2 IHC equivocal breast cancers contain moderate amount (10-50%) of immunostained cells. Relatively small but non-neglectable number (14%) of cases in <10% group was Her2 amplified by FISH which approaches to overall incidence of Her2 overexpression (15-20%) in all breast cancer. Breast cancers with moderate complete/incomplete membrane Her2 IHC staining in <10% of invasive tumor cells should be regarded as equivocal and reflex FISH should be performed in these cases.