Immunohistochemical and molecular analyses of HER2 status in breast cancers are highly concordant and complementary approaches.

Background: Immunohistochemistry (IHC) and fluorescent in situ hybridisation (FISH) are currently the most commonly used methods to assess HER2 status. PCR-based assays allow quantitative determination of HER2 amplification (Q-PCR) or overexpression (Q-RT-PCR), but are not routinely used. We evaluated the relevance of Q-RT-PCR for HER2 status determination.

Methods: We compared IHC and Q-RT-PCR in 466 breast tumours. In discordant or equivocal cases, five additional methods (IHC with two other antibodies, FISH, silver in situ hybridisation (SISH) and Q-PCR) were combined to determine HER2 status. Two cases with HER2 intra-tumour heterogeneity were further explored by allelic profiles analysis and HUMARA clonality determination after microdissection.

Results: We observed 97.3% concordance between Q-RT-PCR and non-equivocal IHC. Twelve out of 466 cases (3%) revealed discordances between the two methods. The power of Q-RT-PCR to predict HER2 status (defined by seven methods) was similar to that of IHC. Although rare, some discordances between techniques might be due to HER2 intra-tumour heterogeneity and we report two examples, one tumour containing two distinct clones, another tumour consisting of HER2 amplified and non-amplified subclones.

Conclusion: Q-RT-PCR and IHC are highly concordant methods for HER2 status assessment, and Q-RT-PCR allows a highly reliable quantitative assessment and could be a useful adjunct to IHC.