Comparison of immunohistochemistry (IHC) and quantitative RT-PCR: ER, PR, and HER2 receptor status.

Abstract

47 Background: IHC is the present standard for measuring estrogen (ER) and progesterone receptor (PR) expression for breast cancer. However, the lack of concordance between testing laboratories and the critical patient treatment decisions made with results prompted ASCO/CAP to recommend guidelines and proficiency testing requirements to ensure accuracy. Quantitative RT-PCR (qRT-PCR) is an alternative method for ER and PR testing and while concordance with IHC has been reported, additional testing is merited. This study compares ER, PR and HER2 status determined by IHC and qRT-PCR. Methods: FFPE tissues of ER(+) tumors collected and tested at the Blumenthal Cancer Center were studied. Expression levels of ESR1, PGR, and ERBB2 were determined by a multiplex qRT-PCR TaqMan assay and by the Oncotype Dx assay. Pre-established cutpoints were used to determine positivity. Only samples with qRT-PCR and corresponding IHC data were used, resulting in 144 ER, 128 PR, and 107 HER2 comparisons. ESR1 and PGR expression levels determined by the two qRT-PCR assays were also compared. Results: Of the144 IHC ER(+) samples, 142 were positive and 2 were negative by both qRT-PCR assays. All 120 IHC PR(+) samples were positive by both qRT-PCR assays. Of the 8 IHC PR(-) samples, 5 were negative and 3 were positive by both qRT-PCR assays. Of the 107 IHC HER2(-) samples, all but 2 were negative by qRT-PCR. One sample was positive by both qRT-PCR assays; one was positive by Oncotype Dx only. The expression levels determined by the qRT-PCR assays showed good correlation for ESR1 (r=0.85) and PGR (r=0.9). Conclusions: Concordance between qRT-PCR and IHC was 98.6% for ER and 97.7% for PR. Except for a single HER2 determination, there was 100% concordance between the two qRT-PCR assays. The discordant sample was HER2 (+) by multiplex qRT-PCR and “equivocal” by Oncotype Dx. Excellent correlation was also observed in the mRNA expression levels. The absence of HER2(+) and ER(-) samples are limitations of this study. These results suggest that qRT-PCR is a promising alternative method to IHC for determining hormone receptor status. Additional testing of samples with endocrine therapy outcomes including ER(-) samples would be beneficial.