# ESR1, PGR, ERBB2, and MKi67 mRNA expression in diagnostic core biopsies from breast cancer patients of the ABCSG Trial 34

## TL;DR

This study compares a new breast cancer test (STRAT4) with traditional methods (IHC) to see how well they agree in measuring key cancer markers.

## Contribution

The study evaluates the STRAT4 assay's performance against IHC in core biopsies and surgical samples from breast cancer patients.

## Key findings

- STRAT4 showed good agreement with IHC for ER and Ki67, and moderate agreement for PR in diagnostic core biopsies.
- STRAT4 and IHC measurements correlated similarly with cancer outcomes like residual cancer burden and survival.
- The STRAT4 assay was tested on both diagnostic biopsies and post-treatment surgical samples.

## Abstract

To investigate the performance of the Xpert® Breast Cancer STRAT4 (CE-IVD∗) Assay (STRAT4) in the neoadjuvant, randomized Austrian Breast and Colorectal Cancer Study Group (ABCSG) Trial 34.

The primary objective of this study was to assess the concordance between STRAT4 mRNA measurements of ESR1, PGR, ERBB2, and MKi67 obtained from diagnostic core biopsies of the ABCSG Trial 34 with central reference laboratory immunohistochemistry (IHC) (and in-situ hybridization for HER2 IHC 2+ samples). For each marker, overall percent agreement (concordance), positive percent agreement (sensitivity), and negative percent agreement (specificity) between STRAT4 and IHC were determined. The secondary objective was to evaluate the concordance of STRAT 4 and IHC in post-treatment surgical samples and the association of both assays with residual cancer burden (RCB), time to distant recurrence (DR), and overall survival (OS). Logistic regression and Cox regression models were used.

A total of 354 formalin-fixed paraffin-embedded diagnostic core biopsies were examined, representing 88.5 % of the available samples. Concordance between STRAT 4 and IHC was 93.7 % for ER, 80.5 % for PR, and 94.1 % for Ki67. All three biomarkers tested by either STRAT4 or central IHC showed similar correlation to RCB, time to DR, and OS.

In diagnostic core biopsies, there was a good agreement between STRAT4 mRNA measurements and centrally assessed IHC measurements of ER, and moderate agreement for PR and Ki67.

•Immunohistochemistry (IHC) is the gold standard for evaluating ER, PR, HER2, and Ki67.•The Xpert® Breast Cancer STRAT4 Assay represents an alternative diagnostic method.•Assay performance was evaluated on diagnostic core biopsies and on surgical samples.•We demonstrated good or moderate agreement between STRAT4 mRNA measurements and central IHC.

Immunohistochemistry (IHC) is the gold standard for evaluating ER, PR, HER2, and Ki67.

The Xpert® Breast Cancer STRAT4 Assay represents an alternative diagnostic method.

Assay performance was evaluated on diagnostic core biopsies and on surgical samples.

We demonstrated good or moderate agreement between STRAT4 mRNA measurements and central IHC.