# Harmonization trial on ESR1 testing strategies in ER+/HER2- breast cancer patients: an Italian experience

## TL;DR

This study evaluates testing methods for ESR1 mutations in breast cancer patients to improve detection consistency across labs in Italy.

## Contribution

The study harmonizes ESR1 mutation detection strategies using reference samples and compares NGS and dPCR performance.

## Key findings

- ESR1 p.D538G mutation was detected in 90% of samples A and B, and 80% of sample C.
- NGS and dPCR showed similar performance in detecting low-frequency ESR1 mutations.
- Harmonized ring trials can standardize procedures for better clinical stratification.

## Abstract

To date, ESR1 activating mutations acts as key player to clinically stratify estrogen receptor (ER)+/HER2-advanced breast cancer (BC) patients eligible to novel new generation oral Selective Estrogen Receptor Degraders (SERD) relapsing after first line aromatase inhibitors. Liquid biopsy represents the most useful biological source to detect ESR1 activating mutations in clinical setting, but the lack of standardized pre-analytical and analytical procedures drastically impacts on detection rate of ESR1 mutations in diagnostic specimens. Here, we sought to harmonize technical procedures comparing technical performance of diagnostically available testing strategies on a series of three reference specimens (sample A, B, C) harboring ESR1 p.D538G mutation at different mutant allele fraction (MAF) (5.0 %, 1.0 %, 0.5 %) shared with n = 10 Italian referral institutions.

A total of 10 μl of gDNA from each reference sample built to mimic clinically detectable ESR1 molecular alteration (5.0 %, 1.0 %, 0.5 % VAF) was shipped by coordinator institution to each participating group to test p.D538G ESR1 alteration leveraging own routinely available testing strategy. Artificial reference sample was previously validated by the University of Naples Federico II before arranging the shipment.

ESR1 exon 10 p.(D538G) hotspot mutation was successfully identified in 90.0 % of samples A, B whereas 8 out of 10 (80.0 %) participating institutions detected sample referenced alteration in sample C. No statistically significant variations were observed between dPCR and NGS based workflows in terms of detectability rate on standard reference samples. A single participating institution (ID#5) failed to detect p.(D538G) ESR1 alteration but supervised procedures by coordinator institution enabled to detect referenced mutation in engineered reference samples set adopting an orthogonal technology (dPCR). In addition, NGS and dPCR platforms displayed a similar technical performance in detecting ESR1 across samples A-C.

NGS and dPCR systems may be considered valid technical solutions to target low frequency ESR1 alterations in diagnostic routine samples. Harmonized ring trials are key weapons to standardize analytical and post-analytical procedures optimizing clinical stratification of BC patients