Abstract RF3-05: Tissue-free circulating tumour DNA detection in patients with early triple negative breast cancer from the c-TRAK-TN trial

Abstract Background: Detection of circulating tumour DNA (ctDNA) in plasma predicts future recurrence in early breast cancer (EBC). Previous studies have relied on tumour-informed approaches with personalised assays. In this study, we performed orthogonal testing on plasma samples collected prospectively in the c-TRAK-TN study to compare a tissue-free assay with digital droplet PCR (ddPCR) for ctDNA analysis. Methods: c-TRAK-TN recruited 161 patients with early triple negative breast cancer (TNBC) deemed clinically high risk of relapse. Patients underwent ctDNA analysis by ddPCR starting within 4 weeks of completion of treatment and continuing every 3 months for up to 2 years. ddPCR tracked 1 or 2 patient specific variants as previously described. We further analysed 1062 timepoints from a total of 159 patients (median 7 per patient, range 1-11) using Guardant Reveal assay, a tissue-free approach that exploits differential methylation patterns in breast cancer compared with normal tissue. Results: Samples were successfully analysed from 95.3% (1012/1062) timepoints from 159 patients with the tissue-free Reveal assay. CtDNA was detected in 34.0% (54/159) of patients in at least 1 timepoint, with a median methylation tumour fraction of 0.266% (range 0.002 to 51.0%). Detection of ctDNA with the tumour agnostic assay strongly associated with future risk of relapse (HR 20.62, p