Abstract 6610: Increasing blood volumes to detect minimal residual disease in neoadjuvant-treated early breast cancer patients

Breast cancer (BC) is the most frequent neoplasia affecting women worldwide normally detected at early stages. In this regard, early diagnosis drastically decreases mortality, however, around 20% of these patients will later relapse. This is mainly caused by undetectable molecular residual disease (MRD) not eliminated by standard primary treatments. Therefore, it is crucial to detect the after-treatment MRD to stratify the patients by their risk of relapse. Liquid biopsies have emerged as non-invasive method to obtain information about tumors and improve clinical cancer management. Regarding this, much has been hypothesized about utilizing high blood volumes to overcome the necessity of complex and resource-intensive next generation sequencing (NGS) methodologies to detect highly diluted blood tumor components in localized cancers. Herein, we employed a combined analysis of circulating tumor DNA (ctDNA) and circulating tumor cells (CTCs) together with high blood volumes and single-assay droplet digital PCR (ddPCR) to detect MRD with ultra-high sensitivity. We prospectively assayed 124 samples extracted at baseline, post-neoadjuvant therapy (NAT), post-surgery and a follow-up on a six-monthly basis. A median of 76.40 mL of blood to detect CTCs and 40 mL of plasma to detect ctDNA per patient from 19 BC women were used in this study. ddPCR assays were performed with a median of 14 partitions per determination to detect ctDNA and 12 partitions for CTCs. Overall, ctDNA, CTCs and ctDNA and/or CTCs were detected in 84.21%, 66.66% and 89.47% respectively in the pre-treatment blood samples. MRD (ctDNA and/or CTCs) was detected in 73.68% of the after NAT blood samples. On the other hand, it was detected in 46.66% and 70.00% of the post-surgery and follow-up samples respectively. Post-NAT MRD was detected in 57.14% (4/7) and 83.33% (10/12) of patients with and without pathological complete response pCR respectively. To note, the discordant patients achieving pCR in tissue with detectable MRD in blood were high-risk BC. Importantly, in one of the two patients without pCR and no MRD detected, not enough sample were available to complete the analysis. The other discordant patient presented a localized disease with residual cancer burden value of 1 and no lymph nodes affected. In 1 out of 19 (5.26%) patient clinically relapsed with a positive MRD detection 6 months earlier. Applying this methodology, we observed a sensitivity of 0.004% in ctDNA detection and 0.224 CTCs p