822PAssessing the clinical utility of circulating tumour DNA through longitudinal liquid biopsy sampling in oesophageal adenocarcinoma

Abstract Background Oesophageal cancer is the eighth most frequently diagnosed form of cancer and has a dismal 5-year survival rate of 12%, with Oesophageal adenocarcinoma (OAC) being the most common sub-type in western countries (Ferlay et al., 2012). Previous studies in many cancer types have demonstrated potential clinical utility of circulating tumour DNA (ctDNA) for detecting minimal residual disease and tumour evolution through therapy, but there has been little investigation of this technology in large cohorts of OAC patients. Consequently, the aim of this study is to evaluate the clinical potential of longitudinal liquid biopsy sampling using 100 OAC cases. Methods Our approach uses the Roche-Avenio expanded panel and ultra-deep sequencing to detect genomic aberrations across 77 cancer genes in the ctDNA of OAC cases. All cases are late disease stage (T3 /T4) and have at least one plasma sample taken before surgery and on average two other samples taken at time points of clinical interest. One third of cases have associated whole genome sequencing of the primary tumour. Results Our results demonstrate that in spite of OAC being a low ctDNA shedding cancer type, ctDNA can be consistently detected in late disease cases. At least one mutation in OAC driver genes was identified in 69% of cases, some of which include TP53, SMAD4, and CDKN2A (Frankell et al., 2019). TP53 was the most frequently mutated gene, which is also observed in whole-genome sequencing (WGS) data. In addition, other OAC driver genes were mutated to a similar extent to the WGS data. Clinically relevant variants were identified in the plasma samples, such as SMAD4 which has been shown to be a prognostic biomarker in OAC. Therapeutically actionable targets not detected in the primary tumour were also captured, including ALK and PIK3CA. Moreover, minimal residual disease was detected after surgery in the majority of patients that went on to relapse. Conclusions These data suggest that ctDNA can be routinely detected in OAC and that clinically relevant alterations can also be identified at late stage of disease, which is when the majority of patients present. Overall our results indicate that there is potential for using targeted sequencing of ctDNA in the clinic. Legal entity responsible for the study The authors. Funding Educational grant from Roche for the library prep kits. Disclosure All authors have declared no conflicts of interest.