The dismal 15% survival rate for oesophageal cancer (EC) is predominantly due to newly diagnosed patients already harbouring distant metastatic disease. This, accompanied by a 600% increase in incidence, and the treatment resistant and highly recurrent nature of the disease, highlights the urgent need for improved therapies and more sensitive methods for screening, surveillance and monitoring.
To this end, we have developed a blood-based ‘liquid biopsy’ to detect and monitor genomic, disease-specific alterations in the circulating tumour DNA (ctDNA) present in the plasma of EC patients.
Tumour, buffy-coat (germline) and cell free plasma DNA extracted from patients at the time of diagnosis were obtained and used to evaluate somatic DNA mutations using targeted amplicon sequencing of 9 commonly mutated genes in EC. Tumour specific mutations were confirmed by droplet digital PCR, which was also used to track patient specific ctDNA in plasma from serially collected blood samples.
Somatic mutations were detected in 19/28 (68%) of tumour biopsies. The same mutations were detected in ctDNA from the plasma of 9/19 (48%) patients. 2 patients presented mutations in plasma DNA that were not detected in DNA from tumour biopsies, highlighting issues of intra-tumoural heterogeneity and sampling bias of tumour biopsies. ctDNA was more frequently detected in patients with advanced stage disease, where it also represented a greater proportion of the total cell free plasma DNA than in patients with localised disease. In patients with serial samples, levels of ctDNA correlated with response to treatment, and rises in ctDNA could be detected prior to clinical evidence of relapse.
Detection of ctDNA using targeted amplicon sequencing and digital PCR is feasible for EC patients, particularly those with advanced disease. CtDNA levels may provide supplementary evidence for monitoring treatment response, serve as a surveillance tool for recurrent disease, and aid in informing therapeutic pathway decisions for many EC patients.