Circulating tumor DNA is a sensitive marker for routine monitoring of treatment response in advanced colorectal cancer.

Accurate assessment of chemotherapy response provides the means to terminate ineffective treatment, trial alternative drug regimens or schedules and reduce dose to minimize toxicity. Here, we have compared circulating tumor DNA (ctDNA) with carcinoembryonic antigen (CEA) for the cycle by cycle assessment of chemotherapy response in 30 patients with metastatic colorectal cancer. CtDNA (quantified using individualized digital droplet PCR (ddPCR) assays) and CEA levels were determined immediately prior to each chemotherapy cycle over time periods ranging from 42-548 days (average of 10 time points/patient).