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Early indicators of metastatic cancer response to therapy are important for evaluating new drugs and stopping ineffective treatment. The Response Evaluation Criteria in Solid Tumors (RECIST) based on repeat cancer imaging are widely adopted in clinical trials, are used to identify active regimens that may change practice, and contribute to regulatory approvals. However, these criteria do not provide insight before 6 - 12 weeks of treatment and typically require that patients have measurable disease. Recent data suggests that measuring on-treatment changes in the amount or proportion of circulating tumor DNA (ctDNA) in peripheral blood plasma may accurately identify responding and non-responding cancers at earlier time points. Over the past year, the RECIST working group has evaluated current evidence for plasma ctDNA kinetics as a treatment response biomarker in metastatic cancers and early endpoint in clinical trials, to identify areas of focus for future research and validation. Here, we outline the requirement for large standardized trial datasets, greater scrutiny of optimal ctDNA collection time points and assay thresholds, and consideration of regulatory body guidelines and patient opinions. In particular, clinically-meaningful changes in plasma ctDNA abundance are likely to differ by cancer type and therapy class, and must be assessed before ctDNA can be considered as a potential pan-cancer response evaluation biomarker. Despite the need for additional data, minimally-invasive on-treatment ctDNA measurements hold promise to build upon existing response assessments such as RECIST, and offer opportunities for developing novel early endpoints for modern clinical trials.