Advances in diagnosis and treatment have resulted in a high rate of survival for many patients with early-stage cancers. However, identifying who is at ongoing risk of relapse remains of high priority to direct subsequent adjuvant therapy. Multiple recent retrospective studies have shown that detection of tumor-derived materials in blood, in particular with circulating tumor DNA (ctDNA) analysis, can identify patients with residual disease before clinical or radiological evidence of metastatic

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... isease, anticipating relapse with relatively high sensitivity and high specificity. We discuss how these emerging technologies are defining new subgroups of patients with "Molecular Residual Disease" and "Molecular Relapse." We outline how novel clinical trials in the adjuvant setting designed for these new subgroups of patients may improve selection for adjuvant therapies, and provide new surrogate endpoints that may allow for early registration of adjuvant therapies and novel clinical trial designs in the adjuvant setting. We discuss the current limitations of these techniques and the routes to clinical implementation.