Circulating Tumor DNA-Guided Adjuvant and Post-Adjuvant Therapy in Resected Colorectal Cancer: From Molecular Residual Disease Detection to Proven Clinical Utility

Colorectal cancer (CRC) kills approximately 850,000 people annually and is the second leading cause of cancer mortality worldwide. After curative-intent surgical resection, recurrence rates of 15–20% in stage II and 30–40% in stage III diseases highlight the fundamental limitations of pathological staging as the sole basis for adjuvant treatment decision-making. Circulating tumor DNA (ctDNA), which comprises tumor-derived cell-free DNA fragments shed into the bloodstream by dying cancer cells, offers a direct and dynamic readout of molecular residual disease (MRD) in the postoperative period. Because ctDNA clears within hours of macroscopically complete surgery, any persistently detectable postoperative signal reflects viable microscopic disease rather than surgical contamination. Over the past decade, evidence has matured rapidly from proof-of-concept observational studies to completed randomized clinical trials, culminating in the recent publication of DYNAMIC-III and ALTAIR in 2025–2026. The DYNAMIC trial established that ctDNA-guided management reduced adjuvant chemotherapy use from 27.9% to 15.3% in stage II colon cancer while maintaining an equivalent five-year recurrence-free survival (88% vs. 87%; difference 1.1%, 95% CI 5.8% to 8.0%). The GALAXY study confirmed ctDNA as the most powerful prognostic biomarker in resected CRC, with a disease-free survival hazard ratio of 11.99. Critically, DYNAMIC-III demonstrated that escalating chemotherapy intensity in ctDNA-positive stage III patients did not improve recurrence-free survival (HR 1.11, p = 0.6), and the phase III ALTAIR trial showed that trifluridine/tipiracil failed to significantly improve disease-free survival at the point of molecular recurrence (HR 0.79, p = 0.107). This review critically synthesizes the current evidence for ctDNA-guided treatment decisions in resected CRC across three therapeutic strategies: de-escalation in ctDNA-negative patients, escalation in ctDNA-positive patients, and post-adjuvant therapy at the point of molecular recurrence. Future progress requires biomarker-matched escalation strategies, adaptive trial designs, and formal validation of ctDNA clearance as a surrogate endpoint for predicting patient outcomes.