Abstract 1121: Detection of postoperative minimal residual disease in colorectal cancer using a novel ultrasensitive whole-genome sequencing-based ctDNA test

Abstract Introduction: Accurate detection of minimal residual disease (MRD) after curative intent surgery remains a major limitation in colorectal cancer (CRC) management. Current ctDNA assays identify only 30-50% of patients who later develop clinical recurrence, restricting their utility for both escalation and safe de-escalation of adjuvant therapy. We evaluated a novel, ultrasensitive, tumor-informed, whole-genome sequencing (WGS)-based ctDNA test engineered to maximize detection sensitivity while simplifying clinical implementation by eliminating the need for bespoke patient-specific capture panels, reducing turnaround time, and requiring only 1mL of plasma. Methods: Pre- and postoperative plasma samples (drawn 14 and 30 days after operation) from 300 stage II-III CRC patients (median follow-up 35.7 months) were collected. At the time of abstract submission, 161 patients have been analyzed, but complete cohort results (n = 300) will be presented at AACR 2026. The test (AccuScan) utilizes rolling-circle amplification to convert single-stranded cfDNA into concatemers, enabling 1000-fold error suppression at modest sequencing depth (median 28×). Tumor-informed variant tracking was derived from paired tumor/germline WGS. ctDNA detection was done blinded to clinical information. Performance was evaluated against recurrence outcomes. Results: Among the 161 analyzed patients, 43 (26.7%) recurred. A median of 11,822 tumor-derived variants per patient were tracked with a mean error rate of 4.97 × 10-7. Preoperatively, the novel WGS test detected ctDNA in 98.1% of samples. Postoperatively, ctDNA was detected in at least one sample (day 14 or 30) in 69.8% of recurrence patients with a specificity of 91.5%. Sensitivity and specificity were 55.8% and 96.6% at day 14 and 62.8% and 94.1% at day 30. ctDNA levels in recurrence patients reached as low as 2.7 ppm (median 162 ppm). Extrapolating the performance to an unselected stage III population indicates a ∼10% 3-year recurrence rate for postoperative ctDNA-negative individuals, comparable to low-risk stage II disease, for which adjuvant therapy is generally not recommended. Postoperative ctDNA status was a strong predictor of recurrence-free survival (HR 11.9; 95% CI, 6.2-23.1; p0.001). In a head-to-head comparison, tumor-informed digital PCR using 8mL plasma aliquots achieved substantially lower sensitivity both preoperative (75.4%) and postoperative (33.3% day 14; 51.5% day 30). Conclusion: The novel WGS-based test delivers exceptional sensitivity in both preoperative and postoperative settings while requiring minimal plasma. The streamlined workflow is compatible with real-world clinical implementation. Sensitive detection at day 14 offers an opportunity for earlier therapeutic intervention, and the assay’s performance supports its use in future therapy de-escalation trials. Citation Format: Maria Hønholt, Tenna V. Henriksen, Mads H. Rasmussen, Christina Demuth, Thomas Kolbro, Peter Bondeven, Jeppe Kildsig, Per V. Andersen, Anders Tøttrup, Nis H. Schlesinger, Claudia Jaensch, Ole Thorlacius-Ussing, Christensen Peter, Alessio Monti, Yingyu Wang, Tam Berntsen, George Yeung, Paul Tang, Tobias Wittkop, Malek Faham, Li Weng, Lene Hjerrild Iversen, Kåre A. Gotschalck, Claus L. Andersen. Detection of postoperative minimal residual disease in colorectal cancer using a novel ultrasensitive whole-genome sequencing-based ctDNA test abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1121.