Abstract 1952: Circulating tumor DNA for assessing neoadjuvant treatment response and recurrence risk in rectal cancer patients

Abstract Introduction: Disease recurrence after surgery is the main factor affecting survival in rectal cancer patients. Hence, identifying high-risk patients is important to optimize postsurgical treatment. In recent years, effective use of neoadjuvant therapy (NAT) has led to increased focus on non-operative “watch-and-wait” strategies. However, clinical modalities for assessing complete response following NAT are not sufficiently accurate for selecting patients suitable for non-operative management. Novel biomarkers such as circulating tumor DNA (ctDNA) offer great potential for predicting neoadjuvant treatment response and identifying patients at risk of recurrence. Materials and Methods: The Danish IMPROVE trial (NCT04084249) recruited 2750 stage I-III colorectal cancer patients between 2018 and 2023 with the purpose of evaluating the clinical utility of ctDNA analyses. Of these, the present study includes 114 patients with locally advanced rectal cancer, who were treated with NAT prior to curative-intent surgery in accordance with national guidelines. Plasma samples were collected before NAT, after NAT, and after surgery (n=319). Analysis of ctDNA was performed using a tumor-agnostic droplet digital PCR test targeting three methylation markers. Recurrence-free survival (RFS) was calculated as the time between surgery and radiological recurrence (local or distant), with censoring at the end of radiological follow-up. Overall survival (OS) was calculated as the time between surgery and death from any cause, with censoring at end of follow-up. The median follow-up was 21.9 months (Interquartile Range (IQR) 11.8-36.0) for recurrence and 34.7 months (IQR 22.6-47.0) for survival. Results: The ctDNA detection rates were 86.8% (n=79/91) pre-treatment, 28.9% (n=33/114) after NAT, and 22.8% (n=26/114) after surgery. Post-NAT ctDNA status was significantly associated with pathologic complete response (pCR; p=0.03, Fisher’s exact). None of the 11 patients with pCR had ctDNA detected following NAT and the sensitivity for detecting residual disease was 32%. Detectable ctDNA was associated with worse RFS and OS, both after NAT (RFS HR 3.2, 95%CI 1.5-6.8, p=0.004; OS HR 3.0, 95%CI 1.1-8.4, p=0.03) and after surgery (RFS HR 8.3, 95%CI 3.7-18.9, p<0.001; OS HR 10.6, 95%CI 3.4-33.4, p<0.001). Conclusions: In conclusion, we showed that ctDNA status after NAT was significantly associated with tumor response to preoperative therapy and could potentially aid in patient selection for nonoperative management. Moreover, our study demonstrated a strong prognostic value of ctDNA analysis in predicting recurrence and survival outcomes, highlighting its potential to improve postoperative management through risk stratification. Citation Format: Malene Krabbe Østergaard, Christina Demuth, Tenna Vesterman Henriksen, Nadia Øgaard, Per Vadgaard Andersen, Kåre Andersson Gotschalck, Lene Hjerrild Iversen, Claudia Jaensch, Mads Falk Klein, Jeppe Kildsig, Nis Hallundbæk Schlesinger, Ole Thorlacius-Ussing, Claus Lindbjerg Andersen. . Circulating tumor DNA for assessing neoadjuvant treatment response and recurrence risk in rectal cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1952.