EP761 - Circulating Tumor DNA in Predicting Treatment Response in Locally Advanced Rectal Cancer - A Prospective Observational Pilot Study

Purpose/Background: Predicting treatment response in rectal cancer remains a challenge. The integration of a blood based biomarker such as Circulating Tumor DNA (ctDNA) with clinical findings and radiological tools could improve accuracy of assessment of tumor response to treatment. The aim of this study is to determine the feasibility and validity of assessing tumor response utilizing ctDNA in patients with locally advanced rectal cancer (LARC) undergoing treatment.

Methods/Interventions: This is a single institution prospective observational pilot study. Patients newly diagnosed with locally advanced rectal cancer (LARC) clinical stage II or III (cT3-4N0, or cTanyN+), were prospectively enrolled from July 2024 to August 2025. ctDNA was assessed using a personalized, tumor-informed, next generation sequencing (NGS) based Signatera assay (NATERA, Inc.). The quantitative ctDNA levels were reported as mean tumor molecules per milliliter (MTM/mL) and categorically as positive/ negative. ctDNA was measured serially at the following time points: time of diagnosis prior to treatment initiation (baseline), during the course of neoadjuvant therapy (NAT)/ total neoadjuvant therapy (TNT) at a monthly interval, upon completion of NAT/TNT, and one month post surgical resection. ctDNA status was compared with treatment response assessed by standard method of restaging (digital rectal examination, endoscopic examination, transrectal ultrasound, and pelvic MRI). Patient demographics and tumor characteristics were reported. Correlation of ctDNA status with clinical endpoints of treatment response were reported (Table 1.). Timeplots of ctDNA values were created to assess the trend of ctDNA during treatment (Figure 1.). Descriptive statistics is used to analyze the results.

Results/Outcomes: A total of 7 patients were enrolled in the study. At baseline, ctDNA detection rate was 100% (7/7). Post-NAT, ctDNA positivity was 60% (3/5). All post-NAT ctDNA positive patients 100% (3/3) showed persistent disease. Of which, 66.6% (2/3) had partial response and 33.3% (1/3) had disease progression. Post NAT, ctDNA resolution was 40% (2/5). Only 50% (1/2) who cleared their ctDNA post NAT had complete clinical response. While the other 50% (1/2) patient, had partial response despite ctDNA resolution post NAT. After definitive surgical resection with total mesorectal excision, 100% (4/4) had ctDNA resolution.

Conclusion/Discussion: ctDNA testing has the potential to serve as a predictive biomarker for treatment response in rectal cancer patients. ctDNA positivity upon completion of neoadjuvant treatment may indicate less favorable response in patients with LARC. Our study also demonstrated a case where the patient may have persistent disease despite having ctDNA resolution. A larger scale clinical trial will help better define the sensitivity and accuracy of ctDNA to predict treatment response and to monitor therapy in LARC.

Figure 1. Timeplot (Partial Responders)
Table 1. Individual Patient Data and Treatment Response
CTX denotes chemotherapy, CRT chemoradiotherapy, DRE digital rectal examination, TRUS transrectal ultrasound, yTN tumor stage after neoadjuvant therapy, yp/pTN tumor stage on pathological staging, NA not available, *Treatment Effect is graded according to the American Joint Committee on Cancer (AJCC) Tumor Regression Grade