ESTRO 2025 - Abstract Book

S1240

Clinical – Lower GI

ESTRO 2025

Material/Methods: From 2019-2024, patients with localised SCCA treated with CRT were prospectively included for data collection and blood sampling. The cfDNA levels (ng/µL) were measured in serum at baseline, mid-therapy and end of treatment (EOT) by DFA. Healthy individuals were included for comparison. Baseline levels were compared to patient characteristics and healthy individuals using Mann-Whitney U test and to mid-therapy and EOT levels with Wilcoxon signed-rank test. Receiver operating characteristics (ROC) was used to estimate the area under the curve (AUC), with cut-off determination by the Youden index. Survival analysis was done by Kaplan-Meier and hazard ratios (HR) were calculated with cox proportional hazard regression. Results: We included 126 patients. Blood samples were available at baseline (n=126), mid-therapy (n=103) and EOT (n=108), with median cfDNA levels of 0.77 95%CI(0.72-0.84), 0.62 ng/µL 95%CI(0.56-0.72) and 0.66 ng/µL 95%CI(0.58-0.73), respectively. The cfDNA levels were elevated in SCCA patients compared to healthy individuals (p<0.001), with a cut off for differentiation at 0.71 ng/µL. Baseline levels were higher in patients with performance status >1 and high-risk disease (T3-T4, N+ or M+) (p<0.05). During treatment, cfDNA levels decreased from baseline to mid-therapy and EOT (p<0.001). Over a median follow-up of 18.5 months, 13 patients experienced treatment failure. While baseline, mid therapy and EOT levels did not correlate with failure, the decline (%) from baseline to lowest cfDNA level during CRT was associated with failure (p<0.05). ROC analysis identified a cut-off at -21.15%, where patients with low cfDNA elimination (>-21.15%) had a significantly higher risk of failure (p<0.05) and shorter disease-free survival (HR=3.36 95%CI(1.28-8.66) p<0.05). Overall survival data were not mature. Conclusion: Our study demonstrated that cfDNA can be quantified in SCCA patients using a simple, inexpensive DFA method. Baseline cfDNA levels correlated with clinical risk factors and low cfDNA elimination during CRT was associated with inferior outcome. Results highlight the clinical potential of cfDNA dynamics as a prognostic biomarker to guide personalized treatment strategies in SCCA. References: 1. Lefèvre AC, Kronborg C, Sørensen BS, Krag SRP, Serup-Hansen E, Spindler KG. Measurement of circulating free DNA in squamous cell carcinoma of the anus and relation to risk factors and recurrence. Radiother Oncol. 2020 Sep;150:211-216. doi: 10.1016/j.radonc.2020.06.045. Epub 2020 Jul 3. Erratum in: Radiother Oncol. 2021 Jul;160:228. doi: 10.1016/j.radonc.2021.05.008 2. Truelsen CG, Kronborg CS, Sørensen BS, Callesen LB, Spindler KG. Circulating cell-free DNA as predictor of pathological complete response in locally advanced rectal cancer patients undergoing preoperative chemoradiotherapy. Clin Transl Radiat Oncol. 2022 Jun 8;36:9-15. doi: 10.1016/j.ctro.2022.06.002. Keywords: circulating free DNA, anal cancer, biomarkers

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Digital Poster External Validation of Contouring Guidelines for Inguinal Lymph Node Irradiation in Anal Canal Cancer Natalia Barogi, Stefania Manfrida, Viola De Luca, Raffaella Michela Rinaldi, Flavia de Giacomo, Bruno Fionda, Rosa Autorino, Nicola Dinapoli, Maria Antonietta Gambacorta Radiation Oncology, Fondazione Policlinico Universitario A. Gemelli IRCCS, Rome, Italy Purpose/Objective: The delineation of clinical target volume (CTV) for inguinal lymph node irradiation in anal canal cancer remains a challenge. Current contouring atlases often lack robust evidence, resulting in inconsistencies in practice. Garda et al. recently proposed guidelines based on the anatomical localization of inguinal lymphadenopathies, recommending