S680
Clinical – Lower GI
ESTRO 2026
adapted fraction, the reference plan was recalculated on the daily CT and compared with the adaptive plan for target (PTV V100, D95) and organ-at-risk (OAR) parameters. Interfractional geometric variation was quantified using Dice similarity coefficient (DSC), 95% Hausdorff distance (HD95), and average symmetric surface distance (ASSD). Workflow times, in-vivo 3D dose verification (3%/2 mm gamma), and acute toxicities were assessed. Results: Of 200 delivered fractions, 48 (24%) required adaptation. Median DSC, HD95 and ASSD for the GTV were 0.75, 15 mm and 4.6 mm, respectively, indicating substantial interfractional variability. Site-specific analysis showed the largest geometric discrepancies in transverse colon tumors (median DSC = 0.70; HD95 = 37.9 mm; ASSD = 6.5 mm), followed by ascending colon tumors (DSC = 0.73; HD95 = 15.9 mm;ASSD=4.9). Sigmoid and descending colon tumors were relatively more stable (DSC ≈ 0.85; HD95 < 10 mm; ASSD ≈ 2.5 mm). Adaptive planning restored prescription-level target coverage (mean PTV V100: 67.9% → 98.3%; D95: 16.9 Gy → 25.2 Gy) while significantly reducing bowel exposure (small bowel V11Gy: 77.9 → 69.7 cc; uninvolved colon V18.5Gy: 30.1 → 19.7 cc). The median total workflow duration was 24.6 min (range 16.5–30.3). In-vivo 3D gamma passing rates (3%/2 mm) exceeded 95% for all fractions. Treatment was well tolerated, with grade ≥ 3 toxicity in 3/40 (7.5%) patients, and no treatment interruptions occurred. Figure 1 illustrates a representative case demonstrating dosimetric improvements, while Figure 2 summarizes the overall workflow timing results.
LARC patients most likely to achieve a complete response and benefit from organ preservation strategies within a TNT framework. The present study supports mid-term tumor response evaluation after INCT in TNT patients and a response-guided treatment strategy. Additional radiotherapy to good responders to INCT may increase the organ-preservation rate of TNT. Prospective validation of this approach is underway. References: 1. Taylor FG, Quirke P, Heald RJ, et al. Preoperative high-resolution magnetic resonance imaging can identify good prognosis stage I, II, and III rectal cancer best managed by surgery alone: a prospective, multicenter, European study. Annals of Surgery.2. Bossé D, Mercer J, Raissouni S, et al. PROSPECT Eligibility and Clinical Outcomes: Results From the Pan- Canadian Rectal Cancer Consortium. Clin Colorectal Cancer. 3. Deng X, Liu P, Jiang D, et al. Neoadjuvant Radiotherapy Versus Surgery Alone for Stage II/III Mid- low Rectal Cancer With or Without High-risk Factors: A Prospective Multicenter Stratified Randomized Trial. Annals of Surgery. 4. Schrag D, et al. Preoperative Treatment of Locally Advanced Rectal Cancer. N Engl J Med. Keywords: TNT; rectal cancer; response-guided therapy CT-Guided Online Adaptive Radiotherapy in the TORCH-C Regimen for Locally Advanced Colon Cancer: A Prospective Feasibility Study Jun Zhao, Hui Zhang, Lei Yu, Fan Xia, Weigang Hu, Zhen Zhang Department of Radiation oncology, Fudan University Shanghai Cancer Center, Shanghai, China Purpose/Objective: The colon is one of the most challenging sites for radiotherapy because of its intrinsic mobility, peristaltic motion and proximity to radiosensitive organs such as the small bowel. As a result, radiotherapy has historically played a limited role in locally advanced colon cancer (LACC). This prospective study evaluated the feasibility, efficiency, and dosimetric benefits of a diagnostic CT–guided online adaptive radiotherapy (ART) workflow implemented within the TORCH-C regimen (short-course RT + CAPOX Digital Poster 418 + PD-1 inhibitor) for LACC, aiming to provide a practical adaptive strategy for highly mobile
abdominal targets. Material/Methods:
Between May 2023 and December 2024, 40 patients with pMMR/MSS LACC were prospectively enrolled; 19 underwent online ART using a CT-linac. For each
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