S723
Clinical – Lower GI
ESTRO 2026
Results: At week 14 (W14), 122 of 141 patients (pts) (87%) underwent CTRE. Among them, 93 (76%) achieved a “good” response (ncCR) and 29 PR. The ncCR rate was 65% (40/62) in arm A and 88% (53/60) in arm B (p = 0.004). MRI at W14 was performed in 82 ncCR patients; 80/82 (98%) were classified as TRG1–2. Among the 30 nCR pts with detailed endoscopic description, findings included ulceration (n = 19), fibrosis (n = 9), small polyp (n = 2).Among 93 pts with ncCR, 76 were managed with a watch-and-wait strategy (W&W) starting at W14 (A: 28; B: 48) (figure 1). At 5 years, OP rates were 75% in A and 83% in B (p = 0.24). The 5-year OP rate was similar for cCR (81%) and nCR (77%) (Figure 2).Between W14 and W24, among 141 pts, 22 had early TME and 23 had LE followed by a W&W approach. At W24, CTRE was performed in 68 pts, among the 15 pts showing PR at W14, 4 achieved nCR by W24 (A: 1/9; B: 3/6). Of the 19 pts without CTRE (W14), 11 achieved ncCR W24 (A: 3/14; B: 8/12).Figure1
Conclusion: CTRE using DRE and rectoscopy is reliable at week 14 in 75% of cases to assess “good” response, followed by W&W. CTRE ncCR at week 14 is strongly concordant with MRI. The prognostic value of cCR and nCR is similar for OP; nCR reflects radiation side effects, and should not justify radical surgery. References: Gerard JP, Barbet N, Schiappa R, Magné N, Martel I, Mineur L, Deberne M, Zilli T, Dhadda A, Myint AS; ICONE group. Neoadjuvant chemoradiotherapy with radiation dose escalation with contact x-ray brachytherapy boost or external beam radiotherapy boost for organ preservation in early cT2-cT3 rectal adenocarcinoma (OPERA): a phase 3, randomised controlled trial. Lancet Gastroenterol Hepatol. 2023 Apr;8(4):356-367. doi: 10.1016/S2468-1253(22)00392-2. Epub 2023 Feb 16. PMID: 36801007. Keywords: Rectal cancer, organ preservation, contact therapy Digital Poster 3372 Positional Variability of Mesorectal Lymph Nodes in Radiotherapy for Rectal Cancer Melisa Bagci Kilic 1 , Hilal Alkis 2 1 Radiation Oncology, Denizli State Hospital, Denizli, Turkey. 2 Radiation Oncology, Marmara University School of Medicine, Istanbul, Turkey Purpose/Objective: Optimal patient positioning for rectal cancer treatment remains controversial due to the toxicities of organs at risk. Simulation with a full bladder in supine position or with a belly board in prone position is widely accepted. However, the clinical target volume of tumor, which is typically created with cranio-caudal
Figure2
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