ESTRO 2025 - Abstract Book

S1164

Clinical – Lower GI

ESTRO 2025

Results: From Jan–Dec 2021, 4pts with metastatic disease completed a safety run in. The main trial recruited 42pts (Mar 2022-May 2023). 11/42 (26%) had cT4 disease and 12/42 (28%) had diverting-ostomies pre-enrolment. Results are presented at 13.3months median follow-up. Intention-to-treat analysis at EOT showed 22/42pts (52%) exhibited pCR/cCR: 12/21(57%) Arm A and 10/21(48%) Arm B. Both arms fulfilled the predefined statistical hypothesis. Of 39pts (18 Arm A, 21 Arm B) reaching EOT, 20 (51%) proceeded to surgery: 8 Arm A and 12 Arm B of which 3 and 2pts had pCRs. Of 19pts not receiving surgery at EOT, 17 (44%) entered active surveillance (9 Arm A, 8 Arm B); 4/17 (1 Arm A, 3 Arm B) subsequently had surgery within 12 months, where 1pt (Arm B) had a pCR. 2pts (1 per Arm) exhibited progressive disease. At 6 months, pCR/cCR rate was 67% (12/18) Arm A and 48% (10/21) Arm B. At 1-year post-EOT, 11(61%) Arm A and 8(38%) Arm B remain pCR/sustained cCR. 4pts (2 per Arm) developed Grade-3 toxicity during treatment. 3pts had MMR-deficient tumours (2 arm B). All had surgery at EOT: 2/3 had pCR. Conclusion: PRIME-RT is the first trial to evaluate SC and LC-TNT with immunotherapy in LARC. TNT regimens incorporating Durv lead to high CR rates in a UK LARC population (52% across all pts). Highest CR rates were seen with SCRT potentially reflecting different immunomodulatory effects with hypo-fractionated RT. Correlative quantification of immune

responses in serial samples will be available at presentation. Keywords: Rectal cancer, immunotherapy, organ preservation

References: Funding information Core funding (Glasgow CRUK CTU) and trial specific funding (AstraZeneca). Trial registration Clinicaltrials.gov NCT04621370 (Registered 9th Nov 2020) ISRCTN18138369 (Registered 27th October 2020)

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Digital Poster Adjuvant chemotherapy after neoadjuvant chemoradiotherapy and surgery for rectal cancer: assessing its continued relevance and effectiveness. Febin Antony 1 , Jamuna Angel Joy 1 , Diya Tessa Thomas 2 , Ashwin Oomen Philips 3 , Jomon Raphael C 1 , Joju Antony Sebastian 4 , Dominic Mathew 5 , Anilkumar Thazhatha Jose 3 , Unnikrishnan P 3 , Rajkrishna B 1 , Mathew Varghese K 1 , Varun Narayan 6 , Sreeja Raju 7 , Donna Liza Thampan 8 , V Ramankutty 8 , Sunu Lazar Cyriac 3 1 Radiation Oncology, Amala Institute of Medical Sciences, Thrissur, India. 2 Medical Student, Amala Institute of Medical Sciences, Thrissur, India. 3 Medical Oncology, Amala Institute of Medical Sciences, Thrissur, India. 4 Surgical Oncology, Amala Institute of Medical Sciences, Thrissur, India. 5 Surgical Gastroenterology, Amala Institute of Medical Sciences, Thrissur, India. 6 Radiology, Amala Institute of Medical Sciences, Thrissur, India. 7 Pathology, Amala Institute of Medical Sciences, Thrissur, India. 8 Clinical Research, Amala Centre for Promotion of Research, Thrissur, India Purpose/Objective: The role of adjuvant chemotherapy (ACT) in patients with locally advanced rectal cancer (LARC) who receive neoadjuvant chemoradiotherapy (NACRT) followed by total mesorectal excision (TME) remains uncertain 1 . This study aimed to evaluate the oncological outcomes of LARC patients who received ACT following NACRT and TME.

Material/Methods: