ESTRO 2025 - Abstract Book

S1171

Clinical – Lower GI

ESTRO 2025

1 Radiotherapy, Netherlands Cancer Institute, Amsterdam, Netherlands. 2 Radiotherapy, Leiden University Medical Center, Leiden, Netherlands. 3 Leeds Institute of Medical Research at St James's, University of Leeds, Leeds, United Kingdom. 4 Cancer Research UK Clinical Trials Unit, University of Birmingham, Birmingham, United Kingdom. 5 Surgery, Odense University Hospital, Odense, Denmark. 6 Surgery, Sahlgrenska University Hospital/Östra, Gothenburg, Sweden. 7 Institute of Clinical Sciences, University of Gothenburg, Gothenburg, Sweden. 8 Clinical Research, University of Southern Denmark, Odense, Denmark. 9 Surgery, Netherlands Cancer Institute, Amsterdam, Netherlands. 10 Surgery, Maastricht University Medical Centre, Maastricht, Netherlands. 11 Surgery, Catharina Hospital Eindhoven, Eindhoven, Netherlands. 12 Experimental Clinical Oncology, Aarhus University, Aarhus, Denmark. 13 Clinical Medicine, Aarhus University, Aarhus, Denmark. 14 Cancer Centre, Queen Elizabeth Hospital, Birmingham, United Kingdom. 15 Radiation Oncology, University Hospital, Leuven, Belgium. 16 Surgery, Leiden University Medical Center, Leiden, Netherlands. 17 Department of Surgery, Queen Elizabeth Hospital, Birmingham, United Kingdom. 18 Oncology, Södersjukhuset, Stockholm, Sweden. 19 Pelvic Cancer, GI oncology and colorectal surgery unit, Karolinska University Hospital, Stockholm, Sweden. 20 Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden. 21 Clinical Oncology, St James University Hospital, Leeds, United Kingdom. 22 Pathology and Data Analytics, Leeds Insititute of Medical Reserach, Leeds, United Kingdom. 23 Abdominal Surgery, University Hospital Leuven, Leuven, Belgium. 24 Surgery, Radboud University Medical Center, Nijmegen, Netherlands. 25 Institute of Cancer & Genomic Sciences, University of Birmingham, Birmingham, United Kingdom Purpose/Objective: Total mesorectal excision (TME) is standard of care for early and intermediate stage rectal cancer. The international STAR-TREC phase II/III trial was designed to compare the effectiveness of two radiotherapy (RT) schedules in achieving organ preservation (OP). Following guidance from the trial steering group, we report 12 month outcomes for the phase II/III patients who where randomised to OP prior to August-2023. Material/Methods: Eligibility included adenocarcinoma of the rectum staged as mrT1-T3b ≤40mm in diameter N0, M0. In phase II, patients were randomised (1:1:1) between TME surgery, short course radiotherapy (SCRT) using 25Gy in five fractions, and chemoradiotherapy (CRT) using 50Gy in 25 fractions with capecitabine (825 mg/m 2 bd on days of radiotherapy). For phase III, patients opted for TME surgery alone or when an OP approach was preferred, randomised (1:1) between SCRT and CRT. A mesorectum-only CTV was used. The two stage response assessment pathway and use of selective local excision for incomplete response is summarised in the figure. Results: Between Jun-2017 and Apr-2024, 503 eligible patients were recruited across 37 international sites. 344 patients (172 SCRT and 172 CRT) are included in this analysis. Median age is 67 years (range:37-89) with 72% male. MR-based T stage was 1% cTX , 8% cT1, 73% cT2, 13% cT3a and 5% cT3b. Of the 172 patients randomised to SCRT, 167 (97%) underwent their randomised treatment, and of the 172 randomised to CRT, 161 (94%) underwent randomised treatment. Grade 3 toxicity up to 4 weeks after completion of RT was observed in 13 (7.7%) patients undergoing SCRT and 10 (6.2%) patients undergoing CRT. No grade 4-5 events occured. At 12 months, organ preservation was achieved with a watch and wait (W&W) strategy in 47 (27.3%) SCRT and 98 (57.0%) CRT patients, and with the addition of local excision (LE)in 58 (33.7%) SCRT and 36 (20.9%) CRT patients. TME free survival estimates at 12 months are 61.5% for SCRT and 79.8% for CRT (Hazard ratio: 1.82; 95% CI 1.26-2.62, Table).