ESTRO 2025 - Abstract Book

S1207

Clinical – Lower GI

ESTRO 2025

2371

Digital Poster Total neo-adjuvant treatment of locally advanced rectal cancer using short-course MRI-guided radiotherapy with simultaneous integrated boost. Koen Kortbeek 1 , Amy De Haar-Holleman 1 , Sven Van Laer 2 , Jacques Bodenst Bezuidenhout 2 , Jens Van Loon 2 , Tim Everaert 2 , Anne-Sophie Bom 2 , Ellen Van Eetvelde 3 , Alexandra Sermeus 4 , Bente van der steen 2 , Kurt Barbé 5 , Thierry Gevaert 2 , Mark De Ridder 2 1 Medical Oncology, Universitair Ziekenhuis Brussel, Brussel, Belgium. 2 Radiotherapy, Universitair Ziekenhuis Brussel, Brussel, Belgium. 3 Surgery, Universitair Ziekenhuis Brussel, Brussel, Belgium. 4 Gastro-enterology, Universitair Ziekenhuis Brussel, Brussel, Belgium. 5 Mathematics, Universitair Ziekenhuis Brussel, Brussel, Belgium Purpose/Objective: The RAPIDO trial demonstrated that short-course radiotherapy (5x5Gy) followed by oxaliplatin/5FU-based chemotherapy and surgery decreased disease related treatment failure compared to long-course chemoradiotherapy (1). However, at 5-year follow-up an increase in locoregional recurrence was observed (2). One of the strategies to improve local control is to increase the dose of on the gross tumor volume (GTV). We present a retrospective analysis of a patient cohort treated with MRI-guided intensified radiotherapy. Material/Methods: Between October 2021 and October 2023, 26 patients with cT3-4 N0-2 rectal cancer underwent daily adaptive radiotherapy using a MRIdian system (ViewRay, Inc.). Radiotherapy consisted of 5 x 5 Gy on the mesorectum with a simultaneous integrated boost till 30 Gy on the GTV, which was gated during treatment. This was followed by chemotherapy consisting of CAPOX (6 cycles) or FOLFOX (9 cycles). In case of complete clinical response (cCR) following neo-adjuvant treatment, a watch-and-wait approach was considered. The primary outcomes are response, local and distant recurrence. This study was sponsored by UZ Brussel and approved its ethics committee. Results: 25 patients were included in the analysis with a median follow-up of 26,5 months. Figure 1 provides consort diagram. One patient was lost to follow-up. 12 patients (48%) had cCR of which 9 chose to enter a watch-and-wait protocol. Three of these patients had suspected local regrowth and had salvage surgery, with one pathologically confirmed complete response. 15 patients had surgery directly following neoadjuvant treatment. Pathological complete response was observed in 25%. Dworak tumor regression grade distribution: 1 (n=4; 25%); 2 (n=3; 19%); 3 (n=5; 31%); 4 (n=4; 25%). Three patients had distant relapse (12%), median time to progression 11 months.