P-374 Total neoadjuvant treatment of rectal cancer by MRI-guided radiotherapy

Short-course radiotherapy (5 x 5 Gy) followed by chemotherapy before total mesorectal excision (TME) decreased the probability of disease-related treatment failure and became new standard care in high-risk locally advanced rectal cancer (Bahadoer et al, Lancet Oncol. 2021). A-five-year follow-up RAPIDO trial recently confirmed reduction distant metastases, but revealed an increase locoregioal recurrence compared to long-course chemoradiotherapy (Dijkstra Ann Surg. 2023). The aim this study was evaluate safety dose-escalation till 6 Gy on gross tumor volume (GTV) using MRI-guided radiotherapy. 33 patients with a were treated between July 2021 March 2023 MRIdian (Viewray, Denver, Colorado). Patients received fractions classic clinical target volume, simultaneous integrated boost GTV. Patient daily adaptive active tracking cine MRI-mode. Radiotherapy 9 cycles FOLFOX or CAPOX. underwent response evaluation rectoscopy MRI, mid-chemotherapy 2 weeks after its completion. This analysis is part that approval from ethical committee UZ Brussel (EC1010135). Of patients, 16 displayed grade I toxicity (48%) 17 showed no (52%) at end Eight excluded current analysis: because comorbidities, 3 did not complete yet had missing data (e.g. waiting for definitive pathology report). 25 remaining 11 (44%) remission offered watchful approach. 14 TME. 1 patient Dworak regression score 0 (4%), (8%), 4 (16%) (24%). neoadjuvant resulted high pathological 68%, within pilot study. may be emerging strategy improve local control organ preservation future.