ESTRO 2023 - Abstract Book

S1130

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ESTRO 2023

C-arm linac (36Gy/20#). This approach utilises the superior MR visualisation to ensure accurate daily adaptation to the primary target by accounting for large interfraction motion. The aim of this work is to present the initial experience for the MRL-delivered fractions for the first 5 patients. Materials and Methods 5 patients treated for rectal cancer under an institutional study (PERMIT trial; NCT03727698) were selected for analysis. Targets volumes were defined as per RCR guidelines [1]. MRL reference plans using an 11-field step and shoot IMRT delivery were created on pre-treatment reference images to meet OAR dose constraints. RCR OAR dose constraints were split between the MRL and C-arm phases in the ratio of 16.5:36 (Monaco 5.40.01). Online adaptation of the reference plan followed an Adapt to Shape (ATS) workflow, with the option of subsequent Adapt to Position (ATP of ATS) if the primary target had moved more than 5mm. We determined the overall treatment session times and the % of dose constraints that were achieved for all online adaptive SIB plans. Results All MRL adaptive ATS SIB plans were successfully optimised and approved by a clinician for treatment. 24/25 plans met all mandatory dose constraints for PTV coverage. The high-dose PTV (PTV1650) V95%>95% dose constraint was not met in one ATS plan (94.8%) due to preferentially sparing adjacent small bowel. The mean PTV1650 V95% across all online ATS plans was 98.4%, with no individual patient mean V95% less than 96.4%. The mean low-dose PTV V95% across all online plans was 99.2%, with all individual patient mean V95% above 98.8%. 99.5% of all online OAR mandatory constraints were met for ATS plans. The median time on couch was 41.3min with a range of individual patient mean time: 35.4–54.4min, reflecting variability in plan complexity in terms of volume delineation and optimisation. The median (range) time between online contouring and treatment delivery was 23.5min (20.6–38.3min). The median (range) treatment delivery time for all online plans was 8min (7–8.6min). Figure 1 illustrates an example of the interfraction motion observed for these patients and shows potential undercoverage of the primary GTV for non-adaptive treatment. Conclusion MRgART for Rectum Cancer on the MRL is feasible and can account for large interfraction changes observed for this indication. Toxicity and pathological outcomes will be presented later. To leverage the full potential of MRgART for rectal cancer, dose escalation strategies or PTV margin reduction could be considered for ATS workflows. Two ATP of ATS plans were required. All mandatory dose constraints were met in both ATP of ATS plans.

PO-1401 Elderly Anal Cancer Patients Treated With Intensity-Modulated Radiation Therapy: A Multicenter Study

L. Caravatta 1 , G. Mantello 2 , F. Valvo 3 , P. Franco 4 , L. Gasparini 5 , F.C. Di Guglielmo 6 , N. Slim 7 , S. Manfrida 8 , M.A. Gambacorta 8 , F. De Felice 9 , M.A. Gerardi 10 , S. Vagge 11 , M. Krengli 12 , E. Palazzari 13 , M.F. Osti 14 , A. Gonnelli 15 , G. Catalano 16 , P. Pittoni 17 , G.B. Ivaldi 18 , A. Galardi 19 , M. Lupattelli 20 , M.E. Rosetto 21 , R.M. Niespolo 22 , A. Guido 23 , O. Durante 24 , G. Macchia 25 , F. Munoz 26 , B.E. Khouzai 27 , M.R. Lucido 28 , R.M. D’Angelillo 29 , A. Porreca 30 , M. Di Nicola 30 , D. Genovesi 31 1 "SS Annunziata" Hospital, "G. d'Annunzio" University, Radiation Oncology Unit, Chieti, Italy; 2 Azienda Ospedaliero Universitaria Ospedali Riuniti, Department of Oncology and Radiotherapy, Ancona, Italy; 3 CNAO National Center for Oncological Hadrontherapy, Radiotherapy Unit, Clinical Department, Pavia, Italy; 4 University of Eastern Piedmont, Department of Oncology, Radiation Oncology, Novara, Italy; 5 "SS Annunziata" Hospital, "G. d'Annunzio" University, Radiation Oncology Unit, Chieti, Italy; 6 "SS Annunziata" Hospital, "G. d'Annunzio" University, Radiation Oncology Unit, Chieti, Italy; 7 IRCCS San Raffaele Scientific Institute, Department of Radiotherapy, Milano, Italy; 8 "A. Gemelli" IRCCS, Fondazione Policlinico Universitario, UOC di Radioterapia Oncologica, Dipartimento di Diagnostica per Immagini,