ESTRO 2025 - Abstract Book

S2842

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

2908

Digital Poster Hypofractionated and dose-escalated radiotherapy for rectal cancer: a feasibility planning study of proton and photon therapy Erik Almhagen 1,2 , Ali Alkhiat 1 , Bruno Sorcini 1 , Freja Alpsten 1,2 , Alexander Valdman 3,2 1 Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, Stockholm, Sweden. 2 Department of Oncology-Pathology, Karolinska Institute, Stockholm, Sweden. 3 Department of Radiation Oncology, Karolinska University Hospital, Stockholm, Sweden Purpose/Objective: The ongoing PRORECT (NCT04525989) trial compares hypofractionated neoadjuvant proton and photon therapy for rectal cancer, with a prescribed dose of 5x5 Gy RBE. However, studies (see Hearn et al. , 2021) have indicated increased pathological complete response rates following higher radiotherapy doses, with acceptable toxicity levels, suggesting dose escalation beyond the current PRORECT fractionation (Pedone et al ., 2023). The purpose of this planning study is to assess the feasibility of dose escalation for proton and photon therapy, in preparation for a dose-escalated PRORECT II trial. Material/Methods: Ten patients already included in the PRORECT study were selected for this planning study. For each patient, four plans were created: two VMAT plans and two proton plans. The first plan in each modality was created using the standard PRORECT fractionation, while the second was created with an escalated prescription of 5x6 Gy RBE dose to the primary tumor, and 5x7 Gy RBE to pathological lymph nodes. All nominal dose distributions were subsequently converted to EQD2-dose distributions with α/β = 5 (Suwinski et al ., 2007) for GTV-volumes, and α/β = 3 for all other tissue. DVH-metrics for pertinent OARs were scored and compared to local recommended tolerance levels. The integral dose was also calculated as the sum of the product of the dose and volume of all voxels with non-zero doses. Results: For the bladder mean dose, the median (range) EQD2 doses were 21.1 (11.9-24.0), 14.5 (5.3-19.5), 23.5 (22.1-31.0) and 17.7 (7.3-22.24) Gy for photon standard, proton standard, photon escalated and proton escalated, respectively, with all but one dose below tolerance level of 30.2 Gy. For the bowel bag D 5cc , median doses were 41.4 (40.6-41.8), 41.2 (40.3-41.7), 43.7 (41.8-45.6) and 45.2 (35.9-54.76) Gy analogously, with a tolerance dose of 52 Gy. For the bowel bag, V 23.8Gy was 159.5 (38.2-283.3), 136.2 (12.1-206.1), 206.0 (153.7-300.2) and 142.5 (90.9-207.4) cm 3 analogously, all below the tolerance level of 450 cm 3 . The median integral photon dose/proton dose ratio was 1.9 and 2.0 for escalated and standard plans, respectively. Conclusion: The feasibility of the hypofractionated dose-escalated plans is evident with most of the plans yielding doses below tolerance levels. The integral dose for photon plans was twice as high compared to proton plans. Prospective evaluation in dose-escalated PRORECT II trial is justified. References: Hearn, N. et al. “Neoadjuvant Radiotherapy Dose Escalation in Locally Advanced Rectal Cancer: a Systematic Review and Meta-analysis of Modern Treatment Approaches and Outcomes”. Clinical Oncology 2021 Rawla P. et al . “Epidemiology of colorectal cancer: incidence, mortality, survival, and risk factors”. Gastroenterology Rev 2019 Suwinski, R. et al . “Moderately low alpha/beta ratio for rectal cancer may best explain the outcome of three fractionation schedules of preoperative radiotherapy”. Int. J. Radiation Oncology Biol. Phys. 2007 Keywords: Proton therapy, Dose escalation, Planning study