S1901
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Maolalaí 5,1 , Suneil Jain 3 , Paul Kelly 4 , Brian O'Neill 1 , Orla Houlihan 6 1 Radiation Oncology, St. Luke’s Radiation Oncology Network, Dublin, Ireland. 2 Radiotherapy Physics, Belfast Health and Social Care Trust (HSCT), Northern Ireland Cancer Centre, Belfast, United Kingdom. 3 Patrick G.Johnston Cancer Centre for Research, Queen’s University Belfast, Belfast, United Kingdom. 4 Radiation Oncology, Bon Secours Radiotherapy Centre in partnership with UPMC Hillman Cancer Centre, Cork, Ireland. 5 Radiotherapy, Irish Research Radiation Oncology Group, Dublin, Ireland. 6 Clinical Oncology, Beacon Centre, Musgrove Park Hospital, Somerset NHS Foundation Trust, Somerset, United Kingdom Purpose/Objective: INSPIRE is a multi-centre, single arm phase II clinical trial evaluating next generation prostate SABR. The primary end point is to reduce ≥ Grade 2 late genitourinary (GU) side effects compared to the PACE B clinical trial (1). This treatment planning study aimed to establish target coverage, and organs of interest (OOI) dose objectives for INSPIRE. Urethral-sparing has shown promise in reducing GU side effects, particularly in the hypofractionated setting, however there remains no consensus on optimal dose guidance or sparing techniques (2). Additionally, erectile dysfunction is an adverse effect of prostate SABR (3). The INSPIRE trial aims to diminish this side effect by sparing the contralateral neurovascular bundle (NVB), in patients motivated to preserve sexual function with a unilateral, radiologically disease-free prostate. Material/Methods: Patients will receive a boost (40-50Gy/5#) to the dominant intra prostatic lesion (DIL), reduced dose to the prostate CTV (35Gy/5#) and prostate PTV (33.25Gy/5#). Urethra and bladder trigone, along with the NVB (when applicable) will be spared. A retrospective planningstudyof 13 patients previously treated at SLRON and Belfast HSCT was completed using Eclipse and Monaco. All patients had rectal spacers and identifiable DILs. DILs and additional OOIs (urethra, bladder trigone and NVB) were contoured. OOI dose criteria were based on the PACE B clinical trial (1). Priorities for target coverage and OOI were outlined to guide the planning process. Multi- disciplinary and multi-centre reviews of the plans took place to develop the INSPIRE trial planning protocol and determine appropriate target coverage and DVCs. Results: A contouring atlas was developed to support consistent delineation of trial OOIs. PTV, CTV Prostate and DIL coverage objectives were achieved for all cases by utilising two urethral sparing strategies, shown in Fig1. Strategy 1 (Primary Urethral Criteria): Urethra V35Gy<0.035cc, PRV V36Gy<0.035cc
Results: For AAA, both profiles showed better agreement when using the ETSS (1,0) mm configuration, as relative slope differences decreased from ~26-42% with (0,0) mm to ~0-9%.For AXB, the best overall results were obtained with the (1.5,1) configuration, with relative slope differences under 7% and distance parameters inferior to 0.6 mm, but the configuration (1.5,0) was
also good, particularly for inplane, where the configurations with X=1 mm were disastrous.
Regarding the OF (Graph 2), the 2x2 cm2 exhibited good agreement between measurement and calculation, but the remaining fields’ OF AAA differences from measurement were below 2.9%, while AXB showed deviations below 1.8%. Conclusion: Adjusting the ETSS significantly enhances the beam model and so it is advised to optimize this parameter. Also, the impact of ETSS on OF should be considered. References: Passal, V., Barreau, M., Tiplica, T. and Dufreneix, S. (2021), Optimizing the effective spot size and the dosimetric leaf gap of the AcurosXB algorithm for VMAT treatment planning. J Appl Clin Med Phys, 22: 154-161. https://doi.org/10.1002/acm2.13256Fogliata A, Stravato A, Pelizzoli M, et al. Effective spot size parameters for Acuros dose calculation algorithm using enhanced leaf modelling: Estimation based on small rectangular MLC fields. J Appl Clin Med Phys. 2025; 26:e70315.Younes T, Chatrie F, Zinutti M, Simon L, Fares G, Vieillevigne L. Optimization of the Eclipse TPS beam configuration parameters for small field dosimetry using Monte Carlo simulations and experimental measurements. Phys Med. 2023 Oct;114:103141. Epub 2023 Oct 30 Keywords: effective target spot size,penumbra,beam modelling
Digital Poster 2799
Establishing Dose Volume Guidance for a next generation prostate SABR trial-INSPIRE-Insights from a pan-island multi-centre treatment planning study Claire Fitzpatrick 1 , Chris Hayes 2 , Sandra Harkin 1 , James Waldron 1 , Conor K McGarry 2,3 , Erica Bennett 4 , Róisín O
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