ESTRO 37 Abstract book

S1025

ESTRO 37

RayStation ® . The mean percentage of spot fluence lateral profiles at 10 cm upstream and 10 cm downstream of the isocentre with >95% pass rate for 1, 1.5, 2 and 3%/1, 1.5, 2 and 3 mm criteria and field width differences between computed and measured are shown in figure 1. Spot elliipticity agreement is shown in figure 2

To benchmark these, spots sizes at depth z = R 100 also calculated for measured in-air data ( σ 0

were

) using

equation 1:

Results Differences between computed spot sizes at R 100

in all

at R 100

, based on measured

TPS and modelled values of σ z

σ 0 data were: < 0.3 mm in x-direction and < ±0.2 mm in y-direction for XiO; < 0.4 mm in x-direction and < 0.3 mm in y-direction for Eclipse; < 0.8 mm in x-direction and < 0.7 mm in y-direction for RayStation; < 1.4 mm in x- direction and < 1.1 mm in y-direction for Pinnacle. Benchmarked variation in TPS computed spot sizes at R 100 for all energies, spot ellipicity agreement, and variation as a function of SSD for 170 MeV are shown in Figure 1.

Conclusion Characteristics of computed in-air spot fluence lateral profiles in both x and y lateral directions, and at five distances along the central axis were compared to measured data for four commercially available TPSs. All were within clinically acceptable tolerances, with XiO showing the closest agreement. Differences observed were attributed to TPS specific beam modelling. Further investigation will assess the cumulative impact of these discrepancies on verified clinical treatment plans. EP-1890 Comparison of multiple Coulomb scattering models for four proton PBS treatment planning systems J. Alshaikhi 1 , D. D’Souza 1 , C. G. Ainsley 2 , I. Rosenberg 2 , G. Royle 3 , R. A. Amos 3 1 UCLH, Radiotherapy Physics, LONDON, United Kingdom 2 UPenn, Roberts Proton Therapy Center- University of Pennsylvania, Philadelphia, USA 3 UCL, Department of Medical Physics and Biomedical Engineering- University College London, London, United Kingdom Purpose or Objective As proton pencil beams propagate through media, multiple Coulomb scattering (MCS) causes lateral spread as a function of depth, thus increasing the spot size at the depth of the Bragg peak ( R 100 ). The purpose of this work is to evaluate computed spot profiles and ellipticity at R 100 for four commercially available proton treatment planning systems (TPS), benchmarked against measured in-air data and a known model. The four TPS (Eclipse TM , XiO ® , Pinnacle 3 , RayStation ® ) were commissioned using pencil beam scanning data from the University of Pennsylvania (UPenn) facility. Material and Methods Individual pencil beam lateral profiles were calculated at depth z = R 100 in each TPS for 27 nominal energies, ranging from 100 to 226.7 MeV. Profiles at five source-to- surface distances (SSD) relative to isocentre (+20, +10, 0, -10, -17cm) were calculated in both x and y directions for all energies. A calculation grid size of 1mm was used.

Conclusion Characteristics of the computed depth dependent spot size σ(z) at R 100 in both lateral directions, and at five different SSD, were compared to modelled values of σ z at R100, based on measured in-air data, for four commercially available TPS. All were within clinically acceptable tolerances, with XiO and Eclipse showing the closest agreement. Differences observed were attributed to TPS specific beam modelling. Further investigation will assess the cumulative impact of these discrepancies on verified clinical treatment plans. EP-1891 Implementation of the optimization algorithm Photon Optimizer in VMAT for prostate cancer treatment I. Birba 1 , M. Robilliard 1 1 Institut Curie, Service de Physique Medicale, paris, France Purpose or Objective The aim of the current study is to evaluate the new optimization algorithm Photon Optimizer version 13 (Varian) (PO13). The purpose is to obtain a treatment planning protocol for prostate cancer treatments, reproducible which can be delivered by the machine and evaluate the dosimetric gain for patients (better organ at risk protection). Material and Methods In our department, the medical prescription dose is 75Gy (2.5Gy/fraction) on the prostate (PTV-T) and 46Gy (2Gy/fraction) on the right and left iliac lymph nodes (PTV-N). Treatment plannings are made of 2 plans: - 1 st plan in integrated boost delivering : 57.5Gy on the PTV-T and 46Gy on the PTV-N in 23 fractions, - 2 nd plan: 17.5Gy on the PTV-T in 7 fractions. Plans are optimized with PO13 and are calculated with