Abstract Book

S1023

ESTRO 37

mean and maximum organ dose and DVH values for CTV, rectum and bladder for cervical treatments and CTV and lung for lung treatment. Results For cervical treatments ACE calculation without heterogeneity density corrections in standard mode shows a difference of -0,4±3,0%, 0,7±0,2%, 0,7±0,2% for D 90 , D 100 and D 95 for CTV respectively. The difference for rectum dose is 0,3±0,5%, 0,8±0,3% and -0,7±1,1% for D2cc, maximum and average dose respectively and the difference for bladder dose is 0,7±0,5%, 0,8±0,6% and - 0,5±0,6% for D2cc, maximum and average dose respectively. The same plans with heterogeneity corrections in standard mode shows a difference of -5,9±2,8%, - 3,6±0,4%, -3,8±0,6% for D 90 , D 100 and D 95 for CTV respectively. The difference for rectum dose is - 4,2±1,2%, -2,4±1,5% and -5,6±1,6% for D2cc, maximum and average dose respectively and the difference for bladder dose is -2,9±0,9%, -2,9±1,2% and -4,5±0,8% for D2cc, maximum and average dose respectively . For lung treatments ACE calculation in standard mode without heterogeneity density corrections shows a difference of 1,0±2,5%, 0,4±0,4%, 0,5±0,5%, -28,0±25,4% and -7,2±8,3% for CTV D 100 , D 90 , D 95, maximum and average dose respectively. The difference for lung dose is 0,1±0.1% and 1,1±0,6% for D2cc and maximum dose respectively. For the same treatments ACE calculation in standard mode with heterogeneity density corrections shows a difference of 0,7±3,4%, 0,2±0,8%, 0,2±1,1%, -28,0±25,4% and -7,4±8,3% for CTV D 100 , D 90 , D 95, maximum and average dose respectively. The difference for lung dose is -1,1±0.6% and 0,2±1,1% for D2cc and maximum dose respectively. Conclusion For the cervical treatments the ACE algorithm shows a good agreement between TG43 and the calculation without density correction, both for standard and high resolution mode. Taking into account the density of the applicator a slight overestimation of the dose for the TG43 algorithm both for CTV and OARs is seen. In the case of lung treatments the ACE algorithm shows the same differences respect to TG43 both for calculation in water and in the case of density correction. EP-1889 Benchmarking spot fluence profiles for four proton treatment planning systems against measured data J. Alshaikhi 1 , D. D’Souza 1 , C. G. Ainsley 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 Accurate beam modelling is an essential function of a treatment planning system (TPS) to ensure that plans can be calculated that are deliverable within clinically acceptable tolerances. The purpose of this work is to evaluate computed in-air spot fluence lateral profiles and spot ellipticity of four commercially available proton TPSs, benchmarked against measured data. The four TPSs (Eclipse TM , XiO ® , Pinnacle 3 , RayStation ® ) were commissioned using pencil beam scanning data from the University of Pennsylvania (UPenn) facility. Material and Methods Individual “in-air” spot fluence profiles for 27 nominal energies, ranging from 100 to 226.7 MeV, were calculated at the surface of a water phantom in each TPS with a calculation grid size of 1mm. Profiles at five source-to- surface distances relative to isocentre (+20, +10, 0, -10, -

17cm) were calculated for all energies. These were all benchmarked against measured data from UPenn, comparing full-width-at-half-maximum (FWHM) and widths at 95, 20, and 10% maximum in both x and y directions. Lateral penumbrae (80-20%) were also compared. Gamma-index analysis with pass criteria of 1, 1.5, 2, and 3%/1, 1.5, 2, and 3 mm were used. Spot ellipticity agreement was calculated using equation 1: Results Mean percentage of spot fluence lateral profiles at the isocentre with >95% pass rate for 1 mm/1% criteria in x – direction were 97.6% (SD 2) for XiO ® , 93.1% (SD 10.4) for Eclipse TM , 93.5% (SD 3.1) for Pinnacle 3 , and 77.6% (SD 12.4) for RayStation ® . For Y – direction at the isocentre were 94.2% (SD 4.8) for XiO ® , 83.9% (SD 7.5) for Eclipse TM , 84.5% (SD 9) for Pinnacle 3 , and 94.5% (SD 6.2) for 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

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