ESTRO 2020 Abstract book

S764 ESTRO 2020

Material and Methods The modular phantom of the OCTAVIUS 4D system (PTW) may be used in two, independent configurations: as a standard phantom with 32 cm diameter or SRS phantom with 17 cm diameter. The Octavius 4D phantom in the SRS configuration is dedicated to the Octavius SRS 1000 (PTW) matrix. Both phantom configurations are made of 1.05 g/cm3 polystyrene. The stereotactic VMAT plans (6X photon beam) for five patients (12 arcs) were prepared. Verification measurements were performed using Versa HD accelerator (Elekta). For each arc, the Octavius SRS 1000 (PTW) matrix measurement was performed twice: in the standard and dedicated phantom for the 1000 SRS matrix. The measured dose distributions for all arcs were compared in the Verisoft v.7.1 (PTW) software with the TPS Monaco v.5.2 (Elekta) calculations. The Verisoft program has The volume analysis option.for gamma analysis parameters: 3mm / 3% and 2mm / 2% (max. and local dose), with 5% threshold was used. Results The results of gamma analysis expressed in gamma passing rate (GPR) for measurements performed in the OCTAVIUS phantom in the standard and SRS configuration for parameters 3mm / 3% (max. and local dose) are not lower than 98.1%. For 2mm / 2% gamma analysis parameters (max. and local dose), GPR results do not reach value lower than 93.1% (Tab. 1). Maximum GPR differences for individual distributions do not exceed 0.8 percentage point (pp) for 3mm / 3% (max. and local dose) and 4.3 pp for 2mm / 2% (max. and local lose). Conclusion The results of gamma analysis for 3mm / 3% (both max. and local dose) parameters do not show a significant difference between the measurement performed in standard and dedicated to measuring stereotactic plans phantoms. In the absence of a phantom dedicated to measurements with the Octavius SRS 1000 matrix, measurements may be performed in a standard configuration or in older (not modular) Octavius phantom, which shape, dimensions, and type of material identical with the modular phantom in standard configuration PO‐1352 Prediction of electron beam parameters of a Monte Carlo model using machine learning A. Wagner 1 , K. Brou Boni 1 , E. Rault 1 , F. Crop 1 , T. Lacornerie 1 , D. Van Gestel 2 , N. Reynaert 3 1 Centre Oscar Lambret, Department of Medical Physics, Lille, France ; 2 Institut Jules Bordet, Department of Radiotherapy, Brussels, Belgium ; 3 Institut Jules Bordet, Department of Medical Physics, Brussels, Belgium Purpose or Objective The objective of this work is to propose a method to predict electron beam characteristics of a linear accelerator Monte Carlo model using machine learning, thus speeding up and automating the modelling process. The method is developed and tested based on a MLC- equipped stereotactic radiotherapy accelerator model. Material and Methods The M6 Cyberknife (Accuray, Sunnyvale CA) equipped with the Incise2 MLC is first modelled in BEAMnrc/DOSXYZnrc using water phantom measurements. The model is then integrated in our home-developed Monte Carlo (MC) 3D dose verification platform Moderato, and validated on patient plans. The general principle of the machine learning (ML) algorithm is described in Figure 1. To generate the data necessary to train the algorithm, a

Results The skin doses obtained for all patients are shown in Fig 2, these doses are lower than those estimated by the TPS (Eclipse, Varian Inc.) calculated with the Axxent specific parameters of TG-43. The doses of the film pieces placed on the balloon surface match the expected, i.e. 20 Gy, the prescribed dose.

Conclusion The skin doses are sufficiently low, which explains why in the 480 patients treated, less than 1% of the cases presented early toxicity of acute grade 3 dermatitis and no case of a higher grade. The in vivo dosimetry procedure initiated will be performed with all patients measuring doses in other areas of interest from now on. PO‐1351 One fits all? The influence of phantom selection on pre‐treatment verification results A. Borda-Ziemacka 1 , M. Gabor 1 , A. Rygielska 1 , D. Pruska- Pich 1 , A. Walewska 1 1 Military Institute od Medicine, Radiotherapy Department- Laboratory of Medical Physics, Warsaw, Poland Purpose or Objective Octavius SRS (PTW) phantom is specially designed and dedicated to using with Octavius SRS 1000 matrix (PTW). Some hospitals were equipped with a new type of matrix for pre-treatment verification of stereotactic (SRS) plans but did not purchase a new phantom. The study aimed to estimate the difference between the results of gamma analysis for stereotactic plans measured with the matrix in the case of using a different phantom than the phantom dedicated to this matrix. As part of the study, the impact of phantom selection on the results of gamma analysis of SRS plans measured with the Octavius SRS 1000 matrix was assessed using the OCTAVIUS phantom in the standard and SRS version.