ESTRO 37 Abstract book

S957

ESTRO 37

Material and Methods An in-house developed pencil beam algorithm based on a look-up table approach was implemented in the open- source treatment planning system matRad. Monte Carlo (MC) simulations using the Geant4/GATE 8.0 toolkit were performed to generate dose distributions maps in a 40x40x40 cm 3 water phantom within magnetic field regions up to 3T. Single monoenergetic parallel beams and a previously validated model for a proton research beam line were employed to generate calibration data for protons within the clinical energy range (62 - 252 MeV). In-depth longitudinal profiles, as well as parameters accounting for lateral deflection of the beam, beam broadening and transverse profiles anisotropy, were interpolated for different materials using a water-equivalent depth scaling. The performance of the algorithm was evaluated in homogeneous and heterogeneous phantoms filled with water, adipose and bone tissue. Dose distributions for central beams as well as volumetric targets using single-field pencil beam scanning proton plans were compared with MC simulations to assess the influence of magnetic fields in the analyzed treatment plans. Results For central beams, a close to perfect agreement was observed for calculations in water in magnetic fields of 0.5, 1.5T and 3.0T. IDD functions showed differences between the PBA and MC of less than 1% before the Bragg-Peak, and deviations of 2-8% in the distal energy falloff region. A good agreement was achieved using slab- like and lateral heterogeneous phantoms with maximal fluctuations in range of 0.7% and mean dose difference lower than 3%. Finally, treatment plans of comparable dosimetric quality to the implemented generic proton beam algorithm in matRad were obtained for box phantoms, see Figure 1.

Conclusion The proposed pencil beam algorithm can accurately describe dose distributions effects induced by external magnetic fields. The successful integration into a treatment planning system make possible next benchmarking stages using in depth Monte Carlo simulations with complex phantoms as well pave the way for experimental measurements and validation. EP-1782 Pre-treatment Patent Specific QA on the Varian Halcyon Linear Accelerator T. Jarema 1 , T. Aland 2 , A. Walsh 2 , R. Murry 1 1 Radiation Oncology Centres, ROC, Toowoomba, Australia 2 Radiation Oncology Centres, ROC, Brisbane, Australia Purpose or Objective The purpose of this study was to compare the results of several different pre-treatment patient specific QA devices as used on the Varian Halcyon Linear Accelerator – including Varian portal dosimetry, point dose measurement in a homogenous slab phantom, and PTW Octavius4D with PTW 1500 detector. Material and Methods Treatment plans for prostate, prostate and nodes, prostate bed, rectum, and head & neck were planned using varying numbers of arcs ranging from 2-6 for VMAT plans, and also with 7 and 9 field IMRT. Verification plans were then created within the Eclipse treatment planning system on phantoms ready for QA. The plans were delivered to the various phantoms in ‘QA mode’ and adjusted for any change in linac output on the day of delivery. All phantoms were imaged by the Halcyon prior to delivery to ensure correct positioning. For the array devices and portal dosimetry, delivered results were then compared with exported results from the planning system using a 2%/2mm and 3%/3mm gamma evaluation (3D for PTW Octavius4D and 2D for Varian portal dosimetry) with a 10% low dose threshold referenced to the global dose maximum. For the point dose measurements (using a PTW 3D pin point chamber), delivered results were compared using local dose difference expressed as a percentage. Total plan dose was assessed, rather than individual fields doses. Results Point dose measurements were in agreement with the treatment planning system to within 3%. Varian Portal dosimetry results were all greater than 90% and 97% for 2%/2mm and 3%/3mm respectively. PTW Octavius4D results were greater than 74% and 92% for 2%/2mm and 3%/3mm with a trend of decreasing gamma result as the number of VMAT arcs increased and also when the gantry speed exceeded the limits of the devices rotation speed. SNC ArcCheck is currently under investigation. Conclusion Point dose measurements, Varian portal dosimetry, and PTW Octavius4D were all used successfully to conduct pre-treatment patient specific QA on a variety of plans delivered using a Varian Halcyon linear accelerator. EP-1783 TPS validation for the Varian Halcyon Linear Accelerator T. Jarema 1 , A. Walsh 2 , T. Aland 2 , R. Murry 1 1 Radiation Oncology Centres, ROC, Toowoomba, Australia 2 Radiation Oncology Centres, ROC, Brisbane, Australia Purpose or Objective The purpose of this study was to validate the preconfigured Varian Eclipse AAA data for the Varian Halcyon Linear Accelerator, as the pre-configured data cannot be edited or manipulated in any way. PDDs, profiles and output factors were used for the initial validation work.

Fig. 1 Spread out Bragg peak (SOPB) (a) and two dimensional dose distribution (b) for a cubic target in a box water phantom. The SOBP for a generic proton beam implemented in matRad is plotted for evaluation of the algorithm performance.