ESTRO 37 Abstract book

S947

ESTRO 37

Nine spherical targets (diameter 1.0 cm) are located on the central frontal plane of the plastic water phantom (30 cm x 30 cm x 10 cm). Individual spheres are set on the longitudinal or the lateral axis with 4 cm offsets. The central target ball is at the isocentre. Plan is prepared with FFF 6 MV beam to prescription dose 10 Gy (PTV(V98%)) for TrueBeam accelerator equipped with 120 leaf MLC. Two other plans are calculated for the sphere size 0.5 cm and 2.0 cm, respectively. On each plan two static MLC fields, with small openings (“the beacons”), are added to enable digitized film image registration to collimator coordinates during image processing. QA plans with static gantry angle (0 deg) are prepared. Dose calculation is done using the AAA algorithm with 1 mm grid size. Two-dimensional dose distribution is detected using Gafchromic EBT3 -film and Vidar Dosimetry Pro Advantage(red) -densitometer. To minimize random errors measurements are repeated three times and films are digitized three times with different lateral positions on the scanner. Images are registered to collimator coordinates using “the beacons” before calculation of the average result. The registration accuracy is similar to the theoretical resolution of the film: 0.1 mm. Scanners lateral and longitudinal response deviations are defined and corrected. Optical density is calibrated to dose. Results Measured dose distribution of HyperArc™ plan with 1 cm diameter sphere targets and the region of interest for gamma and absolute difference evaluations are presented in figure 1. For HyperArc™ plans gamma (2 mm / 2 %) pass rates are: 98.4 % (diameter 2 cm), 99.6 % (diameter 1 cm) and 99.8 % (diameter 0.5 cm) and average absolute differences are: 0.15 Gy (Ø 2 cm), 0.13 Gy (Ø 1 cm) and 0.12 Gy (Ø 0.5 cm). For QA plans corresponding gamma pass rates are: 99.5 % (diameter 2 cm), 99.9 % (diameter 1 cm) and 99.9 % (diameter 0.5 cm) and average absolute differences are: 0.13 Gy (Ø 2 cm), 0.10 Gy (Ø 1 cm) and 0.08 Gy (Ø 0.5 cm).

Conclusion 2D gamma pass rate showed good agreement between Monaco and COMPASS (computed & reconstructed) doses. Therefore COMPASS can be used as patient specific pre- treatment quality assurance and secondary dose calculation tool (for Monaco). EP-1765 Quality assurance of HyperArc™ radiotherapy using a robust film dosimetry protocol A. Kulmala 1 , T. Ikonen 2 , J. Pyyry 2 , M. Tenhunen 3 1 Clinical Research Institute HUCH, Radiotherapy, Helsinki, Finland 2 Varian Medical Systems, Radiotherapy, Helsinki, Finland 3 Helsinki University Hospital, Cancer Center, Helsinki, Finland Purpose or Objective In this study, HyperArc™ (Varian Medical Systems), a novel radiotherapy technique, which enables treatment of multiple small radiotherapy targets using a single isocentre, has been dosimetrically evaluated. HyperArc™ plans are challenging, as many mechanical uncertainties in dose delivery, like table rotation and MLC collimation, might cause non acceptable dose delivery errors. Uncertainties in dose calculation, e.g. source and MLC modelling, might also cause clinically unacceptable errors. The purpose of this study is to verify that calculated and delivered dose agree with sufficient accuracy in cases where target volume consists of multiple small parts distributed on a large area. Material and Methods

Conclusion The agreement between delivered and calculated dose has been found clinically acceptable on mechanically and dosimetrically difficult test cases. The size of target spheres diameter down to 0.5 cm seems not to impact the level of agreement. Radiotherapy plan and QA plan results are found to be similar. EP-1766 Modelling of MR-guided radiotherapy system on Monte Carlo code GEANT4 H. Okamoto 1 , S. Nishioka 1 , K. Iijima 1 , T. Sakasai 1 , Y. Miura 1 , A. Wakita 1 , S. Nakamura 1 , Y. Shibata 1 , Y. Abe 1 , H. Igaki 1 , J. Itami 1