ESTRO 36 Abstract Book

S774 ESTRO 36 _______________________________________________________________________________________________

the advanced acceptance testing procedure was further commissioned. This works consolidates preliminary understanding for range and spot measurement equipment (Grevillot et al, PTCOG2016), using additional data obtained during beam delivery commissioning (e.g. long term reproducibility of QA equipment). Additional detectors were also commissioned, such as a 2D array of ionization chambers and a diamond detector. The commissioning methods developed allowed determining the accuracy of the QA devices in clinical conditions and better define the QA tolerances for periodic quality assurance of the beam delivery system. The purpose of this work is to guide medical physicists in the implementation of dosimetry equipment and associated phantoms as a pre-requisite to acceptance testing, commissioning and further QA checks of the facility itself. Material and Methods A x-ray check was carried out for each ionization chamber to verify the integrity of its construction. Positioning accuracy of the water phantom scanning mechanism was evaluated in 3D against laser tracker measurement. Range measurement equipment was carefully calibrated by measuring the entrance WET of the device. Long term reproducibility of a multi-layer ionization chamber detector used for daily QA of beam ranges was determined and used to define morning QA tolerances of the beam delivery system. Spot measurement equipment was evaluated in terms of spot size, position and 2D homogeneity against radiochromic films. Transverse profiles acquired in water with diamond detector were evaluated against pin-point detectors (Figure 1). An ionization chamber-based 2D array was evaluated in terms of effective depth of measurement, recombination, 2D homogeneity and absolute dose against ROOS chamber. Extensive commissioning procedures were developed specifically for each piece of QA equipment, based on its intended use.

Conclusion The procedures implemented at MedAustron significantly improved the knowledge and the performances of the dosimetric equipment and therefore the quality of medical commissioning activities at the facility. Our experience shows that commissioning of QA equipment is a necessary step towards high precision radiation therapy. EP-1451 Validation of local tolerances for VMAT patient specific QA using the IBA Compass system E. Crees 1 , R. Hulley 1 , G. Kidane 1 , Y. Miao 1 1 Queen's Hospital, Department of Medical Physics, Romford, United Kingdom Purpose or Objective The aim of this study was to review the locally set tolerances for VMAT Head and Neck, Brain and Prostate and Pelvic Nodes patient specific QA using the IBA COMPASS MatriXX evolution measurements and computations with the IBA COMPASS® system. Radiotherapy treatment planning requires independent verification of both the treatment planning system (TPS) dose calculation and the patient dose delivery system. The verification of the dose delivery system can be carried out independently to the TPS dose calculation (in which case individual treatment plans may be verified using a 2 nd calculation alone) or can be incorporated into individual patient specific measurements. The pass/fail tolerances of patient specific QA using the Compass system have been set locally. The calculation method verification tolerance was set for the comparison between the Compass dose calculation and the treatment planning system calculation. The measurement method verification tolerance was set by comparing the MatriXX evolution measured dose to the treatment planning system calculated dose. Material and Methods A retrospective study was performed on Head and Neck, Brain and Prostate and Pelvic Nodes VMAT plans that were produced on the Eclipse TPS using the AAA algorithm. Each treatment plan was re-calculated using the Compass software and the dose distribution of each plan was measured using the Compass MatriXX evolution . Subsequently, the Compass computed and measured dose distributions were compared to the TPS calculated plan using gamma index analysis. Bland Altman statistical analysis was employed to compare gamma index results of the Compass calculated and Compass measured dose distributions. The analyses were performed based on the agreement between the treatment planning system compared to the measurement and Compass calculations. The tolerances were set on absolute dose difference (2% for computation and 3% for measurement on all points) and global gamma index assessment (2%/2 mm criterion for 98% of points – computation and 3%/3mm criterion for 95% of point - measured).

Results The advanced acceptance testing procedures developed resulted in the identification of defects in several devices from different manufacturers before clinical use. The commissioning procedures allowed maximizing the performance of the QA equipment and consequently the quality of medical commissioning activities. Main overall uncertainties are presented in Table 1.