ESTRO 36 Abstract Book

S782 ESTRO 36 _______________________________________________________________________________________________

scanning techniques. The aim of this work is to investigate the influence of variations of the nominal beam width on the dose distribution of cubic dose volumes, which are often part of a typical QA program. Material and Methods For QA purposes, three cubic dose volumes with a spread out bragg peak (SOBP) of 3x3x3 cm³ are optimized with the treatment planning system (TPS), syngo PT Planning (Siemens, Germany). The nominal dose in the SOBP is 0.5 Gy. The depth in water of the centre of the cubes is 50, 125 and 200 mm, respectively. To perform dose calculations on a water phantom with variation of initial beam width, the needed algorithms and base data of our TPS are transfered to MATLAB (The Mathworks Inc., USA). These are mainly the depth dose distributions and the double gaussian parametrization of the beam width. Plans are recalculated with MATLAB with nominal and varied beam width. Dose distributions are analysed performing a 3D gamma index analysis with criterias of 1mm distance to agreement and 5% dose deviation, normalized to global maximum. The maximum negative and positive tolerable beam width deviations are determined where all points still pass the gamma index acceptance criteria (e.g. gamma index < 1). Results The maximum tolerable beam width variations for the dose cube in a depth of 50 mm amounts to -7% and +10%, while for the dose cube in 125 mm depth the found values are -12% and +17%. The most interesting result is found for the dose cube in 200 mm depth. While the high dose region shows comparable large possible beam width variations of -17% to +25%, the maximum tolerable beam width deviation in the entrance region amounts to -8% and +15%. Conclusion The QA plan in small depth shows rather small tolerable deviations of the initial beam width. It is observed, that this is mainly affected by the strong variation of the particle numbers per scan spot in each energy slice, optimized by the TPS to achieve lateral penumbras as small as possible. Following this observation, we created a plan with the same field size parameters consisting uniform scanned energy slices. Applying the described beam width variation, resulting tolerable beam width deviations are -13% and +12% . For the QA plans in medium and large depth, beam width tolerances in the SOBP become larger. One reason is that the additional multiple coulomb scattering in water dilutes the impact of deviations of the initial beam width. Depending on the spot scan pattern, the entrance region can be the region with the highest demands on beam width accuracy for plans in large depths. For TPS commissioning, the spot scan pattern should be inspected with regard to beam spot width variations. EP-1465 A beam matching procedure for Volumetric Modulated Arc Therapy L. Abdullah 1 , C. Constantinescu 1 , M.N. Hussein 1 1 King Faisal Specialist Hospital and Research center, Biomedical Physics, Jeddah, Saudi Arabia Purpose or Objective To describe our experience of commissioning and quality control tests, along with the adjustments performed for beam matching in VMAT. Material and Methods A Synergy linac upgraded to Agility head was matched to a Versa HD linac. Dose calculation for commissioning tests was performed on Monaco TPS version 5.0, with Monte Carlo algorithm, inhomogeneity correction, calculation grid size of 1mm, and statistical uncertainty of 0.5%/control point. The compliance between dose calculation and measurement was assessed for a gamma-index (GI) of 3% dose difference

and (DTA). The manufacturer specifies a routine for acceptance testing of matched linear accelerators which concerns only the beam quality and field size. After appropriate MLC calibration, percentage depth dose and beam quality index were evaluated for both linacs, in similar setups. In-plane and cross-plane beam profiles were acquired for field sizes of 10x10cm², 20x20cm 2 , and depth of 10 cm. All dosimetric parameters appeared identical for both linacs, within a tolerance of 1%. Beam output calibration differed within 0.5%. The quality of matching was found to be valid for 3D-CRT treatments but not for VMAT. Specific test beams were further performed to compare the two linacs for the accuracy of leave and jaws movement, using the “picket fence” and “sliding- window” methodology at different gantry and collimator angles, using electronic portal imaging device (EPID) dosimetry. The MLC minor leaf and jaw offsets of the second linac was finely adjusted mechanically in order to achieve acceptable accuracy of matching. 38 clinical VMAT plans for various sites and different degrees of modulation were verified on both linacs. The correlation between the GI passing rate and plan modulation, assessed by the number of MU and number of segments, was further investigated using a logistic regression test. Results The beam quality index was 0.682 for linac 1 and 0.686 for linac 2 after the vendor's matching procedure was performed. GI analysis of the “picket fence” and “sliding window” test beams indicated the need for mechanical adjustment of MLC minor leaf and jaw offsets of linac 2. After the beam matching accomplished, GI analysis of VMAT clinical treatment plans indicated good agreement between the two linacs. The average passing rate between calculated and delivered dose distribution was 97.7±2.8% (range: 87.5%-100%) for linac 1 and 93.4±4.7% (range: 83%- 99.%) for linac 2. Weak correlations were found between the GI passing rate and number of MU (R 2 =0.043, p=0.146) and segments (R 2 =0.0273, p=0.240), indicating that the degree of modulation is not to be considered in setting acceptance criteria for GI analysis. 3 mm distance-to-agreement