ESTRO 2020 Abstract book

S769 ESTRO 2020

the Eclipse (Varian Medical System) treatment planning system (TPS) and were delivered with TrueBeam STx and HD120 MLC using a 6MV photon beam. Acuros XB algorithm was used for VMAT plan dose calculation with two different grid sizes: 0.25 cm and 0.1 cm. Portal Dosimetry with EPID was used to perform quality control and gamma analyses. Gamma criteria were 3%/2 mm and 2%/2 mm. Analyses were performed in absolute dose mode, global normalization was used and the dose threshold value was 10% (following recommendations from AAPM Task Group 218). Gamma analyses were repeated using the auto alignment tool. Spearman's rank correlation coefficients (r s ) were calculated between the plan normalized monitor units (PMU, MU normalized by the fractional prescription dose) and GPR, maximum gamma, average gamma and percentage of points with gamma > 1.5. Results The average fractional prescription dose was 2.01 Gy (range: 1.8 – 3.02 Gy). The average PMU was 128.6 (range: 81.1 – 223.8). Gamma analyses results are shown in Table 1. The percentage of VMAT plans above the limit of acceptability of 95% is the same with and without automatic alignment, as well as, with a dose calculation grid size of 0.25 cm and 0.1 cm. Spearman's rank correlation coefficients between the PMU and GPR, maximum gamma, average gamma and percentage of points with gamma > 1.5 are shown in Table 2. Average gamma had inverse correlation with PMU (- 0.574 < r s < -0.429). GPR was directly correlated with PMU (0.214 < r s < 0.524). These relationships are displayed in Figure 1. No correlations were found between maximum gamma and percentage of point with gamma > 1.5 with PMU. All these results suggest that increasing PMU, increases the maximum dose in VMAT plan and the gamma criterion becomes less restrictive because of the global normalization. Attending to Figure 1, it seems that for plans with PMU greater than 150, could be more homogeneous to consider normalization to prescribed dose instead of global maximum dose.

compared to the calculated values (see Figure 1). The measurements at fixed field sizes (2x2cm2, 3x3cm2, 4x4cm2 and 8x8cm2) and different MLC configurations also showed a high agreement with an average deviation of 1.2% (see Figure 2). The verification of conformal arc (gamma criteria 1%/1mm) showed a passing rate of 98.8%, for phantom based VMAT plans the passing rate is of the order of 98.4% and finally VMAT patient plans have a passing rate of 99%.

Conclusion Measurements obtained with the SRS MapCHECK - for phantom based as well as for real patient plans - are highly consistent with the calculated dose even using a gamma criteria of 1%/1mm. However the measurements with fixed field size but different MLC configuration systematically show higher measured values than those calculated by the planning system. This deviation is about 1.2%. Overall the study shows that the StereoPHAN with the SRS MapCHECK is a suitable measuring device for dosimetry in stereotactic radiotherapy. PO‐1360 Impact of plan parameters in gamma passing rates in VMAT QA A. Garcia Perez 1 , M. Alonso-Etxarri 2 , A. Gonzalez-Pose 1 , J. Vazquez-Rodriguez 1 , A. Lopez Medina 1 , A.G. Teijeiro- Garcia 1 , B. Alvarez-Andrade 1 , D. Medal-Francesch 1 , F. Salvador-Gomez 1 , M. Salgado-Fernandez 1 1 Hospital Meixoeiro, Radiofisica y PR, VIGO, Spain ; 2 Hospital Universitario Donostia, Radiofísica y PR, Donostia, Spain Purpose or Objective To evaluate the impact of plan parameters (MU and calculation grid size) and spatial shifting on the gamma passing rates (GPR) in patient-specific quality assurance (QA) of volumetric modulated arc therapy (VMAT). Material and Methods A total of 50 head and neck (H&N) VMAT plans were retrospectively selected. VMAT plans were generated by