Impact of plan parameters in gamma passing rates in VMAT QA
PO-1360
Abstract
Impact of plan parameters in gamma passing rates in VMAT QA
Authors: Garcia Perez|, Ana(1)*[ana.garcia.perez@sergas.es];Alonso-Etxarri|, Maddalen(2);Gonzalez-Pose|, Alicia(1);Vazquez-Rodriguez|, Julio(1);Lopez Medina|, Antonio(1);Teijeiro-Garcia|, Antonio Guillermo(1);Alvarez-Andrade|, Benito(1);Medal-Francesch|, Daniela(1);Salvador-Gomez|, Francisco(1);Salgado-Fernandez|, Manuel(1);
(1)Hospital Meixoeiro, Radiofisica y PR, VIGO, Spain;(2)Hospital Universitario Donostia, Radiofísica y PR, Donostia, Spain;
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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 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 (rs) 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 < rs < -0.429). GPR was directly correlated with PMU (0.214 < rs < 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.
Conclusion
The number of VMAT plans that passes the limit acceptability for the analysis gamma is independent of the use of auto alignment tool or the use of a dose calculation grid size of 0.25 cm or 0.1 cm. We have found an inverse correlation between average gamma and PMU; and a direct correlation between GPR and PMU, this could be related to the greater normalization dose associated to a greater value of PMU.
Funded by ISCIII PI17/01735 grant (cofunded by FEDER).