Session Item

The purpose of this study is to present the feasibility of using IBA's MatriXX Resolution array, for dosimetric verification of SBRT plans. Is the use of a limited resolution matrix based on ionization chambers in small field dosimetry a good idea? The portal dosimetry (PD) method based on a-Si detectors provides adequate resolution, but is it a sensitive enough method for SBRT plans. In the presented paper, both methods were compared in an attempt to prove the thesis contained in the title - the usefulness of the investigated method for the verification of SBRT plans.

Dosimetric verification of SBRT plans was performed for selected sites: bone metastases, spine, lung and lymph nodes. All plans were performed using the Eclipse TPS ver. 15.6  for the VMAT technique using an HD MLC collimator. Depending on the location, the dimensions of the applied fields varied from 3x3 to 7x7 cm² (maximum dimensions of dynamically modulated fields). The range of measured doses was 6-18 Gy per fraction. Measurements were performed on the TrueBeam accelerator ver. 2.5. Verification for PD plans was performed in Portal Dosimetry application ver. 15.6, for the tested method in myQA Patients application ver. 2.12. For measurements the following were used: EPID a-Si 1200 from Varian and MartriXX Resolution placed in MiniPhantomR from IBA.

The developed result was statistically compared for two parameters converging in both methods: Area Gamma<1 and Average Gamma (Figure 1).  For the PD method, the resolution of the gamma parameter was set at 2%/2mm. Two data sets were prepared in myQA Patients for the 3%/3mm and 2%/2mm gamma criterion.  The statistics in the graphs (Figure 1) show that the measurements with the MatriXX array are characterized by a higher average gamma, which entails a lower Area Gamma< 1 and thus a lower pass rate. This difference increases as the gamma parameter decreases from 3%/3mm to 2%/2mm. The standard deviation reaches higher values for the tested method and this is a consequence of more outliers. Does this mean a higher sensitivity of the method based on MatriXX array ? A careful analysis of the extreme values obtained for the investigated method, which go beyond the accepted criteria of agreement, shows however that the detector resolution is the critical parameter for fields with a high dose gradient.

The performed measurements showed, that the method of verification of SBRT plans using the MatriXX Resolution array, can be used as an independent method. This is confirmed by the average values of the parameters, Area Gamma < 1 and Average Gamma, presented in Figure 2. In the case of the tested method, the gamma criterion of 2% and 2mm can be successfully applied. However, for the group of plans characterized by a high dose gradient, it is advisable to increase this value to 3% and 3mm.