2%/2mm Gamma index analysis in routine EPID verification of conventional fractionated VMAT plans
PO-1420
Abstract
2%/2mm Gamma index analysis in routine EPID verification of conventional fractionated VMAT plans
Authors: ZALESKA|, Anna(1)*[anna.zaleska@o2.pl];Zawadzka|, Anna(2);Bodzak|, Dominika(2);Kukołowicz|, Paweł(2);
(1)Masovian Oncology Hospital, Medical Physics Department, Wieliszew, Poland;(2)Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Medical Physics Department, Warsaw, Poland;
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Purpose or Objective
Gamma index analysis is currently the standard method of comparing the planned and measured dose distribution. This study was made to consider the possibility of narrowing plan acceptance threshold from more than 95% of measuring points meets the criterion of dose compliance Gamma 3%/3mm (G3%3mm), to 98% and 2%/2mm (G2%/2mm). Study was performed for Electronic Portal Image Devices (EPID) dosimetry.
Material and Methods
Retrospective results of 255 treatment plans with conventional fractionation and various treated locations were analyzed. TrueBeam accelerators: T1, T2 and T3 (Varian) and EPID were used. Plans were prepared in Eclipse Treatment Planning System (Varian) with Anisotropic Analytical Algorithm (ver. 15.6) and PDIP (ver. 13.) algorithm. Analysis was performed with 5% threshold. The region of interest was set as none. Evaluation was done with gamma parameters set to 2% and 2 mm and was accepted if 98% points passed the evaluation. For 22 plans with negative results of G2%/2mm analysis, the new plans were created using the Aperture Shape Controler (ASC) tool witch increase the segments opening in dynamic fields. For all these 22 new plans measurements were performed again. Additionally, to assess the method accuracy, stability for all 3 TrueBeam machines were tested. The same plan was measured every day on each machine for 3 weeks.
Results
Over 75% of the 255 analyzed plans passed G2%/2mm and 98% acceptance criterion. 12% met the G2%/2mm compliance condition for 97% of measuring points. In all 22 re-prepared plans more than 97% measuring points met the 2%/2mm dose criterion. The use of ASCs did not reduce the quality of treatment plans from a clinical point of view. The mean results of stability measurements were T1: 96,9% ± 1,5%, T2: 98,1% ± 0,5% and T3: 98,3% ± 0,8 % points passed the 98% criterion of G2%/2mm in plan composite. The T1 result differs from the other machines.
Conclusion
Narrowing the tolerance criteria in the Gamma analysis assessment allows better indication of treatment plans for which good agreement between calculations and measurements was not obtained. Then new plans can be prepared that are more likely to be implemented correctly. It is important to consider the stability of the machine working when changing the Gamma criterion. ASC tool could be helpful to achieve better verification results.