Correlation between gamma analysis results and monitor units in prostate VMAT
PO-1387
Abstract
Correlation between gamma analysis results and monitor units in prostate VMAT
Authors: Gil Conde|, María(1)*[magilconde@gmail.com];Albertos Fernández|, María José(1);Manuel|, Marín Núñez(1);Campos Morillo|, Fátima(1);Seguro Fernández|, Álvaro(1);
(1)Hospital de Jerez, Radiation Physics Department, Jerez de la Frontera, Spain;
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Purpose or Objective
The introduction of inverse planning techniques has provided the possibility of achieving high quality plans in terms of isodoses and DVH curves, at the expense of an increase in the number of monitor units (MUs) and the complexity of the plan with them. Our purpose in this work is to assess quantitatively the relation between the increase of MUs and the result of the gamma analysis of the pre-treatment QA measurements when using VMAT in one of the most standard diseases in radiotherapy: the prostate cancer case.
Material and Methods
Data of 35 prostate cancer patients that had already been treated at our centre in the last two years were used for this study. The prescribed dose to all of them was 60 Gy in 20 fractions to the PTV encompassing the prostate gland and 48 Gy in 20 fractions to a second PTV encompassing the seminal vesicles. A VMAT plan with two arcs (210º-150º) was designed for each patient using the planning system Pinnacle 9.10 and eventually delivered in a Varian Truebeam. All the plans had to accomplish the same restrictions to the organs at risk.
Pre-treatment QA of each plan was performed in the linac using the OCTAVIUS 4D system (PTW), which allows for volumetric measurements using the Octavius Detector 1500XDR ion chamber matrix. The resulting 3D dose matrix was compared with the one exported from the planning system by a gamma analysis, obtaining for each plan a volumetric gamma index (4D GI).
A correlation analysis between the 2mm-2% local 4D GI and the number of MUs normalized to the maximum prescribed dose in the fraction (MU/D) was performed.
Results
The plot of GI versus MU/D is shown in the figure. Data are fitted by linear regression with R2=0.52. The correlation test shows a Pearson’s coefficient r=-0.72 (p<0.0001). Considering that a 2mm-2% local GI passing rate of 90% would represent at our institution the lowest threshold for a pre-treatment quality result, equation of the line in the figure would give us an upper limit of 3.3 MU/cGy for a prostate VMAT plan of these characteristics.
Conclusion
The upper results support the hypothesis of a correlation between the increase of monitor units and the worsening of pre-treatment QA results. They give an idea of the quantitative relation between these two concepts in the prostate case, which is one of the simplest in radiotherapy. We used gamma 2%-2mm local criteria due to the statistical significance shown by this parameter.