Abstract ID: 84 Development of the 2-dimensional MLC movement technique to improve radiation treatment quality

Abstract

In radiation therapy, leaf width of the multileaf collimator (MLC) determines the resolution of the beam fluence in the vertical direction of the leaf movement. The resolution can be improved by using the MLC with thinner leaves but there is limitation on reducing the width due to spatial constraint of the gantry and increasing leakage dose [1] . The aim of this study was to develop the two-dimensional (2D) MLC movement technique using Monte Carlo (MC) simulation in order to improve the fluence resolution and to minimize unnecessary dose without changing the MLC leaf width. The Varian Trilogy 6MV linear accelerator equipped with the virtual millennium 120 MLC that moves toward 2D direction was modeled by using Geant4 MC tool-kits. The MC-based automatic decision algorithm of the MLC opening was developed to decide the optimal MLC position by comparing the opening at each MLC position. For the patient study to evaluate the developed technique, the patient who received brain 3D conformal radiotherapy was selected. The planning target volume (PTV) was decided with 3-mm-margin and the beam was delivered from 177 directions with the gantry angle from 0 to 360 degree in 2 degree steps. Total 5 × 108 photons were generated in the simulation. By using the 2D optimized MLC, the PTV homogeneity index was improved from 0.123 to 0.125. Mean dose to the PTV was not really changed, from 24.11 Gy to 24.08 Gy when the 2D optimized MLC was used. Meanwhile, the percent volume of the tissue nearby the PTV that was irradiated more than 20.56 Gy (80% of the maximum dose to the tissue with the 1D conventional MLC) was reduced from 5.66% to 0.81% by using the 2D optimized MLC. Mean dose to the normal tissue was also decreased from 11.59 Gy to 9.75 Gy. The results show that the 2D MLC movement technique could improve the radiation treatment quality by reducing normal tissue dose while maintaining the PTV dose.