Stefan Leidenberger

Assessment of a new multileaf collimator concept using GEANT4 Monte Carlo simulations.

The aim of the work was to investigate in advance the dosimetric properties of a new multileaf collimator (MLC) concept with the help of Monte Carlo (MC) simulations prior to the production of a prototype. The geometrical design of the MLC was implemented in the MC code GEANT4. For the simulation of a 6 MV treatment beam, an experimentally validated phase space and a virtual spatial Gaussian-shaped model placed in the origin were used. For the simulation of the geometry in GEANT4, the jaws and the two leaf packages were implemented with the help of computer-aided design data. First, transmission values for different tungsten alloys were extracted using the simulation codes GEANT4 and BEAMnrc and compared to experimental measurements. In a second step, high-resolution simulations were performed to detect the leakage at depth of maximum dose. The 20%-80% penumbra along the travel direction of the leaves was determined using 10 x 10 cm2 fields shifted along the x- and y-axis. The simulated results were compared with measured data. The simulation of the transmission values for different tungsten alloys showed a good agreement with the experimental measurements (within 2.0%). This enabled an accurate estimation of the attenuation coefficient for the various leaf materials. Simulations with varying width of the spatial Gaussian distribution showed that the leakage and the penumbra depend very much on this parameter: for instance, for widths of 2 and 4 mm, the interleaf leakage is below 0.3% and 0.75%, respectively. The results for the leakage and the penumbra (4.7+/-0.5 mm) are in good agreement with the measurements. This study showed that GEANT4 is appropriate for the investigation of the dosimetric properties of a multileaf collimator. In particular, a quantification of the leakage, the penumbra, and the tongue-and-groove effect and an evaluation of the influence of the beam parameters such as the width of the Gaussian distribution was possible.

SU-FF-T-294: Monte Carlo Simulations of the Dosimetric Characteristics of a New Multileaf Collimator

Purpose: The aim of the work was to investigate the dosimetric characteristics of a new multileaf collimator (160MLC™, Siemens) with the help of Monte Carlo(MC) simulations during the design phase. Method and Materials: The MLC was implemented in the MC code Geant4. For the simulation of the 6 MV treatment beam an experimentally validated phase space and a virtual source model were used. For the simulation of the geometry in Geant4 the jaws and the two leaf packages were implemented with the help of CAD data. First, transmission values for different tungsten sinters were extracted using the simulation codes Geant4 and BEAMnrc and compared to experimental measurements. In a second step, high resolution simulations were performed to detect the leakage at depth of maximum dose. The 20%–80% penumbra along the leaf travel direction was determined for different 10×10 cm2 fields shifted along the x‐axis. The simulated results were compared with measured data obtained with a prototype. Results: The simulation of the transmission values for different tungsten sinters showed a good agreement with the experimental measurements (within 2.0%). This gave an accurate estimation of the absorption coefficient for various leaf materials. Simulations with varying source sizes showed that the leakage and the penumbra depended very much on this parameter: e.g. source sizes of 2 mm and 4 mm result in the interleaf leakages below 0.3% and 0.75% respectively. The results for the leakage and the penumbra are in good agreement with the measurements.Conclusion: This study showed that Geant4 is appropriate for the investigation of the dosimetric characteristics of a multileaf collimator. In particular we could quantify the leakage and the penumbra and evaluate the influence of the beam parameters such as the virtual source size. Conflict of Interest: Research supported by Siemens Oncology Care Systems.