AbstractID: 6620 Title: Monte Carlo Simulation of Dynamic MLC Using a Multiple Source Model
The Monte Carlo (MC) technique is being recognized as the most accurate method to predict
patient dose. To benefit from the potential of the MC technique for IMRT delivered with a dynamic
MLC, accurate knowledge about the influence of the MLC on the radiation field is essential.
Therefore, a dynamic MLC for the Varian Clinac 2300 C/D was implemented in a previously
developed MC based multiple source model for photon beams (6 and 15 MV). The MLC model
takes the tongue and groove design into account reducing inter-leaf radiation leakage. The front
ends of the leaves were modeled as plane surfaces, which were inclined according to the beam
divergence. A shift in the leaf position accounted for the rounded form of the leaf ends. Using this
model and the MC code GEANT, lateral dose distributions in several depths of a water phantom
were calculated for different MLC applications: two step and shoot cases and two realistic
treatments (Head and Neck and Bronchus carcinoma; 6 and 15 MV respectively) delivered with
dynamic MLC. The intensity modulated fields were discretized in static fields, which were
computed sequentially. Profiles from the MC dose calculations were compared to measured data
from film dosimetry. The dose profiles for all applications studied show that there is a good
agreement between calculated and measured dose distributions. Thus, the MSM together with the
implementation of the dynamic MLC is an appropriate tool for benchmarking and investigational
purposes or directly for IMRT therapy planning.