  A Geant4 based simulation tool has been developed to perform Monte Carlo
modelling of a 6 MV VarianTM iX clinac. The computer aided design interface of
Geant4 was used to accurately model the LINAC components, including the
Millenium multi-leaf collimators (MLCs). The simulation tool was verified via
simulation of standard commissioning dosimetry data acquired with an ionisation
chamber in a water phantom. Verification of the MLC model was achieved by
simulation of leaf leakage measurements performed using GafchromicTM film in a
solid water phantom. An absolute dose calibration capability was added by
including a virtual monitor chamber into the simulation. Furthermore, a
DICOM-RT interface was integrated with the application to allow the simulation
of treatment plans in radiotherapy. The ability of the simulation tool to
accurately model leaf movements and doses at each control point was verified by
simulation of a widely used intensity-modulated radiation therapy (IMRT)
quality assurance (QA) technique, the chair test.
