  Typically used as a tool for Monte Carlo simulation of high energy physics
experiments, GEANT4 is increasingly being employed for the simulation of
complex radiotherapy treatments. Often the specification of components within a
clinical linear accelerator treatment head is provided in a CAD file format.
Direct import of these CAD files into GEANT4 may not be possible, and complex
components such as individual leaves within a multi-leaf collimator may be
difficult to define via other means. Solutions that allow for users to work
around the limited support in the GEANT4 toolkit for loading predefined CAD
geometries has been presented by others, however these solutions require
intermediate file format conversion using commercial software. Here within we
describe a technique that allows for CAD models to be directly loaded as
geometry without the need for commercial software and intermediate file format
conversion. Robustness of the interface was tested using a set of CAD models of
various complexity; for the models used in testing, no import errors were
reported and all geometry was found to be navigable by GEANT4.
