Purpose: Driven by the need for more accurate and cost-effective therapeutic methods, IMRT is rapidly being deployed at radiation therapy departments. An effecient way of conducting IMRT is by using narrow scanned photon beams, produced from thin bremsstrahlung production targets. Thereby the photon penumbra and pencil beam half width are minimized, while the mean photon energy in the beam is increased, thus enhancing the quality of PET-CT dosimetry. This approach, however, imposes the additional problem of deflecting and stopping an intense stream of high-energy transmitted electrons. In this work, methods for resolving this problem was developed and applied to different treatment unit configurations.
Materials/Method: To optimize the geometrical shape of the electron collector together with the scanning and purging magnetic fields, a composite framework, including CAD (Solid Edge 3D), electromagnetic field simulation (Opera 3D) and Monte Carlo simulation (GEANT4), has been set up. Codes have been developed to integrate the geometry from the CAD software as well as the magnetic fields from the electromagnetic design software in the Monte Carlo simulation, so that all three components have one common description of the physical device investigated. Additionally, software has been developed to enable the results from the Monte Carlo simulation to be directly fed back into the CAD model, permitting the use of analytical models for the electron collector shape as a function of transmitted electron fluence from the target. A major consideration in the choice of GEANT4 as the Monte Carlo simulation software was the ability to simulate all relevant kinds of produced radiation in the electron collector.
Results: The integration of the geometries and the magnetic fields in the different software applications has been thouroughly verified by several means. For example, tracking a 50 MeV electron through the magnetic fields independently in GEANT4 and Opera 3D gives a maximum disagreementof about 500 nm - well within the expected numerical accuracy. The leakage radiation from a number of different collector designs is evaluated and compared to the ICRP 33 recommendations.
Conclusions: A framework integrating CAD, FEM simulations and Monte Carlo transport was developed. The framework, which is more generelly applicable, was here applied to the modelling and simulation of electron collectors for scanned, narrow photon beam IMRT. Relevant physical quantities, including leakage radiation, was computed and evaluated.