Magnetic removal of electron contamination in radiotherapy x-ray beams

Removal of contamination electrons to lower patient skin dose from Linac produced radiotherapy x-ray beams is a serious issue in modern radiotherapy. Such removal can be achieved via the use of a magnetic field and is the subject of investigation in this thesis. The magnetic deflector consists of two separate and adjustable banks of permanent Neodymium-Iron-Boron magnets held in a simple Aluminium frame, which slots into the accessory mount of a conventional Varian Clinac 2100C Linear Accelerator. The deflector allows x-ray beams with field sizes of up to 30x30 cm2 (source to surface distance of 100 cm) to pass through without interference, and weighs less than 20 kg. The deflector generates a maximum field of 0.21 T between the magnets along the central axis for a 10 cm magnet bank separation, and similarly 0.07 T for a 20 cm separation. Using the magnetic deflector, experimental measurements at the central beam axis show entry doses that approach that of the theoretical entry dose without electron contamination (Monte Carlo predictions) for 6 and 10MV x-ray beams. These range from 25% (6MV, 10x10 cm field size) to 55% (10MV, 20x20 cm field size with Perspex block trays) relative dose reduction at the phantom surface. Theoretical modelling has been performed which confirms the removal of the electron contamination for these typical clinical x-ray beam energies and field sizes. Pure electron beam path modelling has also been studied using this technique for determining the accuracy of the modelling technique. Results agree closely with experimentally observed values for 5 clinical electron beam energies between 6 and 20 MeV. The theoretical simulations are based around 3-dimensional modelling of the path of the contamination electrons as they travel through the magnetic field set up by the deflector (MATLAB). The magnetic field data used in modelling has been generated by a finite element package (Maxwell 3D). The experimental verification methods include the use of radiographic film and Attix parallel plate ion chambers with solid water phantoms for both qualitative and quantitative measurements.