Measurement and production of electron deflection using a sweeping magnetic device in radiotherapy

The deflection and removal of high energy electrons produced by a medical linear accelerator has been attained by a Neodymium Iron Boron (NdFeB) permanent magnetic deflector device. This work was performed in an attempt to confirm the theoretical amount of electron deflection which could be produced by a magnetic field for removal of electrons from a clinical x-ray beam. This was performed by monitoring the paths of mostly monoenergetic clinical electron beams (6 MeV to 20 MeV) swept by the magnetic fields using radiographic film and comparing to first order deflection models. Results show that the measured deflection distance for 6 MeV electrons was 18 +/- 6 cm and the calculated deflection distance was 21.3 cm. For 20 MeV electrons, this value was 5 +/- 2 cm for measurement and 5.1 cm for calculation. The magnetic fields produced can thus reduce surface dose in treatment regions of a patient under irradiation by photon beams and we can predict the removal of all electron contaminations up to 6 MeV from a 6 MV photon beam with the radiation field size up to 10 x 10 cm2. The model can also estimate electron contamination still present in the treatment beam at larger field sizes.