The Study of Magnetic Field Effects on the Electron Contamination of Neptun 10 PC

In high energy x-ray radiation therapy, the photons emerging from the target of linac are contaminated by secondary electrons which are referred to as "contaminant electrons". These electrons raise the skin dose when absorbed, therefore, it is desirable to try to remove them. There are different methods including electron filters, He bag and magnetic field which can be used to reduce the contaminant electrons. In this study, the effect of a powerful magnetic field alone and together with a He bag on the x-ray of a Neptun 10 PC linac has been investigated. A magnet holder was designed and constructed to accommodate twenty pieces of NdFeB magnets which produce a very intensive magnetic field (in the order of 0.3 T) in the field center. The percent depth dose value (Ds) at the surface of the phantom was determined in a scanditronix RFA-300 water phantom using a PTW Pin-point chamber. Additionally, a He bag was placed in the air column between the head of the unit and the phantom surface to study the effect of the air column on electron contamination. The obtained measurements showed that the magnetic field causes a reduction in the Ds in 10×10, 20×20 and 25×25 cm2 field sizes by 10.7, 18.3 and 13.3%, respectively. Also the simultaneous application of the magnetic field and the He bag could cause much more reduction in Ds than magnetic field alone.Discussion and Based on other studies, the magnetic field used in this work was intense enough to eliminate almost all of the contaminant electrons emerging from the head of the linac. The share of the electrons produced by the interaction of the photons in the air column between the head and the phantom surface in Ds was only 2-3%, while but for the electrons arising from the head of the linac it was many times higher. To optimize the reduction of the contaminant electrons in the field sizes of 10×10, 20×20 and 25×25 cm2 magnetic field and He bag should be applied. Since the contaminant electrons arising from air column have a minor effect, applying the magnetic field alone is enough to reduce the contaminant electrons.