Reduction of photon contamination in electron therapy of cancer with magnetic fields

Abstract

Introduction: Photon contamination is a restriction on treatment with electron that increase dose to healthy tissue below the tumor. The aim of this study is to reduce the photon contamination using a magnet system. Materials and Methods: A mini-applicator equipped with two neodymium boron permanent magnets was designed which make it possible to adjust the distance between magnets. Mini-applicator was placed in standard applicator of Varian 2100 CD linear accelerator. Mini-applicator was modeled in CST studio finite element software. Deflection angle and displacement of electron beam transporting through magnetic field were calculated. Different transverse magnetic intensities were created by determining the amount of 2 to 5 cm for distance between two poles. The treatment head was rotated to deflected electrons become normal to the water surface. Dosimetry was performed in a water phantom using EBT2 gafchromic film. Films were scanned with HP G3010 reflecting scanner. Optical density in red channel was extracted by programming in MATLAB. Dose curves in presence of magnetic field compared with frame without magnetic field. Results: Simulated and measured magnetic field intensities are consistent. Maximum deflection angle is 32.9 degree for 12 MeV and minimum deflection is 12.1 degree for 15 MeV electron beam. Scanned film showed clearly that photons exit from electron field. The photon contaminations without magnetic field were 4.5 and 2.6 percent for 15 and 12 MeV respectively which using highest magnetic field intensity have decreased to 0.9 and 0.2 percent. Conclusion: A magnetic mini-applicator was created and modeled using finite element method. The deflection angle and displacement of electrons beam was calculated. By applying magnetic field, photon contamination can be reduced. Energetic electrons can be used without concern for adverse effects on healthy depth tissue.