Evaluation of the Electron Contamination in Photon Therapy Using BEAMnrc Code

Today, high-energy photon beams are widely used in the treatment of all types of cancers. However, the production of electrons as contaminants in the photon pathway provides the necessity for research in this field.

Using BEAMnrc code, a Varian2100C/D accelerator head was simulated with 6 MV energy. Applying the advanced techniques of the variance reduction, Depth Dose curves at the Isocenter were obtained and compared to the measurements.</em> We used DOSXYZ code to calculate the Percentage depth dose (PDD) and profile curves. The effect of eliminating electron contamination by helium bag between the accelerator head and treatment bed, Vacuum, Al, Pb, Poly(methyl methacrylate) (PMMA) filters and their composition were also investigated.

By increasing the source to surface distance (SSD) and the air column, we expected an increase in the contribution of electron contamination, but our results showed that increasing the height of air column acted like a filter and scattered low-energy electrons. The presence of helium instead of the air column reduced the surface dose by about 10% compared to the air column. Electron contamination increased for light elements such as Al and PMMA, and decreased by 3.5% for the lead filter.

The use of materials with lower atomic numbers and high densities for filtration of electron contamination of the head of the device and secondary collimators and helium bag is simultaneously proposed.