Evaluation of the dosimetric characteristics of 6 MV flattened and unflattened photon beam

Abstract

This work aims to compare and evaluate the dosimetric properties of 6 MV flattening (FF) and flattening filter free (FFF) photon beams which generated from a Varian 2100 medical accelerator. These properties include percentage depth dose, dose rate, beam profile, out-of-field, energy spectra, scatter factor, and surface dose. This study has been effected by using BEAMnrc and DOSXYZnrc user’s code based on EGSnrc Monte Carlo method. The results obtained showed that the unflattened beams have a dose rate of 2.46 times higher than the flattened beams that would reduce the treatment time. The out-of-field dose of FFF beams at 3.5 cm from the field edge was less than the flattened one due to the reduction of head scatter. The scatter factor and penumbra dose of unflattened beam were found less than that of the flattened one for all field sizes. The unflattened beam has a higher surface dose and build-up dose compared to the flattened beams. Replacing the air column under the jaws by helium leads to the reduction of the surface dose ratio from 1.23 to 1.13, build-up doses and dmax restored as in flattened case. The results of this study have demonstrated that, unflattened beams are very useful for treating cancer cells and sparing the adjacent healthy tissue.

Keywords

• Flattening filter
• Monte Carlo
• Dosimetric properties
• Varian