Monte Carlo study on a flattening filter-free 18-MV photon beam of a medical linear accelerator

PURPOSE: Several studies on the dosimetric properties of unflattened photon beams have shown some advantages for radiotherapy. In this study, the effect of removing the flattening filter from an 18-MV photon beam was investigated using the Monte Carlo method. MATERIALS AND METHODS: The 18-MV photon beam of an Elekta SL25 linear accelerator was simulated using the MCNP4C Monte Carlo (MC) code. Beam dosimetric features, including central axis absorbed doses, beam profiles, and photon energy spectra, were calculated for flattened and unflattened 18-MV photon beams. RESULTS: A 4.24-fold increase in the dose rate was seen for the unflattened beam with a field size of 10 x 10 cm(2). A decrease in the out-of-field dose up to 30% was seen for the unflattened beam. For the unflattened beam, photon energy spectra were softer, and the mean energies of the spectra were higher for a smaller field size. CONCLUSION: Our study showed that the increase in dose rate and lower out-of-field dose can be possible advantages for an unflattened 18-MV beam.