Characteristics of a Flattening Filter Free Photon Beam – Measurements and Monte Carlo Simulations

Abstract: Introduction: In conventional medical linear accelerators, the flattening filter is introduced in the photon beam line to provide a uniform lateral dose profile at a specified depth in water. For some radiotherapy treatments, e.g. intensity modulated radiotherapy, a flat radiation field is not necessary and the flattening filter could be removed. Several studies have shown that removal of the filter improves some properties of the photon beam, which could be beneficial for these treatments. The purpose of this work was to characterise the photon beam from a flattening-filter free linear accelerator by both measurements and Monte Carlo (MC) simulations.Material and Methods: An Elekta Precise medical linear accelerator was modified for flattening-filter free beam delivery, using the initial electron beam energy of a conventional 6 MV beam. The flattening filter was replaced with a copper plate provided by Elekta. The MC model of the flattening-filter free accelerator was validated by comparing calculated depthdose curves and profiles with measurements. The investigated characteristics of this newbeam included depth-dose curves, lateral dose profiles, output factors, beam quality and evaluation of both photon fluence and the origin of photons.Results: The results showed an increased dose output per initial electron at the central axis of 1.75 and a reduced amount of scattered photons from the accelerator head of 12.7±0.6 % and hence a reduced variation of output factors for different field sizes was found. The photon spectrum of the unflattened beam was softer compared to a conventional beam and did not vary significantly off-axis. As a consequence, the shape of the dose profiles varied less withdepth in water for the flattening filter free beam.Conclusions: A MC model of a flattening-filter free accelerator has been developed and verified. Measurements and computer simulations have shown that this new beam possesses several advantages, especially for the delivery of intensity-modulated radiotherapy. It can also be expected that more accurate dose calculations would be possible since corrections in the treatment planning systems, e.g. for scattered radiation and off-axis softening, are less important.