Dosimetric properties of a flattening filter-free 6-MV photon beam: a Monte Carlo study

Abstract

Our study revealed that, for recent radiotherapy techniques, the use of multileaf collimators for beam shaping removing the flattening filter could offer some advantages, including an increased dose rate and decreased out-of-field dose.

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

“…More low photons are absorbed by the thick central part of FF; but with increasing fi eld size, more low photons are allowed to penetrate the thin lateral part of FF and contribute to the photon energy spectrum-thus, the mean energy of spectra is decreased. 1,2 It is seen that the energy spectra for the unfl attened beam is softer than the fl attened beam. The calculated mean energy for all spectra shows that the mean energy for the without-FF case decreases from 4.11 to 3.62 MeV and from 4.03 to 3.20 MeV for 10 × 10 and 30 × 30 cm 2 fi eld size, respectively.…”

“…This was also reported by other studies. 1,2,12 The maximum local difference was 30% for the 30 × 30 cm 2 fi eld size. One must keep in mind that if we normalize this difference to the maximum value of the central axis the difference between fl attened and unfl attened beams in out-of-fi eld region is about 3%.…”

“…[1][2][3] On the other hand, using these beams, the skin-sparing effect of high-energy photon beams is diminished owing to the existence of more contaminating electrons and lowenergy photons. 1 Although, a fl attening fi lter (FF) is primarily designed to produce a fl at beam profi le at a given depth, it decreases the X-ray output considerably and produces quality changes within the primary beam by scattering and absorption of primary photons. On the other hand, with the advent of new techniques such as intensity-modulated radiation therapy (IMRT) and linac-based radiosurgery, higher dose rates are in demand.…”

Dosimetric validation of Monaco treatment planning system on an Elekta VersaHD linear accelerator

“…The MLC‐defined collimation in one axis could lead to higher penumbra and alternate head leakage spectrum 9. Second, the high dose rate FFF beams have a lower out‐of‐field dose, different electron contamination spectra, and possibly lower mean energy 10, 11, 12…”

A review on photoneutrons characteristics in radiation therapy with high-energy photon beams

“…34,35 Removal of flattening filters from linacs has been proposed and studied in several investigations as a way to increase the dose rate for radiosurgery and intensity-modulated radiation therapy (IMRT) treatments. 25,[36][37][38][39][40] It was shown that neutron fluence for a 18 MeV beam of Varian linac was about 69% lower for a flattening filter free beam measured by gold foil activation in neutron moderators. 39 Another MC study by Mesbahi on Elekta SL-25 showed results very close to those of a Varian linac.…”