Monte Carlo study of in-field and out-of-field dose distributions from a linear accelerator operating with and without a flattening-filter.
OBJECTIVE
To compare dosimetric characteristics of 6 MV photon fields originating from a linear accelerator operating with (FF) and without (FFF) a flattening-filter. The main objective is to establish a FFF model that results in similar depth-dose and build-up profiles as the original FF model, and subsequently estimate and compare out-of-field dose distributions.


METHODS
The EGSnrc Monte Carlo user codes BEAMnrc and DOSXYZnrc are used for photon beam simulations of an Elekta linear accelerator and dose calculations in a water phantom, respectively. Three beam models were analyzed: (1) the conventional linear accelerator with the flattening-filter in place and incident electron energy 6.45 MeV (FF 6.45 MeV), (2) similar flattening-filter-free model (FFF 6.45 MeV), and (3) as (2) but with increased electron energy (FFF 8.0 MeV). The field size 5 × 5 cm(2) was used for characterization of dose output, depth dose profiles, and photon spectrum. The field size 40 × 40 cm(2) was used for characterization of cross-field photon energy, photon fluence, and dose distributions. Out-of-field dose distributions were analyzed in both in-plane and cross-plane directions for 5 × 5 cm(2) and 10 × 10 cm(2) fields.


RESULTS
Comparable depth dose distributions, including the build-up region, for FF and FFF fields were achieved by increasing the electron energy from 6.45 MeV to 8.0 MeV for the FFF beam. The FFF beams result in reduced out-of-field dose compared to the FF beam: the reduction was most apparent in the cross-plane direction and more pronounced by the FFF 8.0 MeV beam compared to the FFF 6.45 MeV beam. Differences in out-of-field dose due to direction (in-plane vs cross-plane) were up to 40% for the FF beam; this effect was significantly reduced for the FFF beams. As the flattening-filter is a major source of contaminating electrons, superficial out-of-field dose was expected, and was found to be, reduced for FFF beams.


CONCLUSIONS
The build-up and depth-dose characteristics of a conventional "6 MV" beam can be maintained when changing to a flattening-filter-free modality by increasing the incident electron energy from 6.45 MeV to 8.0 MeV. This will at the same time reduce the out-of-field dose for regions up to 20 cm from the central axis by 10%-30% compared to the original FF situation.
Department of Oncology and Radiotherapy, St. Olavs University Hospital, N-7006 Trondheim, Norway. sigrun.saur@ntnu.no
S S
SS
Almberg
J
J
Frengen
T
T
Lindmo
D000465:Algorithms; D004583:Electrons; D004867:Equipment Design; D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons; D011874:Radiometry / Q000379:methods*; D011878:Radiotherapy / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results; D012984:Software