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The uncertainty was calculated in five different ways, i.
BEAMnrc: software tool to model radiation beams – National Research Council Canada
Data Tables 26 5—E There is another viewpoint concerning PHSP simulations of the linac treatment head, which should be considered.
On the other hand, if one is interested in the most manuual PS simulation note that efficiency is the inverse product of variance and timethen it is necessary to use VRTs.
Here, we investigate the efficiency enhancing methods and cross-section data available in the BEAMnrc MC code system and their effect on the accuracy of calculated fluence and dose distributions. An additional phantom with a 5 cm air gap embedded in water was also simulated for dose computations.
Variance reduction techniques Bremsstrahlung photon splitting, Russian Roulette, and electron splitting The three options for bremsstrahlung photon splitting were used in this study, i. Photon interaction forcing With photon interaction forcing, the user can force photons to interact up to a user-defined number of times, Nin specified component modules within a simulation.
Although significant differences in the mean energy, planar fluence, and angular and spectral distributions were observed when comparing the NIST and BH bremsstrahlung cross-section PHSP files, they were not reflected in the calculated dose distribution in a water phantom with and without an air gap.
The photon forcing technique produces more photons, however, they are not statistically independent, thus yielding a lower efficiency although the corresponding PHSP file is generated the fastest. Estimates of the uncertainty using a significant number of bins e.
The three options for bremsstrahlung photon splitting were used in this study, i. The boundary crossing algorithm together with the electron transport algorithm constitutes the condensed history technique used by a particular MC code system. However, as demonstrated in Sec.
BEAMnrc: software tool to model radiation beams
If the fluence distributions were reconstructed using equal area scoring zones e. The major advantage of using aggressive VRTs is the significant improvement in calculation time without a compromise in the accuracy of the patient dose computation. Three main areas were considered.
The CPU times were scaled to a single 2. Augmented charged particle range rejection is mankal available when used simultaneously with DBS.
Data Tables 20 2— ; Springer-Verlag, Heidelberg,pp. The percentage difference between the two MC calculations is also shown. When using UBS, each bremsstrahlung event produces a predefined number of bremsstrahlung photons, each having a weight equal to the inverse of the splitting number NBRSPL times the weight of the electron that beanmrc the bremsstrahlung event.
When photon splitting was used together with the other techniques, only the cases with improved photon and electron fluence were considered, i. The electron and photon cutoff energies were set to 0. Data Tables 7 6— The tradeoff is that the no-VRT simulation is much slower, although if this simulation is being performed just once for subsequent use then it is a reasonable approach. It has been reported that this option increases the efficiency of homogeneous phantom calculations, 38 however, no photon splitting or charged particle range rejection were employed.
A 3— A systematic study of the influence of efficiency enhancing methods available in BEAMnrc on the accuracy of calculated fluence and dose distributions. The mean energy, planar fluence, and angular and spectral distributions were used as quantities of interest and the estimate of the uncertainty was calculated in bwamnrc different ways, i.
Augmented range rejection accounts correctly for bremsstrahlung production, since charged particles surviving Russian Roulette and with their weight increased by a factor of NBRSPL still have a manuzl to undergo bremsstrahlung events.
The photon forcing parameters can also be passed onto secondary photons and this feature is particularly useful to improve calculation efficiency for bremsstrahlung photon interactions, especially when combined with bremsstrahlung photon splitting.
This is essential if one is considering real-time MC simulation of the treatment head and patient. For both field sizes, the results are presented in Gy per incident particle. Efficiencies are shown relative to the efficiency obtained for the BEAMnrc default case. The efficiency of BEAMnrc PHSP simulations is significantly improved, with directional bremsstrahlung splitting combined with augmented charged particle range rejection, for the two field sizes studied.
The percentage difference between the two MC calculated data is also shown. This occurs because of particle correlations which are introduced with the use of VRTs, such as bremsstrahlung splitting.
The current version of BEAMnrc allows the user to choose three photon cross sections: The default differential cross section for the bremsstrahlung interactions is Bethe—Heitler BH. The following efficiency enhancing methods were considered: This work was supported in part by Grant Nos. Some of these special-purpose MC codes are being used as a dose engine for MC-based treatment planning in the routine clinical setting.
PIRS E In our study, ESAVE was set to 2 MeV in all component modules with the exception of the target where the electron cutoff energy value was used, as recommended in Sheikh—Bagheri et al. Published online Nov 5. A 12 195— This efficiency gain is achieved through the use of variance reduction techniques and faster simulation of the electron transport.
The percentage difference, in the central region of the field and at different depths, between measured and a selection of MC calculated off-axis dose profiles is also shown.
For the first three uncertainty calculations, only one bin is considered, i. For a particular efficiency enhancing technique used in BEAMnrcthe difference between the relative efficiencies obtained using the various estimates of the uncertainty increases as one changes from planar fluence to spectral distribution; this situation is clearly more evident for the larger field size see Figs.