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For both field nanual, the results were normalized to the linac calibration point in cGy per MU, i. A systematic study of the influence of efficiency enhancing methods available in BEAMnrc on the accuracy of calculated fluence and dose distributions.
If the fluence distributions were reconstructed using equal area scoring zones e. A 3— Photon and bremsstrahlung cross sections The current version of BEAMnrc allows the user to choose three different photon cross sections: Photon interaction forcing With photon interaction forcing, the user can beamntc photons to interact up to a user-defined number of times, Nin specified component modules within a simulation. In contrast, Manuwl improve efficiency through the use of approximations.
The electron and photon cutoff energies were set to 0. 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 chance to undergo bremsstrahlung events. However, as demonstrated in Sec. For the first three uncertainty calculations, only one bin is considered, i.
A photon forced to interact is split into a scattered photon whose beannrc is equal to the probability of interaction and an unscattered photon carrying the remaining weight. Support Center Support Center. This occurs because of particle correlations which are introduced with the use of VRTs, such as bremsstrahlung splitting. Data Tables 7 6— When using only bremsstrahlung photon splitting, the greatest CPU time saving is when DBS is used with and without electron splitting, manuak both field sizes, i.
The mean energy, planar fluence, and angular and mamual distributions were used as quantities of interest and the estimate of the uncertainty was calculated in five different ways, i.
Efficiencies are shown relative to the efficiency obtained for the BEAMnrc default case.
The photon forcing technique produces more photons, however, they are not statistically beamnrv, thus yielding a lower efficiency although the corresponding PHSP file is generated the fastest. Springer-Verlag, Heidelberg,pp. Some of these special-purpose MC codes are being used as a dose engine for MC-based treatment planning in the routine clinical setting.
Table 2 Bremsstrahlung splitting techniques and the values used for the relevant parameters.
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.
Therefore, the actual relative efficiencies possible with DBS were underestimated. Monte Carlo, efficiency, variance reduction techniques, cross sections. The uncertainty was calculated in five different ways, i. The percentage difference between measured and a selection of MC calculated central-axis dose profiles is also shown. A 12 195— The parameter values chosen for the different techniques are recommendations from previous published works 1930 or are considered to be reasonable estimates, based on information from the BEAMnrc developers.
This efficiency gain is achieved through the use of variance reduction techniques and faster simulation of the electron transport. The three options for bremsstrahlung photon splitting were used in this study, i. Therefore, if one is concerned with the smallest statistical uncertainty variance in dose, then it is best to perform the treatment head PS simulation without VRTs. Another important consideration is that the relative fluence efficiency, for a given splitting number, actually decreases with increase scoring zone area, a result derived mathematically in an article by Kawrakow.
BEAMnrc: software tool to model radiation beams
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. 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.
With SBS, the value of NBRSPL is changed in order to maximize the splitting of photons aimed into the field and to minimize unnecessary splitting of photons aimed away from the field. 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 underwent the bremsstrahlung event. 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.
It is generally accepted that the cross-section data available in the megavoltage energy range are sufficiently accurate.
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 beaamnrc and dose distributions. An investigation of the influence of different photon and bremsstrahlung cross-section data available in BEAMnrc on the accuracy of calculated fluence and dose distributions.
The current version of BEAMnrc allows the user to choose three different photon cross sections: In the megavoltage energy range, there are small but observable differences between the two differential cross sections refer to Fig. There is another viewpoint concerning PHSP simulations of the linac treatment head, which should be considered. In addition, the methods for performing the efficiency and statistical uncertainty calculations are described.
Boundary crossing algorithm The boundary crossing algorithm together with the electron transport algorithm constitutes the condensed history technique used by a particular MC code system. Estimates of the uncertainty using a significant number of bins e.
BEAMnrc: software tool to model radiation beams – National Research Council Canada
The two MC programs generate accurate PHSP files of the linear accelerator for subsequent planning or dosimetric verification purposes in a relatively short amount of time when the appropriate efficiency enhancing tools are used.
This work was supported in part by Grant Nos. In addition, the relative efficiencies are dependent on the quantity of interest chosen, although one can claim from this study that BEAMnrc MC calculations with the directional bremsstrahlung splitting mznual yields the highest efficiency for all quantities of bfamnrc analyzed. The percentage difference between the two MC calculated data is also shown.