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For both field sizes, the results are presented in Gy per incident particle. Data Tables 7 6— The efficiency of BEAMnrc PHSP simulations is significantly improved, with directional bremsstrahlung splitting combined with augmented charged particle ebamnrc rejection, for the two field sizes studied. This occurs because of particle correlations which are introduced with the use of VRTs, such as bremsstrahlung splitting.
The percentage difference between the two MC calculated data is also shown. Therefore, the actual relative efficiencies possible with DBS were underestimated. When changing the bremsstrahlung differential cross section from BH to NIST, the default photon cross section was used, i.
BEAMnrc, DOSXYZnrc and BEAMDP GUI users manual – NRC Publications Archive –
For the first three uncertainty calculations, only one bin is considered, i. Author information Article notes Copyright and License information Disclaimer.
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.
Estimates of the uncertainty using a significant number of bins e. Published online Nov 5.
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. The default differential cross section for the bremsstrahlung interactions is Bethe—Heitler BH. The results presented emphasize the importance of using a proper measure mabual the uncertainty on the quantity of interest in the calculation of the efficiency of a MC simulation.
The electron and photon cutoff energies were set to 0. Therefore, if one is concerned with kanual smallest statistical uncertainty variance in dose, then it is best to perform the treatment head PS simulation without VRTs.
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 five different ways, i. On the other hand, the scattered photon can be forced to interact again in the forcing zone, depending on how many forced interactions are still available. Monte Carlo MC simulation techniques are presently considered to be the most reliable method for radiation therapy treatment planning.
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. Three main areas were considered. 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.
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.
BEAMnrc: software tool to model radiation beams
Variance reduction techniques Bremsstrahlung photon splitting, Russian Roulette, and electron splitting The three options for bremsstrahlung photon splitting were used in this study, 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 chance to undergo bremsstrahlung events.
Russian Roulette was also considered with the photon splitting techniques. 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.
Medical Physics, Windsor,pp. CPU time is scaled to amnual of a single 2. In contrast, AEITs improve efficiency through the use of approximations.
The percentage difference between measured and a selection of MC calculated central-axis dose profiles is also shown. Monte Carlo, efficiency, variance reduction techniques, cross sections.
Monte Carlo transport parameters Maximum electron step, smax cm 10 Maximum electron loss per step MeV 0. A 3— Academic, New York,Vol. In addition, the relative efficiencies are dependent on the quantity of interest chosen, although one can claim from bdamnrc study that BEAMnrc MC calculations with the directional bremsstrahlung splitting technique yields the highest efficiency for all quantities of interest analyzed.
The parameter values mnaual 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. Data Tables 26 5—E The boundary crossing algorithm together with the electron transport algorithm constitutes the condensed history technique used by a particular MC code system.
The uncertainty was calculated in five different ways, i. The use of sophisticated variance reduction and approximate efficiency improving techniques, combined with parallel processing in computer clusters and the continuing increase in computing power, will help make MC-based treatment planning a mainstay option beannrc radiation oncology departments.
The three options for bremsstrahlung photon splitting were used in this study, i. 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. 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.
BEAMnrc: software tool to model radiation beams – National Research Council Canada
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.
The percentage difference between measured and a selection of MC calculated central and off-axis dose profiles is also shown. If one uses VRTs e. Once it leaves the forcing zone, the unscattered photon may interact again, depending on the sampled path length.