Simple beam models for Monte Carlo photon beam dose calculations in radiotherapy

Abstract

Monte Carlo (code GEANT) produced 6 and 15 MV phase space (PS) data were used to define several simple photon beam models. For creating the PS data the energy of starting electrons hitting the target was tuned to get correct depth dose data compared to measurements. The modeling process used the full PS information within the geometrical boundaries of the beam including all scattered radiation of the accelerator head. Scattered radiation outside the boundaries was neglected. Photons and electrons were assumed to be radiated from point sources. Four different models were investigated which involved different ways to determine the energies and locations of beam particles in the output plane. Depth dose curves, profiles, and relative output factors were calculated with these models for six field sizes from 5x5 to 40x40cm2 and compared to measurements. Model 1 uses a photon energy spectrum independent of location in the PS plane and a constant photon fluence in this plane. Model 2 takes into account the spatial particle fluence distribution in the PS plane. A constant fluence is used again in model 3, but the photon energy spectrum depends upon the off axis position. Model 4, finally uses the spatial particle fluence distribution and off axis dependent photon energy spectra in the PS plane. Depth dose curves and profiles for field sizes up to 10x10cm2 were not model sensitive. Good agreement between measured and calculated depth dose curves and profiles for all field sizes was reached for model 4. However, increasing deviations were found for increasing field sizes for models 1-3. Large deviations resulted for the profiles of models 2 and 3. This is due to the fact that these models overestimate and underestimate the energy fluence at large off axis distances. Relative output factors consistent with measurements resulted only for model 4.

Monte Carlo source model for photon beam radiotherapy: photon source characteristics

“…This contrasts with previous publications, in which SSDs between 80 and 120 cm were studied, 12 but mostly SSDs of 90 or 100 cm were used. 3,11,[15][16][17][18]20,23,33,34 …”

“…Apart form using analytical source models or the PS file itself as a source model, another approach, which overcomes these limitations, is to create a histogram-based source model based on the PSD. 8,[12][13][14][15][16][17] By sampling the initial parameters of a particle from these histogram distributions, virtually no particle is reused, thereby reducing the latent variance. A further advantage of histogram-based source models is that the sampling is faster than reading particles from a PS file.…”

“…This benchmark allows investigating the shape of the dose distribution for each individual source, e.g., the change in depth for the dose maximum for each subsource, compared with other published studies in which the total dose distribution is shown. 11,12,[15][16][17]20 Furthermore, the subsource dose characterization has the potential to ease the commissioning procedure, since information about which subsource needs to be tuned can be obtained by comparing measured dose distributions with those calculated for the individual sources. Dose distribution comparisons are performed for SSDs of 50, 75, 100, and 200 cm to validate the source model for a large range of conditions.…”

“…Previously developed histogram-based source models are closely related to the underlying accelerator. 8,[15][16][17] These source models reconstruct the photon beam below the secondary collimator, i.e., they make use of information about the collimator jaws. Therefore, it will be difficult to use such models if, for example, one pair of jaws is replaced by a multileaf collimator.…”

6-MV photon beam modeling for the Varian Clinac iX by using the Geant4 virtual jaw

“…Thus, it is anticipated that the efficiency of the virtual jaw VSM will increase by approximately 67% compared with the conventional VSM that uses shielding from the jaw. The reduction in the rate of the number of particles was approximately 10% higher than the rate of the shielded area, which is attributed to the increase in the beam fluence due to the increase in the radius from the central axis, arising from the effects of a flattening filter [6,21]. Therefore, because the jaw has the characteristic of shielding from the edges of the radiation field, the rate of reduction in the number of particles appears larger than the rate of the shielded area.…”