Decoupling initial electron beam parameters for Monte Carlo photon beam modelling by removing beam-modifying filters from the beam path

Abstract

A new method is presented to decouple the parameters of the incident e(-) beam hitting the target of the linear accelerator, which consists essentially in optimizing the agreement between measurements and calculations when the difference filter, which is an additional filter inserted in the linac head to obtain uniform lateral dose-profile curves for the high energy photon beam, and flattening filter are removed from the beam path. This leads to lateral dose-profile curves, which depend only on the mean energy of the incident electron beam, since the effect of the radial intensity distribution of the incident e- beam is negligible when both filters are absent. The location of the primary collimator and the thickness and density of the target are not considered as adjustable parameters, since a satisfactory working Monte Carlo model is obtained for the low energy photon beam (6 MV) of the linac using the same target and primary collimator. This method was applied to conclude that the mean energy of the incident e- beam for the high energy photon beam (18 MV) of our Elekta SLi Plus linac is equal to 14.9 MeV. After optimizing the mean energy, the modelling of the filters, in accordance with the information provided by the manufacturer, can be verified by positioning only one filter in the linac head while the other is removed. It is also demonstrated that the parameter setting for Bremsstrahlung angular sampling in BEAMnrc ('Simple' using the leading term of the Koch and Motz equation or 'KM' using the full equation) leads to different dose-profile curves for the same incident electron energy for the studied 18 MV beam. It is therefore important to perform the calculations in 'KM' mode. Note that both filters are not physically removed from the linac head. All filters remain present in the linac head and are only rotated out of the beam. This makes the described method applicable for practical usage since no recommissioning process is required.

Accuracy of patient dose calculation for lung IMRT: A comparison of Monte Carlo, convolution/superposition, and pencil beam computations

“…16 Details on the commissioning and validation of the 6 MV and 18 MV photon beams have been published as well. 25,26 Moreover, none of the data presented in this paper indicate that any systematic errors may still be present when MCDE is used to perform patient dose calculations. The calculations in water that were performed for patient number 4 show that the observed differences mainly arise from differences in particle transport modeling within the patient, as the observed deviations between MCDE and the other dose calculation algorithms strongly decreased compared to the calculations in medium.…”

“…23 Reynaert et al 24 have described the working of MCDE. The commissioning of the 6 MV and 18 MV photon beams of the Elekta SLiplus linear accelerator at Ghent University Hospital has been reported in detail elsewhere, 25,26 including data on the comparison of simulations and measurements. The MLC was modeled using BEAMnrc's MLCE component module ͑CM͒, which takes into account the tongue-andgroove design of the MLC as well as the air gap between the leaves.…”

Benchmarking of Monte Carlo simulation of bremsstrahlung from thick targets at radiotherapy energies

“…At the time of the measurements the energy calibration was based largely on a measurement of the photoneutron threshold in 16 O. Since then, a more detailed energy calibration was carried out using an independent magnetic spectrometer.…”

“…There is an option to select only the leading term of this distribution, but this can lead to differences of a few percent in the angular distribution at 10°off axis. 16 Transport parameters included electron lower energy cutoffs ECUT and AE of 0.7 MeV, and photon lower energy cut-offs PCUT and AP of 1 keV, with the boundary crossing algorithm set to EXACT and the electron-step algorithm set to PRESTA-II. The maximal fractional energy loss per step ESTEPE was 0.25, maximum first elastic moment per step XIMAX of 0.5, and skin depth for BCA 3.0 mean free paths.…”

Calculation of for several small detectors and for two linear accelerators using Monte Carlo simulations

“…These parameters are not known, and several methods on how to determine them have been proposed in the literature. [21][22][23][24][25][26][27][28][29][30] Nevertheless, the use of MC introduces uncertainties, so it is conceivable that away from the simple calculation of the dose to verify the experimental measurements, the systematic uncertainties propagation can "disturb" the final results. In the papers of Muir et al 31 and Wulff et al, 32 the effect of systematic uncertainties of MC calculations of k Q were quantified for several cylindrical ionization chambers.…”