Monte Carlo calculations of an Elekta Precise SL-25 photon beam model

Abstract

BackgroundMonte Carlo (MC) simulations have been used extensively for benchmarking photon dose calculations in modern radiotherapy using linear accelerators (linacs). Moreover, a major barrier to widespread clinical implementation of MC dose calculation is the difficulty in characterising the radiation source using data reported from manufacturers.PurposeThis work aims to develop a generalised full MC histogram source model of an Elekta Precise SL-25 linac (electron exit window, target, flattening filter, monitor chambers and collimators) for 6 MV photon beams used in standard therapies. The inclusion of many different probability processes such as scatter, nuclear reactions, decay, capture cross-sections and more led to more realistic dose calculations in treatment planning and quality assurance.Materials and methodsTwo different codes, MCNPX 2·6 and EGSr-BEAM, were used for the calculation of particle transport, first in the geometry of the internal/external accelerator source, and then followed by tracking the transport and energy deposition in phantom-equivalent tissues. A full phase space file was scored directly above the upper multilayer collimator’s jaws to derive the beam characteristics such as planar fluence, angular distribution and energy spectrum. To check the quality of the generated photon beam, its depth dose curves and cross-beam profiles were calculated and compared with measured data.ResultsIn-field dose distributions calculated using the accelerator models were tuned to match measurement data with preliminary calculations performed using the accelerator information provided by the manufacturer. Field sizes of 3×3, 5×5, 10×10, 15×15 and 20×20 cm2were analysed. Local differences between calculated and measured curve doses beneath 2% were obtained for all the studied field sizes. Higher discrepancies were obtained in the air–water interface, where measurements of dose distributions with the ionisation chamber need to be shifted for the effective point of measurement.ConclusionThe agreements between MC-calculated and measured dose distributions were excellent for both codes, showing the strength and stability of the proposed model. Beam reconstruction methods as direct input to dose-calculation codes using the recorded histograms can be implemented for more accurate patient dose estimation.

Empirical modeling of the percent depth dose for megavoltage photon beams

“…The PDD for four high-energy photon energies of 4 MV, 6 MV, 10 MV, 18 MV were modeled in this study. The measurements data were 6 MV, 10 MV (Elekta infinity, Stockholm, Sweden, and Varian trueBeam, Palo Alto, Ca), while 4 MV and 18 MV were published data [ 7 , 8 ].…”

Monte-Carlo techniques for radiotherapy applications I: introduction and overview of the different Monte-Carlo codes

“…72 The MCNP code is available to the worldwide user community, subject to USA national security restrictions and distributed through the Radiation Safety Information Computational Center, part of Oak Ridge National Laboratory. MCNP has been applied to a wide range of medical physics problems 73 including the modelling of medical linear accelerators, [74][75][76][77][78] and imaging systems such as radiotherapy electronic portal imaging devices (EPIDs), [79][80][81] CT 82 and PET/CT. 83 MCNP has also been used to develop models for kV intraoperative radiotherapy, 84,85 Brachytherapy, [86][87][88] proton therapy 89,90 and gamma irradiator devices 91,92 including the Leksell Gamma Knife 93,94…”

Monte-Carlo techniques for radiotherapy applications II: equipment and source modelling, dose calculations and radiobiology

“…The MC models were compared to measure dosimetry profile data in homogeneous water and homogeneous phantoms containing air, bone and lung equivalent materials. 18 Toossi et al used the MCNPX code to simulate the 8, 12 and 14 MeV electron beams generated by a Siemens Primus linear accelerator. Simulated dosimetry profile data were compared to measured data for different field sizes including applicators as part of a commissioning process.…”