Monte Carlo simulation of small electron fields collimated by the integrated photon MLC

Abstract

In this study, a Monte Carlo (MC)-based beam model for an ELEKTA linear accelerator was established. The beam model is based on the EGSnrc Monte Carlo code, whereby electron beams with nominal energies of 10, 12 and 15 MeV were considered. For collimation of the electron beam, only the integrated photon multi-leaf-collimators (MLCs) were used. No additional secondary or tertiary add-ons like applicators, cutouts or dedicated electron MLCs were included. The source parameters of the initial electron beam were derived semi-automatically from measurements of depth-dose curves and lateral profiles in a water phantom. A routine to determine the initial electron energy spectra was developed which fits a Gaussian spectrum to the most prominent features of depth-dose curves. The comparisons of calculated and measured depth-dose curves demonstrated agreement within 1%/1 mm. The source divergence angle of initial electrons was fitted to lateral dose profiles beyond the range of electrons, where the imparted dose is mainly due to bremsstrahlung produced in the scattering foils. For accurate modelling of narrow beam segments, the influence of air density on dose calculation was studied. The air density for simulations was adjusted to local values (433 m above sea level) and compared with the standard air supplied by the ICRU data set. The results indicate that the air density is an influential parameter for dose calculations. Furthermore, the default value of the BEAMnrc parameter 'skin depth' for the boundary crossing algorithm was found to be inadequate for the modelling of small electron fields. A higher value for this parameter eliminated discrepancies in too broad dose profiles and an increased dose along the central axis. The beam model was validated with measurements, whereby an agreement mostly within 3%/3 mm was found.

A Monte Carlo simulation framework for electron beam dose calculations using Varian phase space files for TrueBeam Linacs

“…These parameters are iteratively adjusted to achieve an agreement with measurement data. 20,29,32,33 It should be noted that inaccuracies in modeling the source distribution and the geometry, position, and material composition of Linac components, especially of the in-field treatment head components such as the ionization chamber and scattering foil can affect the final dose distribution. 34 While direct measurement of electron beam T VI.…”

Delivery validation of an automated modulated electron radiotherapy plan

“…Various groups [1][2][3][4][5] have reported on the feasibility of using the photon MLC as a possible collimation device, while others [6][7][8][9][10][11][12] have manufactured add-on electron MLCs that are positioned closer to the patient in an effort to minimize the penumbra width due to air scatter. It was reported that acceptable penumbra can be obtained with photon MLCs by using SSDs in the 70 cm range.…”

Measurement and Monte Carlo simulation for energy‐ and intensity‐modulated electron radiotherapy delivered by a computer‐controlled electron multileaf collimator

“…Various groups have been investigating an efficient and clinically practical means to deliver MERT. Previous research on the electron MLC (eMLC) for MERT delivery is documented in representative literatures by Ma et al, (2) Gauer et al, (10) Hogstrom et al, (11) Al-Yahya et al, (12) Vatanen et al, (13) and Eldib et al, (14) while efforts on using photon MLC (pMLC) to deliver MERT were also made by many investigators including du Plessis et al, (15) Klein et al, (16) Jin et al, (17) Karlsson et al, (18) and Mihaljevic et al (19) Recently, Connell et al (20) published a study on the use of scattering foil free beams for modulated electron radiotherapy, an experimental feasibility study on the use of scattering foil free beams for modulated electron radiotherapy, exploring the potential benefit of removing the scattering foil from the beam line due to the significant reduction of the bremsstrahlung tail dose.…”