Development and validation of an electron Monte Carlo model for an Elekta Synergy® linear accelerator
Sachse, Karl Nicholas
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Background: The objective for this study was to develop a Monte Carlo (MC) EGSnrc based electron model for an Elekta Synergy® 160-leaf Agility™ linear accelerator (linac) and to validate it against measurements. The requirement was that the developed model should be able to reproduce central axis (CAX) percentage depth dose (PDD) curves, off-axis profiles (OAPs) and relative output factors (ROFs) within 2%/2mm of a subset of measured linac data. Methods: EGSnrc/BEAMnrc component modules were used to model the linac according to vendor supplied specifications, where multi-leaf collimator and Jaw positions for each electron energy-applicator combination were obtained from log files. Since the initial electron beam properties (focal spot size and shape, energy spectrum) were unknown, the effects of these parameters on electron CAX PDDs and OAPs were investigated and by means of iterations the set of parameters producing the best match with measured water tank data were identified. Phase space files generated by these models were used as source input in EGSnrc/DOSXYZnrc where a unit-density water phantom was modelled, and dose distributions were calculated and extracted accordingly. Six electron nominal energies, 11 field sizes and two source-to-surface distances (SSDs) were evaluated. MATLAB® scripts were developed to process and analyze both simulated and measured data. Results: BEAMnrc could successfully be used to model each component in the path of the initial electron beam. The electron focal spot shape was determined from measured inline and crossline profiles and was found to be circular since secondary scattering foil geometries exemplified radial symmetry. The full width at half maximum (FWHM) of the focal spot (assuming a Gaussian intensity distribution) was determined iteratively from simulations and a set value of 1.50 mm was chosen. A monoenergetic and two different poly-energetic energy spectrums (symmetrical Gaussian and asymmetrical experimental spectrum) were investigated for their effects on CAX PDDs and OAPs. The asymmetrical energy spectrum with a low-energy tail produced satisfactory results within component dimensional tolerances and solved the match in the build-up region for all electron energies. Simulated data complied to measured data with a 100 % pass rate using a 2%/2mm criterion. Conclusions: The developed MC EGSnrc electron model was able to predict dose distributions within 2%/2mm of measured PDDs and OAPs, and ROFs within 3 %. The underlying success of the model is embedded in the experimental energy spectrum which provided a valuable free parameter which, by fine adjustment, improved the match in the build-up region of dose distributions. Furthermore, focal spot parameters could be determined by means of simulations and thereby circumvented the difficulty associated with the measurement of the focal spot.