A virtual photon source model of an Elekta linear accelerator with integrated mini MLC for Monte Carlo based IMRT dose calculation

Abstract

A dedicated, efficient Monte Carlo (MC) accelerator head model for intensity modulated stereotactic radiosurgery treatment planning is needed to afford a highly accurate simulation of tiny IMRT fields. A virtual source model (VSM) of a mini multi-leaf collimator (MLC) (the Elekta Beam Modulator (EBM)) is presented, allowing efficient generation of particles even for small fields. The VSM of the EBM is based on a previously published virtual photon energy fluence model (VEF) (Fippel et al 2003 Med. Phys. 30 301) commissioned with large field measurements in air and in water. The original commissioning procedure of the VEF, based on large field measurements only, leads to inaccuracies for small fields. In order to improve the VSM, it was necessary to change the VEF model by developing (1) a method to determine the primary photon source diameter, relevant for output factor calculations, (2) a model of the influence of the flattening filter on the secondary photon spectrum and (3) a more realistic primary photon spectrum. The VSM model is used to generate the source phase space data above the mini-MLC. Later the particles are transmitted through the mini-MLC by a passive filter function which significantly speeds up the time of generation of the phase space data after the mini-MLC, used for calculation of the dose distribution in the patient. The improved VSM model was commissioned for 6 and 15 MV beams. The results of MC simulation are in very good agreement with measurements. Less than 2% of local difference between the MC simulation and the diamond detector measurement of the output factors in water was achieved. The X, Y and Z profiles measured in water with an ion chamber (V = 0.125 cm(3)) and a diamond detector were used to validate the models. An overall agreement of 2%/2 mm for high dose regions and 3%/2 mm in low dose regions between measurement and MC simulation for field sizes from 0.8 x 0.8 cm(2) to 16 x 21 cm(2) was achieved. An IMRT plan film verification was performed for two cases: 6 MV head&neck and 15 MV prostate. The simulation is in agreement with film measurements within 2%/2 mm in the high dose regions (> or = 0.1 Gy = 5% D(max)) and 5%/2 mm in low dose regions (<0.1 Gy).

Stereotactic, Single-Dose Irradiation of Lung Tumors: A Comparison of Absolute Dose and Dose Distribution Between Pencil Beam and Monte Carlo Algorithms Based on Actual Patient CT Scans

“…The differences between XVMC calculations and measurements in water were within 2%/1 mm for standard square fields from 2 Â 2 cm 2 to 30 Â 30 cm 2 (data not shown). Details of XVMC and the improved virtual photon energy fluence model were described previously (27,28).…”

Agility MLC transmission optimization in the Monaco treatment planning system

“…First, the linear accelerator photon beam is approximated using a virtual source model (VSM) consisting of a primary photon source, a scatter photon source, and an electron contamination source 6, 7, 8, 9. The VSM is used instead of MC transport through the components of the linear accelerator to speed up the calculation.…”

“…The VSM is used instead of MC transport through the components of the linear accelerator to speed up the calculation. Second, the primary collimator, jaws, and MLC are modeled using transmission probability filters (TPFs) 6, 7, 8, 9. Similar to the VSM, the TPFs are used instead of direct MC simulation to significantly reduce calculation times.…”

Multisource modeling of flattening filter free (FFF) beam and the optimization of model parameters

“…Cashmore 9 also reported similar results of dosimetric properties for a 6 MV FFF beam using an Elekta Precise linear accelerator. Vassiliev [5][6][7][8]10,11 In addition, some of them suggested novel methods of beam source modeling that could be used to improve the speed of MC-based dose calculation. 8,10 However, these methods are computationally intensive and it is difficult to apply them in clinical treatment planning.…”