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General purpose Monte Carlo code for simulation of particle transport is used to study the basic dosimetric parameters like percentage depth dose and dose profiles and compared with the experimental measurements from commercial dual energy medical linear accelerator. Varian Clinac iX medical linear accelerator with dual energy photon beams (6 and 15 MV) is simulated using FLUKA. FLAIR is used to visualize and edit the geometry. Experimental measurements are taken for 100 cm source-to-surface (SSD) in 50 × 50 × 50 cm^{3} PTW water phantom using 0.12 cc cylindrical ionization chamber. Percentage depth dose for standard square field sizes and dose profiles for various depths are studied in detail. The analysis was carried out using ROOT (a DATA analysis frame work developed at CERN) system. Simulation result shows good agreement in percentage depth dose and beam profiles with the experimental measurements for Varian Clinac iX dual energy medical linear accelerator.

Monte Carlo (MC) method has become a powerful tool in radiation therapy for studying the dosimetric parameters. It is a common objective of medical physics to achieve an accuracy of better than ±5% for the delivery of dose. But this can be realized only if the dose calculation accuracy is better than ±2%. Therefore, in the future, Monte Carlo algorithms will have a clear preference compared with all other methods of dose calculation. At present, different Monte Carlo codes are used widely for modeling medical linear accelerators [

Monte Carlo methods are applied in radiation therapy to analyze the adequacy of linac head components, to benchmark dose calculation models, and to study the beam characteristics [

Monte Carlo for treatment planning is practically impossible in Indian scenarios, where there are a large number of patients with limited facilities. A Monte Carlo model takes considerable time to optimize individual treatment plans. However, the beam characteristic and other parameters can be tuned using any available Monte Carlo codes for all clinical institutions [

In this study, we modeled a commercial medical linear accelerator, Varian Clinac iX, using FLUKA [

Dual energy photon beam from Varian Clinac iX (Varian Medical Systems, Palo Alto, CA, USA) is modeled using FLUKA Monte Carlo. We simplified this model without altering any components in the linac head as per manufactures description. Version 2011.2b.4 (updated in 11 September 2013) of FLUKA code was utilized in the present research. This code is a multipurpose Monte Carlo code which has been developed for accurate simulation of the interaction and propagation in matter of about 60 different particles, including photons and electrons from 1 keV to thousands of TeV, neutrinos, muons of any energy, hadrons of energies up to 20 TeV and all the corresponding antiparticles, neutrons down to thermal energies, and heavy ions. A well-described physical phenomenon in our energy range is the power of this tool.

Figure ^{3} cube is defined at 100 cm from the target with 1 cm plastic outer covering layer except in beam entrance plane.

The schematic representation of Varian Clinac iX medical linear accelerator and water phantom simulated in our study.

FLAIR gives wide options to choose desired FLUKA input cards (the type of interaction and transport thresholds can be set in the physics and transport section of the cards). EM-CASCA card is used as a default in both cases. Number of primary photons for this study was set at

All the measurements were carried out by Scanditronix automatic water phantom (Blue Phantom, Scanditronix Wellhofer AB, Sweden) and a cylindrical ionization chamber of 0.12 cm^{3} active volume. Both PDD curves and beam profiles were taken using 1 mm step size for measurements. For comparison 2 mm readings were considered to obtain better illustration in PDD and 1 mm binning for profile plots. Beam quality measurements were carried out using TPS 20/10 on both energies before the data taking. Room lasers and standard scalars are used for accurate position of water phantom.

Standard 10 × 10 cm^{2} field size is used to obtain the beam energy for both photon energies. Percentage depth dose for this field size is compared between Monte Carlo calculated and the measurements. This is a common approach by many authors [

Three independent measurements were taken to reduce the uncertainties in maximum dose point and the comparison is performed to this normalized point. Initial energy value of photon beam is taken as the default value as set by the manufacturer. Further, the value was increased or decreased according to the result of the first comparison between calculations and measurements. In 6 MV photon beam, the energies of 6.0, 6.2, 6.3, and 6.4 MeV were simulated and for 15 MV photon beam, the energies of 15.0, 15.2, 15.3, and 15.4 MeV were simulated. Local differences between simulation and measurement results were calculated for accurate comparison between results.

Using the modeled dual energy linac, percentage depth dose and beam profiles were generated. PDD of both energies for 5 × 5 cm^{2}, 10 × 10 cm^{2}, 15 × 15 cm^{2}, and 20 × 20 cm^{2} were generated using FLAIR of FLUKA Monte Carlo. This study estimates the accuracy and adequacy of our simplified model. The beam profiles of 6 and 15 MV photon, including sharp dose fall off penumbra region (dose region between 80% and 20%) for 10 × 10 cm^{2} at depth of maximum dose and at 5 cm depth is compared in this study.

The Monte Carlo simulated percent depth dose curves and profiles across the central axis for flattened beams at different depths were compared against the measured data to verify the goodness of the MC model. The PDD curves are only sensitive to the mean energy of the incident electron, but dose profile curves are both dependent on the mean energy. Therefore, logically, one can use PDD to find the optimal energy. In our study 10 × 10 cm^{2} field size is used to optimize the beam energy. The optimum beam energy for 6 MV and 15 MV clinical photon is found as 6.2 MeV and 15.3 MeV. These values are largely dependent on the Monte Carlo models where linac head components are simulated.

A comparison for the depth dose curves is shown in Figures ^{2}, 10 × 10 cm^{2}, 15 × 15 cm^{2}, and 20 × 20 cm^{2} show good agreement between measurement and simulation. Relative errors ((calculated − measured)/calculated) over the depth are plotted separately in the previously mentioned figures. This small variation may be due to the changes in the geometry, especially in the truncated shape of flattening filter. Larger field sizes are not considered in this work due the limited use of such fields in our routine clinical practice

Comparison of calculated and measured percentage depth dose for 6 MV photon beam for field sizes 5 × 5 cm^{2} (a), 10 × 10 cm^{2} (b), 15 × 15 cm^{2} (c), and 20 × 20 cm^{2} (d). Relative error is shown on right side.

Comparison of calculated and measured percentage depth dose for 15 MV photon beam for field sizes 5 × 5 cm^{2} (a) 10 × 10 cm^{2} (b), 15 × 15 cm^{2} (c), and 20 × 20 cm^{2} (d). Relative error is shown on right side.

A cross-plane dose profiles comparison is shown in Figure

Comparison of calculated and measured depth dose profile and relative error (right side) for 6 MV photon beam for depth of maximum dose (a) and 5 cm depths (b).

Comparison of calculated and measured depth dose profile and relative error (right side) for 15 MV photon beam for depth of maximum dose (a) and 5 cm depths (b).

This simplified FLUKA Monte Carlo model (avoiding the multileaf collimator) of Varian Clinac iX linac, based on the manufacturer’s information, gives good agreement in percentage depth dose curves and beam profile plots. Monte Carlo model with retracted multileaf collimator shows perfect agreement for basic physical properties in standard field sizes. Although percentage depth dose values in the build-up region show more dependence on the field size (Figures

The authors declare no conflict of interests.

The authors sincerely acknowledge the assistance provided by Professor Roberto Cirio (INFN and University of Torino, Italy). The authors are grateful to Dr. Vincenzo Moncao (INFN and University of Torino, Italy) for his periodic suggestions and fruitful discussion.