Developement a simple point source model for Elekta SL-25 linear accelerator using MCNP4C Monte Carlo code

Monte Carlo (MC) method is the most accurate method for dose calculations in radiotherapy. It has been used in many applications including external photon and electron beam therapy, brachytherapy, etc. (1). MC method has overcome the deficiencies of analytical dose calculation methods in conditions such as tissue inhomogeneities and electronic disequilibrium with small fields(2-5). For the application of MC calculations in external beam radiotherapy, a 2-step development approach is followed. The first step requires a realistic model of the linear accelerator or Cobalt-60 machine treatment head to quantify the energy, angular, and positional, distribution of photons and electrons entering the patient (6-10). This initial simulation is performed once and requires a reformat of the treatment head Phase-Space (PS) distribution into a virtual source model (9, 10). This virtual source must recreate the distribution of photons and electrons exiting the treatment head without the time-consuming process of simulating individual photon and electron trajectories through the various patient-independent structures of the accelerator treatment head, such as the flattening filter. The second step utilizes this virtual source to simulate the depth dose and profile characteristics incident upon a water phantom. A phase space file containing the energy, angular, and spatial distribution of photons and electrons crossing a plane normal to the beam central axis contains all particle information necessary for calculating the dose in a patient and the computer storage requirements can be quite large, requiring up to 1 Gigabyte of storage space. Bearing in mind that several beam qualities and a large number of field sizes are available per linac, the need to compress PS data becomes obvious. Therefore, beam models were constructed(6-7). The feasibility of a beam model for radiotherapy treatment planning has to be *Corresponding author: Dr. Asghar Mesbahi, Department of Medical Physics, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran. Fax: +98 411 3364660 E-mail: asgharmesbahi@yahoo.com Background: Monte Carlo (MC) modeling of a linear accelerator is a prerequisite for Monte Carlo dose calculations in external beam radiotherapy. In this study, a simple and efficient model was developed for Elekta SL-25 linear accelerator using MCNP4C Monte Carlo code. Materials and Methods: The head of Elekta SL-25 linac was simulated for 6 and 18 MV photon beams using MCNP4C MC code. Energy spectra and fluence distribution of photons crossing the phase space plane were calculated. A simple point source model was developed based on calculated photon spectra and spatial distribution. Using this model, percent depth doses (PDDs), and beam profiles were calculated for different field sizes. The results of MC calculations were compared with measurements. Results: There was a good agreement between MC calculations and measurement for descending part of PDD curves. However, comparing calculated PDDs with measurement showed up to 10% differences for build up region of PDD curves for both energies. For beam profiles, there was 2% difference in flat region and up to 15% difference was seen for out of field region. These results were acceptable according to the recommended criteria. Using this model, the run time was decreased 24 times in comparison to original full Monte Carlo method. Conclusion: Our study showed the presented model to be accurate and effective for MC calculations in radiotherapy treatment planning. Also, it substantially lowers MC runtime for radiotherapy purposes. Iran. J. Radiat. Res., 2006; 4 (1): 7-14