Document Type : Original Paper
Authors
1
Professor, Medical Physics Research Center, Bu-Ali Research Institute, Mashhad University of Medical Sciences, Mashhad, Iran.
2
Ph.D. Student in Medical Physics, Mashhad university of Medical Sciences, Mashhad, Iran
3
Assistant Professor, Medical Physics Research Center, Bu-Ali Research Institute, Mashhad University of Medical Sciences, Mashhad, Iran.
4
Assistant Professor, Radiation Oncology Dept., Imam Reza Hospital, Mashhad University of Medical Sciences, Mashhad, Iran.
5
Assistant Professor, Nuclear Physics Dept., Ferdowsi University, Mashhad, Iran.
Abstract
ntroduction: Monte Carlo simulation is the most accurate method of simulating radiation transport and
predicting doses at different points of interest in radiotherapy. A great advantage of the Monte Carlo method
compared to the deterministic methods is the ability to deal accurately with any complex geometry. Its
disadvantage is the extremely long computing time required to obtain a dose distribution with good statistical
accuracy.
Materials and Methods: The MCNP-4C Monte Carlo code was used to simulate a 9 MV photon beam from
a Neptun 10PC linear accelerator. The accelerator was modeled as a complete unit consisting of a target, exit
window, initial collimator, primary collimator, flattening filter, monitor chamber and secondary collimator.
The geometrical details and the composition of each component was either obtained from the manufacturer or
was directly measured. The simulation of the source was performed in a two step process. Initially, the
electron source was defined. Secondly, the bremsstrahlung energy spectra and the fluence distribution at the
scoring planes were used to define the photon source. The simulated electron beam energy followed a
Gaussian distribution, with FWHM equal to 12% in nominal energy. The used intensity distribution of the
electron beam also followed a Gaussian distribution with a FWHM equal to 0.34 cm. To compute the photon
beam data a 50 × 50 × 40 cm
3
water phantom located at SSD = 100 cm was simulated. The depth dose and
the dose profile curves were calculated for four different field sizes (5×5, 10×10, 20×20 and 30×30 cm
2
) and
compared against the measured values. The low-energy cut-off for the photons and electrons was 10 and 500
KeV, respectively. The measurements were carried out by using a Scanditronix dose scanning system and a
0.12 cm
3
RK ionization chamber.
Results: To verify the simulated model, the calculated Monte Carlo dose data were compared against the
corresponding measured values. The energy spectra and the angular distribution of the x-ray beam generated
by the Neptun 10PC linac was examined. The result showed an efficiency of about 73% for the production of
bermsstrahlung photon by the target. The agreement between the calculated and the measured depth dose and
the dose profile was generally better than 2% for all the fields.
Discussion and Conclusion: The simulation of the Neptun 10PC linac performed in this work is capable of
computing the depth dose data and the beam profiles in water phantom for all the predefined fields including
5×5, 10×10, 20×20 and 30×30 cm
2
. Therefore, it can be concluded that MCNP-4C is a suitable tool for the
dose calculation in radiotherapy. The simulated linac machine and the resulting data can be used to predict
the dose distribution in all complex fields.
Keywords
Main Subjects
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