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Medical Physics / Volume 39 / Issue 1 / RADIATION THERAPY PHYSICS

Med. Phys. 39, 40 (2012); http://dx.doi.org/10.1118/1.3668315 (8 pages)

Monte Carlo linear accelerator simulation of megavoltage photon beams: Independent determination of initial beam parameters

Sigrun Saur Almberg

Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway and Department of Oncology and Radiotherapy, St. Olavs University Hospital, NO-7006 Trondheim, Norway

Jomar Frengen

Department of Oncology and Radiotherapy, St. Olavs University Hospital, NO-7006 Trondheim, Norway

Arve Kylling

Department of Oncology and Radiotherapy, Ålesund Hospital, NO-6026 Ålesund, Norway

Tore Lindmo

Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway

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(Received 18 May 2011; accepted 21 November 2011; revised 24 October 2011; published online 9 December 2011)

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Purpose: To individually benchmark the incident electron parameters in a Monte Carlo model of an Elekta linear accelerator operating at 6 and 15 MV. The main objective is to establish a simplified but still precise benchmarking procedure that allows accurate dose calculations of advanced treatment techniques.
Methods: The EGSnrc Monte Carlo user codes BEAMnrc and DOSXYZnrc are used for photon beam simulations and dose calculations, respectively. A 5 × 5 cm2 field is used to determine both the incident electron energy and the electron radial intensity. First, the electron energy is adjusted to match the calculated depth dose to the measured one. Second, the electron radial intensity is adjusted to make the calculated dose profile in the penumbrae region match the penumbrae measured by GafChromic EBT film. Finally, the mean angular spread of the incident electron beam is determined by matching calculated and measured cross-field profiles of large fields. The beam parameters are verified for various field sizes and shapes.
Results: The penumbrae measurements revealed a non-circular electron radial intensity distribution for the 6 MV beam, while a circular electron radial intensity distribution could best describe the 15 MV beam. These electron radial intensity distributions, given as the standard deviation of a Gaussian distribution, were found to be 0.25 mm (in-plane) and 1.0 mm (cross-plane) for the 6 MV beam and 0.5 mm (both in-plane and cross-plane) for the 15 MV beam. Introducing a small mean angular spread of the incident electron beam has a considerable impact on the lateral dose profiles of large fields. The mean angular spread was found to be 0.7° and 0.5° for the 6 and 15 MV beams, respectively.
Conclusions: The incident electron beam parameters in a Monte Carlo model of a linear accelerator could be precisely and independently determined by the benchmarking procedure proposed. As the dose distribution in the penumbra region is insensitive to moderate changes in electron energy and angular spread, accurate penumbra measurements is feasible for benchmarking the electron radial intensity distribution. This parameter is particularly important for accurate dosimetry of mlc-shaped fields and small fields.

© 2012 American Association of Physicists in Medicine

ACKNOWLEDGMENT

The study was made possible by the financial support from the Strategic Research Area Medical Technology at the Norwegian University of Science and Technology.

Article Outline

  1. INTRODUCTION
  2. MATERIALS AND METHODS
    1. B EAM nrc accelerator model
    2. Phantom simulations in D OSXYZ nrc
      1. Depth doses
      2. Penumbra profiles
      3. Cross-field profiles of large fields
    3. Dosimetric measurements
      1. Depth and cross-field dose profile measurements
      2. Penumbra, buildup, and MLC-field measurements
  3. RESULTS
    1. Depth doses
    2. Penumbra profiles
    3. Cross-field profiles
    4. Build-up region
    5. MLC-shaped fields
  4. DISCUSSION
  5. CONCLUSION
Digital Object Identifier
http://dx.doi.org/10.1118/1.3668315

KEYWORDS and PACS

Keywords

dosimetry, Gaussian distribution, linear accelerators, Monte Carlo methods, radiation therapy, Monte Carlo, photon beams, penumbra

PACS

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
William R. Hendee, Editor
Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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