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Medical Physics / Volume 38 / Issue 12 / RADIATION THERAPY PHYSICS

Med. Phys. 38, 6513 (2011); http://dx.doi.org/10.1118/1.3660770 (15 pages)

Calculation of kQclin,Qmsrfclin,fmsr for several small detectors and for two linear accelerators using Monte Carlo simulations

P. Francescon, S. Cora, and N. Satariano

Department of Medical Physics, ULSS 6 – 36100 Vicenza, Italy

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(Received 4 July 2011; accepted 25 October 2011; revised 20 October 2011; published online 17 November 2011)

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Purpose: The scope of this study was to determine a complete set of correction factors for several detectors in static small photon fields for two linear accelerators (linacs) and for several detectors.
Methods: Measurements for Monte Carlo (MC) commissioning were performed for two linacs, Siemens Primus and Elekta Synergy. After having determined the source parameters that best fit the measurements of field specific output factors, profiles, and tissue-phantom ratio, the generalized version of the classical beam quality correction factor for static small fields, kQclin,Qmsrfclin,fmsr, were determined for several types of detectors by using the egs_chamber Monte Carlo user code which can accurately reproduce the geometry and the material composition of the detector. The influence of many parameters (energy and radial FWHM of the electron beam source, field dimensions, type of accelerator) on the value of kQclin,Qmsrfclin,fmsr was evaluated. Moreover, a MC analysis of the parameters that influence the change of kQclin,Qmsrfclin,fmsr as a function of field dimension was performed. A detailed analysis of uncertainties related to the measurements of the field specific output factor and to the Monte Carlo calculation of kQclin,Qmsrfclin,fmsr was done.
Results: The simulations demonstrated that the correction factor kQclin,Qmsrfclin,fmsr can be considered independent from the quality beam factor Q in the range 0.68 ± 0.01 for all the detectors analyzed. The kQclin,Qmsrfclin,fmsr of PTW 60012 and EDGE diodes can be assumed dependent only on the field size, for fields down to 0.5 × 0.5 cm2. The microLion, and the microchambers, instead, must be used with some caution because they exhibit a slight dependence on the radial FWHM of the electron source, and therefore, a correction factor only dependent on field size can be used for fields ≥0.75 × 0.75 and ≥1.0 × 1.0 cm2, respectively. The analysis of uncertainties gave an estimate of uncertainty for the 0.5 × 0.5 cm2 field of about 0.7% (1σ) for kQclin,Qmsrfclin,fmsr factor and of about 1.0% (1σ) for the field output factor, ΩQclin,Qmsrfclin,fmsr, of diodes, microchambers, and microLion.
Conclusions: Stereotactic diodes with the appropriate kQclin,Qmsrfclin,fmsr are recommended for determining ΩQclin,Qmsrfclin,fmsr of small photon beams.

© 2011 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

The contribution of Dr. Iwan Kawrakow (Siemens AG, Heidelberg, Germany) to the Monte Carlo commissioning of the accelerators as well as to the definition of several egs_chamber input files, was fundamental to obtain a very high accuracy in the determination of the source parameters and in the simulations of the detectors. Moreover, Dr. Iwan Kawrakow and Dr. Hugo Bouchard (Centre Hospitalier de l’Universitè de Montreal, Canada) provided very useful hints on the determination of uncertainties.

Article Outline

  1. INTRODUCTION
  2. MATERIAL AND METHODS
    1. Monte Carlo commissioning
      1. Measurements
      2. Monte Carlo simulations of OFdetfclin , OAR det , and TPR det
    2. Monte Carlo correction factors kQclin,Q10×10fclin,f10×10 for static fields
    3. Estimate of uncertainties
      1. Experimental uncertainties
      2. Monte Carlo Uncertainties of OFdetfclin and of kQclin,Qmsrfclin,fmsr
        1. Statistical uncertainties (type A).
        2. Systematic uncertainties (type B)
  3. RESULTS AND DISCUSSION
    1. MC commissioning
    2. Correction Factors
    3. Uncertainties
      1. Experimental uncertainties
      2. Monte Carlo Uncertainties OFdetfclin and of kQclin,Q10×10fclin,f10×10
  4. CONCLUSIONS
Digital Object Identifier
http://dx.doi.org/10.1118/1.3660770

KEYWORDS and PACS

Keywords

biological tissues, biomedical measurement, diodes, linear accelerators, Monte Carlo methods, phantoms, radiation therapy, sensors, small beams, correction factor, output factor, Monte Carlo, detector

PACS

  • 87.53.Jw

    Therapeutic applications, including brachytherapy

  • 07.07.Df

    Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

  • 87.55.K-

    Monte Carlo methods

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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