Monte Carlo calculations for reference dosimetry of electron beams with the PTW Roos and NE2571 ion chambers.

PURPOSE To investigate recommendations for reference dosimetry of electron beams and gradient effects for the NE2571 chamber and to provide beam quality conversion factors using Monte Carlo simulations of the PTW Roos and NE2571 ion chambers. METHODS The EGSnrc code system is used to calculate the absorbed dose-to-water and the dose to the gas in fully modeled ion chambers as a function of depth in water. Electron beams are modeled using realistic accelerator simulations as well as beams modeled as collimated point sources from realistic electron beam spectra or monoenergetic electrons. Beam quality conversion factors are calculated with ratios of the doses to water and to the air in the ion chamber in electron beams and a cobalt-60 reference field. The overall ion chamber correction factor is studied using calculations of water-to-air stopping power ratios. RESULTS The use of an effective point of measurement shift of 1.55 mm from the front face of the PTW Roos chamber, which places the point of measurement inside the chamber cavity, minimizes the difference between R50, the beam quality specifier, calculated from chamber simulations compared to that obtained using depth-dose calculations in water. A similar shift minimizes the variation of the overall ion chamber correction factor with depth to the practical range and reduces the root-mean-square deviation of a fit to calculated beam quality conversion factors at the reference depth as a function of R50. Similarly, an upstream shift of 0.34 rcav allows a more accurate determination of R50 from NE2571 chamber calculations and reduces the variation of the overall ion chamber correction factor with depth. The determination of the gradient correction using a shift of 0.22 rcav optimizes the root-mean-square deviation of a fit to calculated beam quality conversion factors if all beams investigated are considered. However, if only clinical beams are considered, a good fit to results for beam quality conversion factors is obtained without explicitly correcting for gradient effects. The inadequacy of R50 to uniquely specify beam quality for the accurate selection of kQ factors is discussed. Systematic uncertainties in beam quality conversion factors are analyzed for the NE2571 chamber and amount to between 0.4% and 1.2% depending on assumptions used. CONCLUSIONS The calculated beam quality conversion factors for the PTW Roos chamber obtained here are in good agreement with literature data. These results characterize the use of an NE2571 ion chamber for reference dosimetry of electron beams even in low-energy beams.

[1]  Replacement correction factors for cylindrical ion chambers in electron beams. , 2009, Medical physics.

[2]  I Kawrakow,et al.  Efficiency improvements for ion chamber calculations in high energy photon beams. , 2008, Medical physics.

[3]  Alan Dusautoy,et al.  Primary standards of absorbed dose for electron beams , 2009 .

[4]  I Kawrakow,et al.  Investigation of systematic uncertainties in Monte Carlo-calculated beam quality correction factors , 2010, Physics in medicine and biology.

[5]  D. Rogers,et al.  Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams. , 2012, Medical physics.

[6]  Iwan Kawrakow,et al.  Calculations for plane-parallel ion chambers in 60Co beams using the EGSnrc Monte Carlo code. , 2003, Medical physics.

[7]  Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector. , 2010, Medical physics.

[8]  D. Rogers,et al.  Replacement correction factors for plane-parallel ion chambers in electron beams. , 2010, Medical physics.

[9]  A. Brahme,et al.  Recent Advances in Electron and Photon Dosimetry , 1986 .

[10]  G X Ding,et al.  R50 as a beam quality specifier for selecting stopping-power ratios and reference depths for electron dosimetry. , 1996, Medical physics.

[11]  J. Seuntjens,et al.  Modelling ionization chamber response to nonstandard beam configurations , 2008 .

[12]  Josep Sempau,et al.  Electron beam quality correction factors for plane-parallel ionization chambers: Monte Carlo calculations using the PENELOPE system. , 2004, Physics in medicine and biology.

[13]  M R McEwen,et al.  The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration. , 2003, Physics in medicine and biology.

[14]  Iwan Kawrakow,et al.  Effective point of measurement of thimble ion chambers in megavoltage photon beams. , 2009, Medical physics.

[15]  I Kawrakow,et al.  CSnrc: correlated sampling Monte Carlo calculations using EGSnrc. , 2004, Medical physics.

[16]  I. Kawrakow.  On the effective point of measurement in megavoltage photon beams. , 2006, Medical physics.

[17]  I. Kawrakow.  Accurate condensed history Monte Carlo simulation of electron transport. I. EGSnrc, the new EGS4 version. , 2000, Medical physics.

[18]  M R McEwen,et al.  Measured and Monte Carlo calculated kQ factors: Accuracy and comparison. , 2011, Medical physics.

[19]  Study of the effective point of measurement for ion chambers in electron beams by Monte Carlo simulation. , 2009, Medical physics.

[20]  Calculation of the replacement correction factors for ion chambers in megavoltage beams by Monte Carlo simulation. , 2008, Medical physics.

[21]  Experimental determination of the effective point of measurement for various detectors used in photon and electron beam dosimetry. , 2011, Physics in medicine and biology.

[22]  A. Maio,et al.  Monte Carlo simulation of a typical 60Co therapy source. , 1999, Medical physics.

[23]  D. Rogers,et al.  The central electrode correction factor for high-Z electrodes in small ionization chambers. , 2011, Medical physics.

[24]  G X Ding,et al.  Calculation of stopping-power ratios using realistic clinical electron beams. , 1995, Medical physics.

[25]  C. Ma,et al.  BEAM: a Monte Carlo code to simulate radiotherapy treatment units. , 1995, Medical physics.

[26]  D. Rogers,et al.  Wall correction factors, Pwall, for thimble ionization chambers. , 2006, Medical physics.

[27]  K. Zink,et al.  Beam quality corrections for parallel-plate ion chambers in electron reference dosimetry , 2012, Physics in medicine and biology.

[28]  D. Rogers,et al.  Monte Carlo calculations of kQ, the beam quality conversion factor. , 2010, Medical physics.

[29]  I. Kawrakow.  Accurate condensed history Monte Carlo simulation of electron transport. II. Application to ion chamber response simulations. , 2000, Medical physics.

[30]  D W Rogers,et al.  Effects of changes in stopping-power ratios with field size on electron beam relative output factors. , 1998, Medical physics.

[31]  J. Heverhagen,et al.  Monte-Carlo-based perturbation and beam quality correction factors for thimble ionization chambers in high-energy photon beams , 2008, Physics in medicine and biology.

[32]  Positioning of a plane-parallel ionization chamber in clinical electron beams and the impact on perturbation factors. , 2009, Physics in medicine and biology.

[33]  D. Rogers,et al.  Wall correction factors, Pwall, for parallel-plate ionization chambers. , 2006, Medical physics.

[34]  J. H. Hubbell,et al.  Review of photon interaction cross section data in the medical and biological context. , 1999, Physics in medicine and biology.

[35]  M R McEwen,et al.  Determination of absorbed dose calibration factors for therapy level electron beam ionization chambers. , 2001, Physics in medicine and biology.

[36]  D. Rogers,et al.  AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. , 1999, Medical physics.

[37]  Monte Carlo calculations of beam quality correction factors kQ for electron dosimetry with a parallel-plate Roos chamber. , 2008, Physics in medicine and biology.

[38]  The determination of beam quality correction factors: Monte Carlo simulations and measurements. , 2009, Physics in medicine and biology.