Characterization of electron contamination in megavoltage photon beams.

The purpose of the present study is to characterize electron contamination in photon beams in different clinical situations. Variations with field size, beam modifier (tray, shaping block) and source-surface distance (SSD) were studied. Percentage depth dose measurements with and without a purging magnet and replacing the air by helium were performed to identify the two electron sources that are clearly differentiated: air and treatment head. Previous analytical methods were used to fit the measured data, exploring the validity of these models. Electrons generated in the treatment head are more energetic and more important for larger field sizes, shorter SSD, and greater depths. This difference is much more noticeable for the 18 MV beam than for the 6 MV beam. If a tray is used as beam modifier, electron contamination increases, but the energy of these electrons is similar to that of electrons coming from the treatment head. Electron contamination could be fitted to a modified exponential curve. For machine modeling in a treatment planning system, setting SSD at 90 cm for input data could reduce errors for most isocentric treatments, because they will be delivered for SSD ranging from 80 to 100 cm. For very small field sizes, air-generated electrons must be considered independently, because of their different energetic spectrum and dosimetric influence.

[1]  J. Purdy,et al.  Entrance and exit dose regions for a Clinac-2100C. , 1993, International journal of radiation oncology, biology, physics.

[2]  J. Purdy.  Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator. , 1986, Medical Physics (Lancaster).

[3]  B M Rao,et al.  Investigations on the near surface dose for three 10-MV x-ray beam accelerators with emphasis on the reduction of electron contamination. , 1988, Medical physics.

[4]  B. Bjärngard,et al.  Analysis of central-axis doses for high-energy x rays. , 1995, Medical physics.

[5]  T. Zhu,et al.  Doses near the surface in high-energy x-ray beams. , 1995, Medical physics.

[6]  P Andreo,et al.  Determination of effective bremsstrahlung spectra and electron contamination for photon dose calculations. , 1989, Physics in medicine and biology.

[7]  P J Biggs,et al.  Electrons as the cause of the observed dmax shift with field size in high energy photon beams. , 1979, Medical physics.

[8]  M. Karlsson,et al.  Electron contamination in clinical high energy photon beams. , 1996, Medical physics.

[9]  B. Faddegon,et al.  Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom. , 2001, Medical physics.

[10]  J Yang,et al.  Modelling of electron contamination in clinical photon beams for Monte Carlo dose calculation. , 2004, Physics in medicine and biology.

[11]  A. Dutreix,et al.  Characteristics of contamination electrons in high energy photon beams. , 1993, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[12]  J W Scrimger,et al.  Contamination of a 15-MV photon beam by electrons and scattered photons. , 1982, Radiology.

[13]  P. Biggs.  The forward production of high-energy electrons from megavoltage photon beams. , 1987, Medical physics.

[14]  A Fogliata,et al.  Dosimetric features of linac head and phantom scattered radiation outside the clinical photon beam: experimental measurements and comparison with treatment planning system calculations. , 2001, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[15]  R Mohan,et al.  Sources of electron contamination for the Clinac-35 25-MV photon beam. , 1983, Medical physics.

[16]  A. Ahnesjö,et al.  Dose calculations for external photon beams in radiotherapy. , 1999, Physics in medicine and biology.

[17]  Correcting for electron contamination at dose maximum in photon beams. , 1999, Medical physics.

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

[19]  R Mohan,et al.  Investigation of buildup dose from electron contamination of clinical photon beams. , 1983, Medical physics.

[20]  D P Fontenla,et al.  Effects of beam modifiers and immobilization devices on the dose in the build-up region. , 1994, International journal of radiation oncology, biology, physics.

[21]  I. Rosen,et al.  Beam‐commissioning methodology for a three‐dimensional convolution/superposition photon dose algorithm , 2000, Journal of applied clinical medical physics.

[22]  T C Zhu,et al.  Electron contamination in 8 and 18 MV photon beams. , 1998, Medical physics.

[23]  C C Ling,et al.  Magnetic analysis of the radiation components of a 10 MV photon beam. , 1982, Medical physics.

[24]  P J Biggs,et al.  An investigation into the presence of secondary electrons in megavoltage photon beams. , 1983, Physics in medicine and biology.

[25]  F. Nüsslin,et al.  A virtual photon energy fluence model for Monte Carlo dose calculation. , 2003, Medical physics.

[26]  T C Zhu,et al.  Photon beam skin dose analyses for different clinical setups. , 1998, Medical physics.

[27]  M. Butson,et al.  Lepton contamination and photon scatter produced by open field X-ray beams in the build-up region , 2002 .

[28]  M. Altschuler,et al.  Calculation of the dose in the build-up region for high energy photon beam. Treatment planning when beam spoilers are employed. , 1995, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[29]  C K Ross,et al.  Comparison of measured and Monte Carlo calculated dose distributions from the NRC linac. , 2000, Medical physics.

[30]  T. Solberg,et al.  A virtual source model for Monte Carlo modeling of arbitrary intensity distributions. , 2000, Medical physics.

[31]  C Kappas,et al.  A monte carlo approach to electron contamination sources in the Saturne-25 and -41. , 2001, Physics in medicine and biology.

[32]  M Miften,et al.  Implementation of FFT convolution and multigrid superposition models in the FOCUS RTP system. , 2000, Physics in medicine and biology.

[33]  J M Wilkinson,et al.  Electron contamination and build-up doses in conformal radiotherapy fields. , 1999, Physics in medicine and biology.

[34]  C C Ling,et al.  Contaminant electrons in the build-up region of a 4 MV photon beam. , 1983, Physics in medicine and biology.

[35]  D W O Rogers,et al.  Monte Carlo calculation of nine megavoltage photon beam spectra using the BEAM code. , 2002, Medical physics.

[36]  P. Storchi,et al.  Calculation of a pencil beam kernel from measured photon beam data. , 1999, Physics in medicine and biology.

[37]  A. Ahnesjö,et al.  A pencil beam model for photon dose calculation. , 1992, Medical physics.

[38]  C. Ceberg,et al.  A simplistic formalism for calculating entrance dose in high-energy x-ray beams. , 2002, Physics in medicine and biology.

[39]  M. Carrión,et al.  Comparison between TG-51 and TRS-398: electron contamination effect on photon beam-quality specification. , 2004, Physics in medicine and biology.