Reconstruction of megavoltage photon spectra from electronic portal imager derived transmission measurements.

Three variants of the Schiff equation are investigated to model the spectra produced by megavoltage linear accelerators. These models are tested against well-validated Monte Carlo (MC) generated spectra on the central axis of large-area fields, and show excellent agreement. Numerical reconstructions of 6 and 10 MV spectra using the same models are then presented, using experimental attenuation data derived from an electronic portal imager. The process of deriving spectra from experimental attenuation data is shown to be inherently badly constrained mathematically, with the derived spectrum being highly sensitive to noise in the source data, and non-unique. By placing a priori constraints on the Schiff model from both physical knowledge of the construction of the accelerator and MC data, physically useful results are gained and presented for both the energy dependence and off-axis behaviour of photon spectra.

[1]  P. Huang,et al.  Reconstruction of diagnostic x-ray spectra by numerical analysis of transmission data. , 1986, Medical physics.

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

[3]  M Partridge,et al.  The practical implementation of a scatter model for portal imaging at 10 MV. , 1998, Physics in medicine and biology.

[4]  L. C. Baird.  X-ray spectra vs attenuation data: a theoretical analysis. , 1981, Medical physics.

[5]  K. Peck,et al.  Reconstruction of 6 MV photon spectra from measured transmission including maximum energy estimation. , 1997, Physics in medicine and biology.

[6]  A L Boyer,et al.  An analytic calculation of the energy fluence spectrum of a linear accelerator. , 1994, Medical physics.

[7]  P. Evans,et al.  Radiological thickness measurement using a liquid ionization chamber electronic portal imaging device. , 1999, Physics in medicine and biology.

[8]  L. Silberstein.  XXXIV. Spectral composition of an X-ray radiation determined from its filtration curve , 1933 .

[9]  C. Bacci,et al.  A study of quality of bremsstrahlung spectra reconstructed from transmission measurements. , 1990, Medical physics.

[10]  Hans A. Bethe,et al.  On the Stopping of Fast Particles and on the Creation of Positive Electrons , 1934 .

[11]  R M Harrison,et al.  A method and contrast-detail phantom for the quantitative assessment of radiotherapy portal imaging systems. , 1994, The British journal of radiology.

[12]  A L Boyer,et al.  Bremsstrahlung review: an analysis of the Schiff spectrum. , 1991, Medical physics.

[13]  P M Evans,et al.  Scattered radiation in portal images: a Monte Carlo simulation and a simple physical model. , 1996, Medical physics.

[14]  C Kleinschmidt,et al.  Analytical considerations of beam hardening in medical accelerator photon spectra. , 1999, Medical physics.

[15]  G. Adams.  The Absorption of High Energy Quanta. I , 1948 .

[16]  P. Huang,et al.  Spectral characterization of 4 MV Bremsstrahlung by attenuation analysis. , 1981, Medical physics.

[17]  P. Huang,et al.  Simulation studies of 4-MV x-ray spectral reconstruction by numerical analysis of transmission data. , 1982, Medical physics.

[18]  Frank Verhaegen,et al.  Considerations for modelling MLCs with Monte Carlo techniques , 2000 .

[19]  M van Herk,et al.  A matrix ionisation chamber imaging device for on-line patient setup verification during radiotherapy. , 1988, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[20]  C. Baker,et al.  Reconstruction of megavoltage photon spectra by attenuation analysis. , 1995, Physics in medicine and biology.

[21]  S. Lillicrap,et al.  X-Ray Spectra from Accelerators in the range 2 to 6 MeV , 1967 .

[22]  D. Marquardt.  An Algorithm for Least-Squares Estimation of Nonlinear Parameters , 1963 .

[23]  L. Schiff.  Energy-Angle Distribution of Thin Target Bremsstrahlung , 1951 .