Optimization of accelerator target and detector for portal imaging using Monte Carlo simulation and experiment.

Megavoltage portal images suffer from poor quality compared to those produced with kilovoltage x-rays. Several authors have shown that the image quality can be improved by modifying the linear accelerator to generate more low-energy photons. This work addresses the problem of using Monte Carlo simulation and experiment to optimize the beam and detector combination to maximize image quality for a given patient thickness. A simple model of the whole imaging chain was developed for investigation of the effect of the target parameters on the quality of the image. The optimum targets (6 mm thick aluminium and 1.6 mm copper) were installed in an Elekta SL25 accelerator. The first beam will be referred to as A16 and the second as Cu1.6. A tissue-equivalent contrast phantom was imaged with the 6 MV standard photon beam and the experimental beams with standard radiotherapy and mammography film/screen systems. The arrangement with a thin Al target/mammography system improved the contrast from 1.4 cm bone in 5 cm water to 19% compared with 2% for the standard arrangement of a thick, high-Z target/radiotherapy verification system. The linac/phantom/detector system was simulated with the BEAM/EGS4 Monte Carlo code. Contrast calculated from the predicted images was in good agreement with the experiment (to within 2.5%). The use of MC techniques to predict images accurately, taking into account the whole imaging system, is a powerful new method for portal imaging system design optimization.

[1]  O Dössel,et al.  Monte Carlo simulations of the imaging performance of metal plate/phosphor screens used in radiotherapy. , 1999, Medical physics.

[2]  D. G. Lewis,et al.  Monte Carlo simulation and dosimetric verification of radiotherapy beam modifiers. , 2001, Physics in medicine and biology.

[3]  R. Boellaard,et al.  A convolution model to convert transmission dose images to exit dose distributions. , 1997, Medical physics.

[4]  A F Bielajew,et al.  A thin target approach for portal imaging in medical accelerators. , 1998, Physics in medicine and biology.

[5]  P F O'Brien,et al.  Megavoltage imaging with low Z targets: implementation and characterization of an investigational system. , 1998, Medical physics.

[6]  W. Lutz,et al.  Determination of effective electron source size using multislit and pinhole cameras. , 2000, Medical physics.

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

[8]  J A Rawlinson,et al.  Low-energy imaging with high-energy bremsstrahlung beams: analysis and scatter reduction. , 1993, Medical physics.

[9]  G. Cho,et al.  Monte Carlo studies of metal/phosphor screen in therapeutic X-ray imaging , 1999 .

[10]  B. Bjärngard,et al.  Evaluation of a beam-spot camera for megavoltage x rays. , 1988, Medical physics.

[11]  D M Galbraith.  Low-energy imaging with high-energy bremsstrahlung beams. , 1989, Medical physics.

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

[13]  B. G. Szabó,et al.  Comparison of three rat strains for development of radiation-induced lung injury after hemithoracic irradiation. , 2001, Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.

[14]  A Fenster,et al.  X-ray scatter in megavoltage transmission radiography: physical characteristics and influence on image quality. , 1994, Medical physics.

[15]  D. Rogers,et al.  EGS4 code system , 1985 .

[16]  D. Sheikh-Bagheri,et al.  Sensitivity of megavoltage photon beam Monte Carlo simulations to electron beam and other parameters. , 2002, Medical physics.