An investigation of accelerator head scatter and output factor in air.

Our purpose in this study was to investigate whether the Monte Carlo simulation can accurately predict output factors in air. Secondary goals were to study the head scatter components and investigate the collimator exchange effect. The Monte Carlo code, BEAMnrc, was used in the study. Photon beams of 6 and 18 MV were from a Varian Clinac 2100EX accelerator and the measurements were performed using an ionization chamber in a mini-phantom. The Monte Carlo calculated in air output factors was within 1% of measured values. The simulation provided information of the origin and the magnitude of the collimator exchange effect. It was shown that the collimator backscatter to the beam monitor chamber played a significant role in the beam output factors. However the magnitude of the scattered dose contributions from the collimator at the isocenter is negligible. The maximum scattered dose contribution from the collimators was about 0.15% and 0.4% of the total dose at the isocenter for a 6 and 18 MV beam, respectively. The scattered dose contributions from the flattening filter at the isocenter were about 0.9-3% and 0.2-6% of the total dose for field sizes of 4x4 cm2-40x40 cm2 for the 6 and 18 MV beam, respectively. The study suggests that measurements of head scatter factors be done at large depth well beyond the depth of electron contamination. The insight information may have some implications for developing generalized empirical models to calculate the head scatter.

[1]  J. Cygler,et al.  Evaluation of the first commercial Monte Carlo dose calculation engine for electron beam treatment planning. , 2003, Medical physics.

[2]  G. Ding.  Using Monte Carlo simulations to commission photon beam output factors--a feasibility study. , 2003, Physics in medicine and biology.

[3]  Frank Verhaegen,et al.  Monte Carlo modelling of external radiotherapy photon beams. , 2003, Physics in medicine and biology.

[4]  G. Ding.  Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of monte carlo simulations for a varian 2100EX accelerator. , 2002, Physics in medicine and biology.

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

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

[7]  T C Zhu,et al.  Modeling the output ratio in air for megavoltage photon beams. , 2001, Medical physics.

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

[9]  T. Mackie,et al.  Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique. , 2000, Medical physics.

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

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

[12]  J. Palta,et al.  Two-effective-source method for the calculation of in-air output at various source-to-detector distances in wedged fields. , 1999, Medical physics.

[13]  T. Zhu,et al.  The equivalent square concept for the head scatter factor based on scatter from flattening filter. , 1998, Physics in medicine and biology.

[14]  K L Lam,et al.  Measurement of backscatter to the monitor chamber of medical accelerators using target charge. , 1998, Medical physics.

[15]  T C Zhu,et al.  An equivalent square field formula for determining head scatter factors of rectangular fields. , 1997, Medical physics.

[16]  K L Lam,et al.  Flattening-filter-based empirical methods to parametrize the head scatter factor. , 1996, Medical physics.

[17]  R. Sloboda,et al.  Analytical representation of head scatter factors for shaped photon beams using a two-component x-ray source model. , 1995, Medical physics.

[18]  R. Sloboda,et al.  Parametrization of head-scatter factors for rectangular photon fields using an equivalent square formalism. , 1995, Medical physics.

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

[20]  A Ahnesjö,et al.  Collimator scatter in photon therapy beams. , 1995, Medical physics.

[21]  E L Chaney,et al.  A Monte Carlo study of accelerator head scatter. , 1994, Medical physics.

[22]  B. Bjärngard,et al.  Equivalent squares of irregular photon fields. , 1993, Medical physics.

[23]  B. Bjärngard,et al.  An equivalent-square formula for head-scatter factors. , 1993, Medical physics.

[24]  C Field,et al.  Backscatter into the beam monitor chamber: implications for dosimetry of asymmetric collimators. , 1993, Medical physics.

[25]  A. Gorączko,et al.  Prediction of Saturne II+ 10 MV and 23 MV photon beam output factors. , 1991, International journal of radiation oncology, biology, physics.

[26]  C. Sibata,et al.  Scatter factor corrections for elongated fields. , 1989, Medical physics.

[27]  K. Kase,et al.  Head scatter data for several linear accelerators (4-18 MV). , 1986, Medical physics.

[28]  M S Patterson,et al.  Characteristics of an 18 MV photon beam from a Therac 20 Medical Linear Accelerator. , 1981, Medical physics.

[29]  W Sewchand,et al.  Revision of tissue-maximum ratio and scatter-maximum ratio concepts for cobalt 60 and higher energy x-ray beams. , 1980, Medical physics.

[30]  Holt Jg,et al.  The extension of the concept of tissue-air ratios (TAR) to high-energy x-ray beams. , 1970 .

[31]  M. Tatcher,et al.  Head-scatter factors in rectangular photon fields. , 1993, Medical physics.