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A virtual source model for Monte Carlo modeling of arbitrary intensity distributions
Author(s) -
Chetty I.,
DeMarco J. J.,
Solberg T. D.
Publication year - 2000
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598881
Subject(s) - monte carlo method , physics , linear particle accelerator , photon , fluence , computational physics , optics , beam (structure) , monte carlo method for photon transport , scattering , photon energy , dynamic monte carlo method , mathematics , statistics , direct simulation monte carlo , laser
A photon virtual source model was developed for simulating arbitrary, external beam, intensity distributions using the Monte Carlo method. The source model consists of a photon fluence grid composed of a matrix of square elements, located 25‐cm downstream from the linear accelerator target. Each particle originating from the fluence map is characterized by the seven phase space parameters, position ( x , y , z ) , direction ( u , v , w ) , and energy. The map was reconstructed from fluence and energy spectra acquired by modeling components of the linear accelerator treatment head using the Monte Carlo code MCNP4B . The effect of contaminant electrons is accounted for by the use of a sub‐source derived from a phase‐space simulation of a 25‐MV linac treatment head using the code BEAM . The BEAM sub‐source was incorporated into the MCNP4B phase‐space model and is sampled using a field‐size dependent sampling ratio. A Gaussian blurring kernel is convolved with the photon fluence map to account for the finite focal spot size and scattering effects from structures such as the flattening filter and MLC leaves. Depth dose and profile source calculations for 6‐MV and 25‐MV photon beams, for 5 × 5 cm 2 ,10 × 10 cm 2 , and 15 × 15 cm 2field sizes, are in good agreement with measurement and are well within acceptability criteria suggested by the AAPM Task Group Report No. 53. Irregular field calculations compared with film measurement and with a 3‐D pencil beam algorithm show that the source model is capable of accurately simulating arbitrary MLC fields.