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Absorbed dose conversion factors for therapeutic kilovoltage and megavoltage x‐ray beams calculated by the Monte Carlo method
Author(s) -
Knight Richard T.
Publication year - 1997
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 - 10.1118/1.598084
Subject(s) - kerma , monte carlo method , physics , dosimetry , absorbed dose , bremsstrahlung , computational physics , fluence , calibration , beam (structure) , laser beam quality , optics , nuclear medicine , materials science , radiation , photon , mathematics , statistics , medicine , laser , quantum mechanics , laser beams
This thesis describes techniques for performing calculations of the backscatter factor, B, and the ratio of mean mass energy absorption coefficients water‐to‐air,( μ ̄ en/ ρ )w , air , for kilovoltage x‐ray beams, and the ratio of mean stopping powers water‐to‐air, s w , air , for megavoltage x‐ray beams, through application of the Monte Carlo method. Consistent theoretical expressions for absorbed dose to water, D w , in low‐ and medium‐energy x‐rays have been proposed. Bremsstrahlung spectra were calculated by detailed Monte Carlo simulations of the U.K. National Physical Laboratory (NPL) standard accelerator, a Philips SL series accelerator and a Siemens Stabilipan x‐ray unit. Values of polyenergetic B w( w )were calculated by a kerma‐weighted averaging technique which utilizes precalculated monoenergetic values and primary fluence spectra. Comprehensive sets of data for[ ( μ ̄ en/ ρ )w , air ] p ,( μ ̄ en( z , f ) / ρ )w , airand B w( w )are presented, appropriate for insertion into the proposed expressions for D w . Values of the Spencer–Attix s w , airwere calculated for the accelerator beams by a dose‐weighted averaging technique which utilizes precalculated monoenergetic dose distributions and primary fluence spectra. Accurate values of TPR 10 20 , accounting for spectrum quality shift off‐axis, were acquired by convolving water terma distributions with point‐energy‐deposition kernels. The beam quality dependence of the NPL absorbed dose‐to‐water calibration factor, N W , was subsequently investigated.