Absorbed dose conversion factors for therapeutic kilovoltage and megavoltage x‐ray beams calculated by the Monte Carlo method

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.