Monte Carlo modelling of radiotherapy kV x-ray units

To obtain accurate information for absorbed dose calculations in water for kilovoltage x-rays, the photon spectrum, planar fluence and the angular dis tribution of the photons at the collimator exit of the x-ray unit have to b e known. The only way to obtain this information is by Monte Carlo (MC) sim ulation. Compared with the situation for high-energy photons and electrons, where in recent years numerous papers have been devoted to MC modelling of complete clinical accelerator units, there is a lack of similar work for k V x-ray units. A reliable MC model for a kV x-ray unit would allow the outp ut information to be used in a treatment planning system for regular and ir regular treatment fields. Furthermore, with MC simulation, perturbation fac tors of dose-measuring devices, such as those specified in codes of practic e, can be calculated. In this work, the MC code EGS4/BEAM was used to build realistic models of t wo complete x-ray units. The tungsten target, exit window, collimator, addi tional filtration and applicator were taken into account. For some aspects of the work, a comparison was made with the simulations from another MC cod e, MCNP4B. The contribution to the characteristic radiation from electron i mpact ionization and from the photoelectric effect of reabsorbed bremsstrah lung photons was studied. Calculated and measured photon fluence spectra in air and half-value layers for a Philips MCN410 tube were compared for seve ral anode voltages and additional filtrations. Results from the two codes a greed well, and the agreement with measured spectra was found to be good fo r energies above 50 keV but rather less good below that energy. For a Sieme ns Stabilipan 2 Th300 x-ray tube, HVLs and dose distributions in water were compared with measurements for several clinical x-ray qualities. For most of the combinations of radiation qualities and applicators, good agreement was obtained, although there were also some cases where the agreement was n ot so good. Electron contamination and photon build-up at the water surface were studied using MC simulation. The influence of depth on the photon spe ctral distribution was investigated. Both EGS4/BEAM and MCNP4B, in their default versions, handle inadequately t he production of characteristic x-rays. This was found to have only a minor influence on the calculated dosimetric quantities. Simulations with MCNP4B required the use of several variance reduction techniques in order to obta in results within reasonable calculation times.