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Dose distributions in the buildup region of megavoltage linear accelerator photon beams

Mubata, Cephas D.; (1994) Dose distributions in the buildup region of megavoltage linear accelerator photon beams. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The purpose of the investigation was to obtain accurate values for dose distributions in the buildup region of megavoltage photon beams from linear accelerators. Most of the work was carried out using Monte Carlo calculation techniques. These techniques were used to calculate bremsstrahlung photon spectra, electron contamination in the photon beams, depth-dependent mass collision stopping-power ratios and in the investigation of sources of fluence perturbation in plane parallel chambers. The calculations were carried out for the 5-, 6-, 8-, 10- and 21MV photon beams from Philips linear accelerators. By simulating the treatment machine heads of the Philips linacs, the bremsstrahlung photon spectra were calculated. The calculated spectra were benchmarked by comparing calculated depth dose curves and beam quality indices with measured data, with agreement better than 2% and 1% respectively. An assessment of the electron contamination in the above photon beams was also carried out in order to obtain the correct distributions of the electron spectra per incident primary photon, for chamber simulations. The spectra and distribution of the electrons from the different sources in the machine heads, are presented. Using the calculated photon and electron spectra and distributions, the depth-dependent stopping-power ratios were calculated for ionization-dose conversion in the buildup region of the various photon beams. The calculated stopping power ratios have shown that below l0MV, the buildup region ratios are invariant with depth, within 1% but the variation in the ratios increases with energy to non-negligible values for higher energies with up to 3% at 21MV. The problems with fluence perturbation with fixed-plate chambers in buildup region meaurements led to the construction of a Perspex extrapolation chamber. This chamber, together with Monte Carlo calculations, was used to investigate the sources of fluence perturbation in plane-parallel chambers. The commercial fixed-plate chambers used in work were the PTW Markus, Scanditronix NACP, Vinten Model-640 and the Klevenhagen (1982) electron chambers. The contributions of the electrons from the front window and backscattered electrons have been shown to influence the slops of the extrapolation curves at small plate separation, with the sidewall electrons taking over at large separations. Extrapolation curves for chambers of different diameters have also been calculated to give guidelines for chamber designs.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Dose distributions in the buildup region of megavoltage linear accelerator photon beams
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Health and environmental sciences; Megavoltage; Photon beams
URI: https://discovery.ucl.ac.uk/id/eprint/10104198
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