The dosimetric accuracy of megavoltage photon and electron beams in radiotherapy
In this thesis, a detailed review and analysis is undertaken of the numerical and theoretical data employed in the various National and International Dosimetry Protocols and Codes of Practice currently in use. Air kerma to dose conversion factors for megavoltage photon and electron beams are then derived, using each of the protocols, for a variety of commonly used ionisation chambers and it is found that differences in the resultant values are present for both photon and electron beams. For example, for a NE2571 graphite walled cylindrical ionisation chamber, the maximum variation in the air kerma to dose conversion factor for photons Cw,λ occurs at a "nominal" energy of 25MV, where the difference between the maximum and minimum values of Cw,λ is 5.1%, while the maximum difference between the different protocols in the air kerma to dose conversion factor for electrons Cw,e occurs at a mean incident energy of 9 MeV, where the difference between the maximum and minimum values of Cw,e is 2.7%. Analysis of the conversion factors indicates that the cause of these differences is due to a combination of both the numerical data and theoretical factors employed, although the major contributions to the differences arise from the use of different sets of stopping power ratios in the different protocols; in turn the use of different sets of stopping power ratios arises partly from different methods of specifying beam quality. An experimental approach was, therefore, developed to determine the consistency of each protocol's air kerma to dose conversion factors with an absolute value for one quality of beam, i.e. the relationship to a photon calibration standard, and also to examine the consistency between photon and electron beam modalities. When Cobalt-60 gamma rays were used as a calibration quality and comparative measurements were made in photon beams of nominal energies from 6MV to 25MV, and electron beams of nominal incident energies from 5MeV to 20MeV, inconsistencies were found in the majority of protocols and, for photon beams this was found to be attributable to inadequate description of photon beam quality.