Characterization of the water-equivalent material WTe for use in electron beam dosimetry

This paper describes the characterization of the water-equivalent material WTe (produced by St Bartholomew's Hospital, London). The use of epoxy resin phantoms offers a number of advantages over water for radiotherapy dosimetry in terms of robustness and ease of use, but the published uncertainties in the fluence corrections for such phantoms significantly increase the overall uncertainty in the measurement of absorbed dose to water at the reference point. Depth-ionization data were obtained in water and WTe for electron beams in the range 4 MeV to 16 MeV and it was found that the measured fluence in the WTe phantom was approximately 0.4% higher than in a water phantom at the same depth. For measurements only at the reference depth this difference was less, with the fluence in the WTe phantom being 0.2% higher. The standard uncertainty on this value is estimated to be +/- 0.12%, which represents a significant improvement over previous measurements. It was also found that the range scaling factor is not equal to unity, as previously recommended for this material, but that the data was best fitted by the relation 1 mm WTe = 1.01 mm water (with an uncertainty of +/- 0.2%). The results obtained confirm previous investigations of WTe as to its suitability for reference ion chamber dosimetry in the radiotherapy clinic. However, the recommendation is still to use a water phantom wherever possible.