| Abstract | The aim of this work was to develop and trial a mailed audit system for external beam radiation therapy, traceable to the Canadian National Metrology Institute, with an overall uncertainty comparable to calibrated secondary standard reference detectors.
Alanine was chosen as the dosimeter to be used for the audit system. After a detailed investigation of influence quantities, a complete uncertainty budget was constructed, indicating a relative standard uncertainty in the desired quantity – absorbed dose to water – of around 0.9 %.
The complete system was then validated by comparison with the standard maintained by the National Physical Laboratory in the UK. The comparison between the two alanine systems at the two laboratories showed agreement at the 0.5 % level.
A hermitically-sealed dosimeter holder was developed for simplicity of use and to reproduce the geometry of standard clinical dosimeters. Eleven cancer centres across Canada participated in the initial trial. The mean ratio of the dose measured using alanine, relative to the dose delivered was found to be 1.010 with a relative standard uncertainty of 0.6 %.
These investigations have confirmed the suitability of the alanine system, both in terms of ease-of-use and accuracy, for mailed audit dose measurements in Canada. |
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