Po‐Poster ‐ 13: A physical explanation for the failure of chamber calibrations for low energy brachytherapy seeds

Abstract

In the last few years it has become apparent that for low energy sources, chamber calibration factors are in error when used at sites with significant elevations. Empirical correction factors have been measured and published, but not a physical explanation of the effect. As the effect is only for low energy and not high energy sources, the explanation must lie in the relative ranges of the electrons set in motion in the wall. If all electrons stop in the chamber cavity, no correction for temperature and pressure need be applied. If all electrons cross the cavity then the usual Ctp correction is appropriate. For Ir‐192 or Cs‐137 sources, applying Ctp to account for changes of ambient pressure is appropriate. However, for lower energy sources (I‐125 or Pd‐103) a simple model predicts a significant number of electrons stop in the chamber and Ctp over‐corrects for the difference in ambient pressure. An analytic solution for a semi‐infinite parallel plate chamber predicts a behaviour similar to that described by the empirical correction factor. A simple numerical model of a well chamber shows that geometry is a second order effect, and that the ratio of cavity dimension to electron range is the principle physical consideration.