AbstractID: 7819 Title: EGSnrc calculations of a dag-coated, aluminum-walled thimble chamber
Ionization chambers are the most commonly used tools for absorbed dose determination in radiotherapy. The accuracy of the dose determination
protocols depends largely on the validity of the cavity theory upon which they are based. In current practice, many chambers use a coating of graphite
dag on the inner wall. Previous experimental results using this type of chamber showed poor agreement with dosimetry theory. Recent improvements
in Monte Carlo simulations permit an accurate investigation of this effect using the EGSnrc code. In this study, the Monte Carlo code EGSnrc was
used to compare the response of an aluminum-walled thimble ionization chamber to that of a graphite-walled thimble chamber when exposed to a
⁶⁰Co beam. This ratio was also calculated for varying thicknesses of a graphite dag layer on the inside of the aluminum wall. The
results were compared to earlier experimental data and although there was an offset between the two sets of values, they showed the same underlying
behavior as a function of the dag thickness. The results were also compared to the theory used in TG-21 and TG-51 and showed that while the
calculated results agree with the predictions of the dosimetry theory for the bare aluminum wall, the theory does not correctly predict the relative
responses of the chambers in the presence of the graphite dag layer.