Experimental p(wall) and p(cel) correction factors for ionization chambers in low-energy clinical proton beams

Current dosimetry protocols for clinical protons using ionization chambers do not take into account ionization chamber-dependent perturbation correction factors. In the present investigation, the relative response of 17 cylindrical ionization chambers was evaluated at three proton beam qualities: at two points in a modulated beam and one point in a non-modulated beam, all with an incident energy of 75 MeV. Thirteen of the ionization chambers had a Farmer-type geometry but consisted of different combinations of wall and central electrode materials. All ionization chambers were calibrated in terms of air kerma as well as in terms of absorbed dose to water in a 60Co beam. The relative response of the ionization chambers was compared with results of Monte Carlo simulations of proton and secondary electron transport in the phantom and the ionization chamber geometry. The results of the measurements for cylindrical ionization chambers show relative perturbation effects that are limited to 0.5-1%, resulting in perturbation correction factors that are larger than unity compared with an NE2571 ionization chamber. The experimental relative wall and total perturbation correction factors agree with Monte Carlo calculated values, indicating that the differences between the responses of different ionization chambers are due to secondary electron effects. This conclusion is supported by the comparison of our results with those from other investigators after re-analysis of their data. The central electrode perturbation correction factor for an aluminium electrode in a Farmer-type geometry was found to be unity within the experimental uncertainties.