Experimental pwall and pcel correction factors forionization chambers in low-energy clinical proton beams

Abstract

Current dosimetry protocols for clinical protons using ionization chambers donot take into account ionization chamber-dependent perturbation correctionfactors. In the present investigation, the relative response of 17 cylindricalionization chambers was evaluated at three proton beam qualities: at twopoints in a modulated beam and one point in a non-modulated beam, all with anincident energy of 75 MeV. Thirteen of the ionization chambers had aFarmer-type geometry but consisted of different combinations of wall andcentral electrode materials. All ionization chambers were calibrated in termsof 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 withresults of Monte Carlo simulations of proton and secondary electron transportin the phantom and the ionization chamber geometry.The results of the measurements for cylindrical ionization chambers showrelative perturbation effects that are limited to 0.5-1%, resulting inperturbation correction factors that are larger than unity compared withan NE2571 ionization chamber.The experimental relative wall and total perturbation correction factors agreewith Monte Carlo calculated values, indicating that the differences betweenthe responses of different ionization chambers are due to secondary electroneffects. This conclusion is supported by the comparison of our results withthose from other investigators after re-analysis of their data.The central electrode perturbation correction factor for an aluminiumelectrode in a Farmer-type geometry was found to be unity within theexperimental uncertainties.