Experimental p(wall) and p(cel) correction factors for ionization chambersin low-energy clinical proton bearras

Current dosimetry protocols for clinical protons using ionization chambers do not take into account ionization chamber-dependent perturbation correcti on factors. In the present investigation, the relative response of 17 cylin drical 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 ter ms of air kerma as well as in terms of absorbed dose to water in a Co-60 be am. The relative response of the ionization chambers was compared with resu lts of Monte Carlo simulations of proton and secondary electron transport i n the phantom and the ionization chamber geometry. The results of the measurements for cylindrical ionization chambers show re lative perturbation effects that are limited to 0.5-1%, resulting in pertur bation correction factors that are larger than unity compared with an NE257 1 ionization chamber. The experimental relative wall and total perturbation correction factors ag ree with Monte Carlo calculated values, indicating that the differences bet ween the responses of different ionization chambers are due to secondary el ectron effects, This conclusion is supported by the comparison of our resul ts with those from other investigators after re-analysis of their data. The central electrode perturbation correction factor for an aluminium elect rode in a Farmer-type geometry was found to be unity within the experimenta l uncertainties.