Ionization chamber dosimetry of proton beams using cylindrical and plane parallel chambers. Nwversus NKion chamber calibrations

Abstract

Determinations of the absorbed dose in a 170 MeV proton beam have been performed using seven ionization chambers of different types: five cylindrical (two FWT IC-18 and three NE-2571, of which one was modified to have the central electrode made of graphite) and two plane parallel (NACP-02 and Roos FK-6). The ionization was converted into absorbed dose in the proton beam according to the generalization of the formalism provided by the IAEA Code of Practice (TRS 277), which enables the use of the same equations for all kinds of beam used in radiotherapy. The absorbed dose obtained with the two IC-18 chambers, a chamber type commonly used as a reference in proton beams, was up to 1.5% lower than that obtained with the Farmer NE-2571 chamber, which was used as the reference in this work when calibration factors in terms of N_K were used. To investigate this difference, experimental No factors for six chambers (the two IC-18 chambers, the NACP-02, the FK-6 and two of the NE-2571 chambers) were determined in a high-energy electron beam. The procedure commonly recommended for plane parallel ion chambers was used for all the chambers, using the same reference chamber, a Farmer NE-2571. In the 170 MeV proton beam all the N_D factors yielded consistent absorbed dose determinations within the estimated experimental uncertainties. This finding calls into question the value of the product k_attk_m for the IC-18 chamber given by the IAEA Code of Practice used in this comparison, and points at possible chamber to chamber variations that theoretical k_attk_m factors cannot predict. The investigations enabled the determination of the p_wall(⁶⁰Co) factor of the Roos FK-6 plane parallel chamber, yielding 1.003+or-0.5%, and a correction for the effect of the aluminium central electrode of NE-2571 chambers in proton beams, equal to 1.003+or-0.4%. Two of the chambers (the plane parallel FK-6 and the modified cylindrical NE-2571) were provided with calibration factors in terms of absorbed dose to water, N_w, at the quality of ⁶⁰Co by the Primary Standard Dosimetry Laboratory in Germany (PTB). Using the N_w formalism excellent agreement was found with the determination based on the experimental N_D, giving support to the implementation of the N_w procedure in therapeutic proton beams.