IONIZATION-CHAMBER DOSIMETRY OF PROTON-BEAMS USING CYLINDRICAL AND PLANE-PARALLEL CHAMBERS - N-W VERSUS N-K ION-CHAMBER CALIBRATIONS

Determinations of the absorbed dose in a 170 MeV proton beam have been performed using seven ionization chambers of different types: five cy lindrical (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 d ose in the proton beam according to the generalization of the formalis m provided by the IAEA Code of Practice (TRS 277), which enables the u se of the same equations for all kinds of beam used in radiotherapy. T he 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 tha n 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 N-D factors for six chambers (the two IC-18 chambers, the NACP-02, the FK-6 and two of th e NE-2571 chambers) were determined in a high-energy electron beam. Th e procedure commonly recommended for plane parallel ion chambers was u sed for all the chambers, using the same reference chamber, a Farmer N E-2571. In the 170 MeV proton beam all the N-D factors yielded consist ent absorbed dose determinations within the estimated experimental unc ertainties. This finding calls into question the value of the product k(att)k(m) for the IC-18 chamber given by the IAEA Code of Practice us ed in this comparison, and points at possible chamber to chamber varia tions that theoretical k(att)k(m) factors cannot predict. The investig ations enabled the determination of the p(wall)(Co-60) factor of the R oos FK-6 plane parallel chamber, yielding 1.003 +/- 0.5%, and a correc tion for the effect of the aluminium central electrode of NE-2571 cham bers in proton beams, equal to 1.003 +/- 0.4%. Two of the chambers (th e plane parallel FK-6 and the modified cylindrical NE-2571) were provi ded with calibration factors in terms of absorbed dose to water, N-W, at the quality of Co-60 by the Primary Standard Dosimetry Laboratory i n 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 .