Ionization chamber dosimetry of proton beams

In a recent study the absorbed dose was determined in a 170 MeV proton beam using seven different ionization chambers. The absorbed dose obtained with two IC-18 chambers, a chamber type commonly used as reference in proton beams, was found to be up to 1.5 % lower than that obtained with a Farmer NE-2571, when calibration factors in terms of NK were used. When the IC-18 chambers instead were calibrated in a high-energy electron beam, using the Farmer NE-2571 as a reference, consistent results were obtained in the proton beam within the estimated experimental uncertainties. This finding questions the value of the product kattkm for the IC-18 chamber given by the IAEA Code of Practice which was used in this comparison, and points at possible chamber-to-chamber variations that theoretical kattkm factors cannot predict. This second conclusion is confirmed by the results obtained at a recent ionization chamber intercomparison between different proton therapy centers and was presented at the XXII PTCOG meeting