Determination of k Q factors for cylindrical and plane-parallel ionization chambers in a scanned carbon ion beam by means of cross calibration.

ABSTRACT

The accuracy in the dosimetry of therapeutically used carbon ion beams is predominantly affected by the large uncertainty of the so-called k Q factor of the ionization chamber used for the measurements. Due to a lack of experimental data, the k Q factor of ionization chambers in carbon ion beams is still derived by calculation, and, for instance, a standard uncertainty of about 3% is given for k Q factors tabulated in the TRS-398 dosimetric protocol. Recently, k Q factors for two Farmer-type ionization chambers have been determined experimentally in the entrance channel of 429 MeV/u carbon ions, achieving about a threefold reduction of the uncertainty. To further improve the data basis on experimental k Q factors with low uncertainties, k Q factors for the same irradiation condition have now been determined for eight different cylindrical ionization chambers (NE2571, FC65-P, FC23-C, CC25, CC13, TM30010, TM30011, TM30012) and three different plane-parallel ionization chambers (PPC-40, PPC-05, TM34001) by means of a cross-calibration procedure. Generally, standard measurement uncertainties of 1.1% could be achieved. Deviations of less than 1.2% were found between the experimental and the tabulated k Q values. Moreover, the consideration of the experimental values with their smaller uncertainties in updated versions of the dosimetric protocols might enable a substantial reduction of the uncertainties in the dosimetry of carbon ion beams.