Experimental determination of the beam quality dependence factors, kQ, for ionization chambers used in photon and electron dosimetry.

Dosimetry in radiotherapy with ionization chambers calibrated in 60Co gamma beams in terms of absorbed dose to water, DW, can be performed if a factor conventionally denoted as kQ is known. The factor kQ depends on the beam quality and the chamber characteristics. Calculated values of the kQ factors for many types of ionization chamber have been recently published. In this work the experimental determination of the kQ factors for various ionization chambers was performed for 6 MV and 15 MV photon beams and for a 14 MeV electron beam. The kQ factors were determined by a procedure based on relative measurements performed with the ionization chamber and ferrous sulphate solution in 60Co gamma radiation and accelerator beams, respectively. The experimental kQ values are compared with the calculated values so far published. Theoretical and experimental kQ values are in fairly good agreement. The uncertainty in the experimental kQ factors determined in this work is less than about 1%, that is, appreciably smaller than the uncertainty of about 1.5% reported for the calculated values.