Investigation of ionization chamber-specific beam quality correction factor (kQ,Q0) used for absorbed dose determination in megavoltage photon beams

In the recent past (Andreo et al., 2020), new ionization chamber-specific beam quality correction factors (k_Q,Q0) were published in the literature for the determination of the absorbed dose to water in high energy radiotherapy photon beams. In a previous experimental investigation, made at our laboratory, five various Farmer-type cylindrical ionization chambers, i.e. Exradin A12, IBA FC65-G, NE-2571, PTW 30011 and PTW 30012, were studied to examine their responses with reference to the determination of the absorbed dose to water in an inter-chamber comparison. In the present work, measurements were made to experimentally study the relation or ratio of the k_Q,Q0 correction factors using the five various types of the ionization chambers employed in our previous investigation. In total, nine ionization chambers, two of each type, were included in the present study, with the exception of a single PTW 30011 ionization chamber. The photon beams employed were the 6 MV and the 15 MV beam. The outcomes of this work displayed excellent agreement between the ratio of the ionization chamber-specific k_Q,Q0 correction factors, for the ionization chambers utilized, and the corresponding ratio calculated using the published k_Q,Q0 correction factors. The discrepancy obtained in these comparisons was within 0.1%, for both the photon beams examined, which is much smaller than the uncertainty associated with the determination of this relationship.