On the impact of ICRU report 90 recommendations on kQ factors for high‐energy photon beams

Purpose To assess the impact of the ICRU report 90 recommendations on the beam‐quality conversion factor, kQ, used for clinical reference dosimetry of megavoltage linac photon beams. Methods The absorbed dose to water and the absorbed dose to the air in ionization chambers representative of those typically used for linac photon reference dosimetry are calculated at the reference depth in a water phantom using Monte Carlo simulations. Depth‐dose calculations in water are also performed to investigate changes in beam quality specifiers. The calculations are performed in a cobalt‐60 beam and MV photon beams with nominal energy between 6 MV and 25 MV using the EGSnrc simulation toolkit. Inputs to the calculations use stopping‐power data for graphite and water from the original ICRU‐37 report and the new proposed values from the recently published ICRU‐90 report. Calculated kQ factors are compared using the two different recommendations for key dosimetry data and measured kQ factors. Results Less than about 0.1% effects from ICRU‐90 recommendations on the beam quality specifiers, the photon component of the percentage depth‐dose at 10 cm, %dd(10)x, and the tissue‐phantom ratio at 20 cm and 10 cm, TPR1020, are observed. Although using different recommendations for key dosimetric data impact water‐to‐air stopping‐power ratios and ion chamber perturbation corrections by up to 0.54% and 0.40%, respectively, we observe little difference (≤0.14%) in calculated kQ factors. This is contradictory to the predictions in ICRU‐90 that suggest differences up to 0.5% in high‐energy photon beams. A slightly better agreement with experimental values is obtained when using ICRU‐90 recommendations. Conclusion Users of the addendum to the TG‐51 protocol for reference dosimetry of high‐energy photon beams, which recommends Monte Carlo calculated kQ factors, can rest assured that the recommendations of ICRU report 90 on basic data have little impact on this central dosimetric parameter.