| Résumé | 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 two ionization chambers representative of those typically used for linac photon reference dosimetry, a graphite- and a plasticwalled chamber, 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 % impacts from ICRU-90 recommendations on the beam quality specifiers, the photon component of the percentage depth-dose at 10 cm, %d d (10)x, and the tissuephantom ratio at 20 cm and 10 cm, TPR20 10, 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 0.6 % and 0.4 %, 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 highenergy 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 ICRUreport 90 on basic data have little impact on this central dosimetric parameter. |
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