Mainegra-Hing Ernesto, Muir Bryan R
Measurement Science and Standards, National Research Council of Canada, Ottawa, ON, K1A 0R6, Canada.
Med Phys. 2018 Jun 3. doi: 10.1002/mp.13027.
To assess the impact of the ICRU report 90 recommendations on the beam-quality conversion factor, k , used for clinical reference dosimetry of megavoltage linac photon beams.
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 k factors are compared using the two different recommendations for key dosimetry data and measured k factors.
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) , 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 k 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.
Users of the addendum to the TG-51 protocol for reference dosimetry of high-energy photon beams, which recommends Monte Carlo calculated k factors, can rest assured that the recommendations of ICRU report 90 on basic data have little impact on this central dosimetric parameter.
评估国际辐射单位与测量委员会（ICRU）第90号报告的建议对用于兆伏级直线加速器光子束临床参考剂量测定的射束质量转换因子k的影响。
使用蒙特卡罗模拟，在水模体的参考深度处计算代表直线加速器光子参考剂量测定中通常使用的电离室中的水吸收剂量和空气吸收剂量。还进行了水中深度剂量计算，以研究射束质量指标的变化。使用EGSnrc模拟工具包在钴-60射束和标称能量在6兆伏至25兆伏之间的兆伏光子射束中进行计算。计算的输入使用来自原始ICRU-37报告的石墨和水的阻止本领数据以及最近发布的ICRU-90报告中提出的新值。使用关于关键剂量测定数据的两种不同建议和测量的k因子比较计算的k因子。
观察到ICRU-90建议对射束质量指标、10厘米处深度剂量百分比的光子分量%dd(10)以及20厘米和10厘米处的组织模体比TPR1020的影响小于约0.1%。尽管对关键剂量测定数据使用不同的建议分别对水-空气阻止本领比和电离室扰动校正产生高达0.54%和0.40%的影响，但我们观察到计算的k因子几乎没有差异（≤0.14%）。这与ICRU-90中的预测相矛盾，该预测表明在高能光子束中差异高达0.5%。使用ICRU-90建议时与实验值的一致性稍好。
对于高能光子束参考剂量测定的TG-51协议附录的用户，该附录推荐蒙特卡罗计算的k因子，可以放心，ICRU第90号报告关于基本数据的建议对这个核心剂量测定参数影响很小。
