Li X A, Rogers D W
Institute for National Measurement Standards, National Research Council, Canada, Ottawa, Canada.
Med Phys. 1994 Jun;21(6):791-7. doi: 10.1118/1.597395.
The percentage depth dose at 10 cm in a 10 x 10-cm2 photon beam at an SSD of 100 cm, %dd(10), is a better beam-quality specifier for radiotherapy beams than the commonly used values of TPR10(20) or nominal accelerating potential. The presence of electron contamination affects the measurement of %dd(10) but can be removed by the use of a 0.1-cm lead filter, which reduces surface dose from contaminant electrons from the accelerator by more than 95% for radiotherapy beams with energies from 60Co to 50 MV. The filter performs best when it is placed immediately below the head. An electron-contamination correction factor is introduced to correct for electron contamination from the filter and air. It converts the %dd(10) which includes the electron contamination with the filter in place [hereafter %dd(10)m], into %dd(10) for just the photons in the filtered beam. The correction factor is a linear function of %dd(10)m for all filtered beams with %dd(10)m > 70%. A small correction for the photon filtering effect converts the pure photon %dd(10) for the filtered beam into that for the unfiltered beam, which can be used to determine stopping-power ratio. Calculations show that the values of water-to-air stopping power ratio in the unfiltered beam are related to the values of %dd(10)m in the filtered beam by a cubic function. The uncertainty of stopping-power ratios in unfiltered beams for the same value of the %dd(10)m is within 0.2% for all beams.
在源皮距为100 cm的10×10 cm²光子束中，10 cm深度处的百分深度剂量%dd(10)，对于放射治疗束而言，是比常用的TPR10(20)值或标称加速电位更好的射束质量指标。电子污染的存在会影响%dd(10)的测量，但可通过使用0.1 cm的铅滤过器将其去除，对于能量从60Co到50 MV的放射治疗束，该滤过器可使加速器中污染电子产生的表面剂量降低超过95%。当滤过器直接置于机头下方时效果最佳。引入一个电子污染校正因子，以校正来自滤过器和空气的电子污染。它将包含滤过器存在时电子污染的%dd(10) [以下简称%dd(10)m]，转换为仅针对滤过射束中光子的%dd(10)。对于所有%dd(10)m > 70%的滤过射束，校正因子是%dd(10)m的线性函数。对光子滤过效应进行的小校正，可将滤过射束的纯光子%dd(10)转换为未滤过射束的纯光子%dd(10)，后者可用于确定阻止本领比。计算表明，未滤过射束中水对空气的阻止本领比值与滤过射束中%dd(10)m的值通过一个三次函数相关。对于相同的%dd(10)m值，所有射束未滤过射束中阻止本领比的不确定度在0.2%以内。
