Department of Medical Physics, ULSS 6 - 36100 Vicenza, Italy.
Med Phys. 2011 Dec;38(12):6513-27. doi: 10.1118/1.3660770.
The scope of this study was to determine a complete set of correction factors for several detectors in static small photon fields for two linear accelerators (linacs) and for several detectors.
Measurements for Monte Carlo (MC) commissioning were performed for two linacs, Siemens Primus and Elekta Synergy. After having determined the source parameters that best fit the measurements of field specific output factors, profiles, and tissue-phantom ratio, the generalized version of the classical beam quality correction factor for static small fields, k(Q(clin),Q(msr) ) (f(clin),f(msr) ), were determined for several types of detectors by using the egs_chamber Monte Carlo user code which can accurately reproduce the geometry and the material composition of the detector. The influence of many parameters (energy and radial FWHM of the electron beam source, field dimensions, type of accelerator) on the value of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) was evaluated. Moreover, a MC analysis of the parameters that influence the change of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) as a function of field dimension was performed. A detailed analysis of uncertainties related to the measurements of the field specific output factor and to the Monte Carlo calculation of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) was done.
The simulations demonstrated that the correction factor k(Q(clin),Q(msr) ) (f(clin),f(msr) ) can be considered independent from the quality beam factor Q in the range 0.68 ± 0.01 for all the detectors analyzed. The k(Q(clin),Q(msr) ) (f(clin),f(msr) ) of PTW 60012 and EDGE diodes can be assumed dependent only on the field size, for fields down to 0.5 × 0.5 cm². The microLion, and the microchambers, instead, must be used with some caution because they exhibit a slight dependence on the radial FWHM of the electron source, and therefore, a correction factor only dependent on field size can be used for fields ≥ 0.75 × 0.75 and ≥ 1.0 × 1.0 cm², respectively. The analysis of uncertainties gave an estimate of uncertainty for the 0.5 × 0.5 cm² field of about 0.7% (1σ) for k(Q(clin),Q(msr) ) (f(clin),f(msr) ) factor and of about 1.0% (1σ) for the field output factor, Ω(Q(clin),Q(msr) ) (f(clin),f(msr) ), of diodes, microchambers, and microLion.
Stereotactic diodes with the appropriate k(Q(clin),Q(msr) ) (f(clin),f(msr) ) are recommended for determining Ω(Q(clin),Q(msr) ) (f(clin),f(msr) ) of small photon beams.
本研究旨在为两台线性加速器(linac)和几种探测器确定一组完整的静态小光子场中几个探测器的修正因子。
对两台直线加速器(Siemens Primus 和 Elekta Synergy)进行了用于蒙特卡罗(MC)调试的测量。在确定了最能拟合场特定输出因子、轮廓和组织-体模比测量的源参数后,使用能够准确再现探测器几何形状和材料组成的 egs_chamber Monte Carlo 用户代码,为几种类型的探测器确定了静态小场经典束质修正因子的广义版本,即 k(Q(clin),Q(msr) )(f(clin),f(msr) )。评估了许多参数(电子束源的能量和径向半高全宽、场尺寸、加速器类型)对 k(Q(clin),Q(msr) )(f(clin),f(msr) )值的影响。此外,还对作为场尺寸函数变化的 k(Q(clin),Q(msr) )(f(clin),f(msr) )参数的 MC 分析进行了研究。对场特定输出因子测量和 k(Q(clin),Q(msr) )(f(clin),f(msr) )的蒙特卡罗计算相关的不确定性进行了详细分析。
模拟表明,对于所有分析的探测器,修正因子 k(Q(clin),Q(msr) )(f(clin),f(msr) )可以认为独立于束质因子 Q,在 0.68 ± 0.01 的范围内。PTW 60012 和 EDGE 二极管的 k(Q(clin),Q(msr) )(f(clin),f(msr) )可以仅假定依赖于场大小,对于低至 0.5×0.5 cm²的场。然而,microLion 和 microchambers 则需要谨慎使用,因为它们表现出对电子源径向半高全宽的轻微依赖性,因此,仅依赖于场大小的修正因子可用于分别为 0.75×0.75 和≥1.0×1.0 cm²的场。不确定性分析估计了 0.5×0.5 cm² 场的 k(Q(clin),Q(msr) )(f(clin),f(msr) )因子的不确定性约为 0.7%(1σ),二极管、microchambers 和 microLion 的场输出因子,Ω(Q(clin),Q(msr) )(f(clin),f(msr) )的不确定性约为 1.0%(1σ)。
对于确定小光子束的Ω(Q(clin),Q(msr) )(f(clin),f(msr) ),建议使用具有适当 k(Q(clin),Q(msr) )(f(clin),f(msr) )的立体定向二极管。