Boyd Robert A, Hogstrom Kenneth R, White R Allen, Antolak John A
Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, USA.
Med Phys. 2002 Nov;29(11):2571-83. doi: 10.1118/1.1517293.
Currently, the pencil-beam redefinition algorithm (PBRA) utilizes a single electron source to model clinical electron beams. In the single-source model, the electrons appear to originate from a virtual source located near the scattering foils. Although this approach may be acceptable for most treatment machines, previous studies have shown dose differences as high as 8% relative to the given dose for small fields for some machines such as the Varian Clinac 1800. In such machines collimation-scattered electrons originating from the photon jaws and the applicator give rise to extra-focal electron sources. In this study, we examined the impact of modeling an additional electron source to better account for the collimator-scattered electrons. The desired dose calculation accuracy in water throughout the dose distribution is 3% or better relative to the given dose. We present here a methodology for determining the electron-source parameters for the dual-source model using a minimal set of data, that is, two central-axis depth-dose curves and two off-axis profiles. A Varian Clinac 1800 accelerator was modeled for beam energies of 20 and 9 MeV and applicator sizes of 15 x 15 and 6 x 6 cm2. The improvement in the accuracy of PBRA-calculated dose, evaluated using measured two-dimensional dose distributions in water, was characterized using the figure of merit, FA3%, which represents the fractional area containing dose differences greater than 3%. For the 15 x 15 cm2 field the evaluation was restricted to the penumbral region, and for the 6 x 6 cm2 field the central region of the beam was included as it was impacted by the penumbra. The greatest improvement in dose accuracy was for the 6 x 6 cm2 applicator. At 9 MeV, FA3% decreased from 15% to 0% at 100 cm SSD and from 34% to 4% at 110 cm SSD. At 20 MeV, FA3% decreased from 17% to 2% at 100 cm SSD and from 41% to 10% at 110 cm SSD. In the penumbra of the 15 x 15 cm2 applicator, the improvement was less, but still significant. At 9 MeV, FA3% changed from 11% to 1% at 100 cm SSD and from 10% to 12% at 110 cm SSD. At 20 MeV, FA3% decreased from 12% to 8% at 100 cm SSD and from 14% to 5% at 110 cm SSD. Results demonstrate that use of a dual-source beam model can provide significantly improved accuracy in the PBRA-calculated dose distribution that was not achievable with a single-source beam model when modeling the Varian Clinac 1800 electron beams. Time of PBRA dose calculation was approximately doubled; however, dual-source beam modeling of newer accelerators (e.g., the Varian Clinac 2100) may not be necessary because of less impact of collimator-scattered electrons on dosimetry.
目前，笔形束重新定义算法（PBRA）利用单个电子源来模拟临床电子束。在单源模型中，电子似乎源自位于散射箔附近的虚拟源。尽管这种方法对大多数治疗机来说可能是可接受的，但先前的研究表明，对于某些机器，如瓦里安Clinac 1800，相对于给定剂量，小射野的剂量差异高达8%。在这类机器中，源自光子准直器和限束器的准直散射电子会产生焦外电子源。在本研究中，我们研究了对额外电子源进行建模以更好地考虑准直器散射电子的影响。在整个剂量分布中，水中所需的剂量计算精度相对于给定剂量为3%或更高。我们在此提出一种使用最少数据集（即两条中心轴深度剂量曲线和两条离轴剂量分布曲线）来确定双源模型电子源参数的方法。对一台瓦里安Clinac 1800加速器进行建模，模拟20和9 MeV的束能量以及15×15和6×6 cm²的限束器尺寸。使用水中测量的二维剂量分布来评估PBRA计算剂量的精度提高情况，用品质因数FA3%来表征，它表示剂量差异大于3%的部分面积。对于15×15 cm²射野，评估仅限于半值层区域，对于6×6 cm²射野，由于其受半值层影响，所以包括束的中心区域。剂量精度的最大提高出现在6×6 cm²限束器的情况下。在9 MeV时，在100 cm源皮距处FA3%从15%降至0%，在110 cm源皮距处从34%降至4%。在20 MeV时，在100 cm源皮距处FA3%从17%降至2%，在110 cm源皮距处从41%降至10%。在15×15 cm²限束器的半值层区域，改善较小，但仍然显著。在9 MeV时，在100 cm源皮距处FA3%从11%变为1%，在110 cm源皮距处从10%变为12%。在20 MeV时，在100 cm源皮距处FA3%从12%降至8%，在110 cm源皮距处从14%降至5%。结果表明，当对瓦里安Clinac 1800电子束进行建模时，使用双源束模型可以显著提高PBRA计算剂量分布的精度，这是单源束模型无法实现的。PBRA剂量计算时间大约增加了一倍；然而，对于更新的加速器（如瓦里安Clinac 2100），由于准直器散射电子对剂量测定的影响较小，可能不需要进行双源束建模。
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