Almberg S S, Frengen J, Lindmo T
Department of Oncology and Radiotherapy, St. Olavs University Hospital, N-7006 Trondheim, Norway.
Med Phys. 2012 Aug;39(8):5194-203. doi: 10.1118/1.4738963.
To compare dosimetric characteristics of 6 MV photon fields originating from a linear accelerator operating with (FF) and without (FFF) a flattening-filter. The main objective is to establish a FFF model that results in similar depth-dose and build-up profiles as the original FF model, and subsequently estimate and compare out-of-field dose distributions.
The EGSnrc Monte Carlo user codes BEAMnrc and DOSXYZnrc are used for photon beam simulations of an Elekta linear accelerator and dose calculations in a water phantom, respectively. Three beam models were analyzed: (1) the conventional linear accelerator with the flattening-filter in place and incident electron energy 6.45 MeV (FF 6.45 MeV), (2) similar flattening-filter-free model (FFF 6.45 MeV), and (3) as (2) but with increased electron energy (FFF 8.0 MeV). The field size 5 × 5 cm(2) was used for characterization of dose output, depth dose profiles, and photon spectrum. The field size 40 × 40 cm(2) was used for characterization of cross-field photon energy, photon fluence, and dose distributions. Out-of-field dose distributions were analyzed in both in-plane and cross-plane directions for 5 × 5 cm(2) and 10 × 10 cm(2) fields.
Comparable depth dose distributions, including the build-up region, for FF and FFF fields were achieved by increasing the electron energy from 6.45 MeV to 8.0 MeV for the FFF beam. The FFF beams result in reduced out-of-field dose compared to the FF beam: the reduction was most apparent in the cross-plane direction and more pronounced by the FFF 8.0 MeV beam compared to the FFF 6.45 MeV beam. Differences in out-of-field dose due to direction (in-plane vs cross-plane) were up to 40% for the FF beam; this effect was significantly reduced for the FFF beams. As the flattening-filter is a major source of contaminating electrons, superficial out-of-field dose was expected, and was found to be, reduced for FFF beams.
The build-up and depth-dose characteristics of a conventional "6 MV" beam can be maintained when changing to a flattening-filter-free modality by increasing the incident electron energy from 6.45 MeV to 8.0 MeV. This will at the same time reduce the out-of-field dose for regions up to 20 cm from the central axis by 10%-30% compared to the original FF situation.
比较源自带有均整器（FF）和不带有均整器（FFF）运行的直线加速器的6兆伏光子射野的剂量学特征。主要目标是建立一个FFF模型，使其产生与原始FF模型相似的深度剂量和剂量建成曲线，随后估算并比较射野外剂量分布。
分别使用EGSnrc蒙特卡罗用户代码BEAMnrc和DOSXYZnrc对医科达直线加速器进行光子束模拟，并在水模体中进行剂量计算。分析了三种射束模型：（1）带有均整器且入射电子能量为6.45兆伏的传统直线加速器（FF 6.45兆伏），（2）类似的无均整器模型（FFF 6.45兆伏），以及（3）与（2）相同但电子能量增加的模型（FFF 8.0兆伏）。使用5×5平方厘米的射野尺寸来表征剂量输出、深度剂量曲线和光子能谱。使用40×40平方厘米的射野尺寸来表征射野交叉方向的光子能量、光子注量和剂量分布。针对5×5平方厘米和10×10平方厘米的射野，在平面内和平面交叉方向上分析射野外剂量分布。
对于FFF射束，通过将电子能量从6.45兆伏增加到8.0兆伏，实现了与FF射野相当的深度剂量分布，包括剂量建成区。与FF射束相比，FFF射束导致射野外剂量降低：这种降低在平面交叉方向最为明显，并且与FFF 6.45兆伏射束相比，FFF 8.0兆伏射束更为显著。对于FF射束，由于方向（平面内与平面交叉）导致的射野外剂量差异高达40%；对于FFF射束，这种效应显著降低。由于均整器是污染电子的主要来源，预计并发现FFF射束的浅表射野外剂量会降低。
当通过将入射电子能量从6.45兆伏增加到8.0兆伏转变为无均整器模式时，可以保持传统“6兆伏”射束的剂量建成和深度剂量特征。与原始FF情况相比，这同时将距中心轴20厘米范围内区域的射野外剂量降低10% - 30%。
