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J Appl Clin Med Phys. 2016 May 8;17(3):358-370. doi: 10.1120/jacmp.v17i3.6173.
This study presents the basic dosimetric properties of photon beams of a Versa HD linear accelerator (linac), which is capable of delivering flattening filter-free (FFF) beams with a beam quality equivalent to the corresponding flattened beams based on comprehensive beam data measurement. The analyzed data included the PDDs, profiles, penumbra, out-of-field doses, surface doses, output factors, head and phantom scatter factors, and MLC transmissions for both FFF and flattened beams of 6 MV and 10 MV energy from an Elekta Versa HD linac. The 6MVFFF and 10MVFFF beams had an equivalent mean energy to the flattened beams and showed less PDD variations with the field sizes. Compared with their corresponding flattened beams, Dmax was deeper for FFF beams for all field sizes; the ionization ratio variations with the field size were lower for FFF beams; the out-of-field doses were lower and the penumbras were sharper for the FFF beams; the off-axis profile variations with the depths were lesser for the FFF beams. Further, the 6MVFFF and 10MVFFF beams had 35.7% and 40.9% less variations in output factor with the field size, respectively. The collimator exchange effect was reduced in the FFF mode. The head scatter factor showed 59.1% and 73.6% less variations, on average, for the 6MVFFF and 10MVFFF beams, respectively; the variations in the phantom scatter factor were also smaller. The surface doses for all beams increased linearly with the field size. The 6MVFFF and 10MVFFF beams had higher surface doses than the corresponding flattened beams for field sizes of up to 10 ×10cm2 but had lower surface doses for larger fields. Both FFF beams had lower average MLC transmissions than the flattened beams. The finding that the FFF beams were of equivalent quality to the corresponding flattened beams indicates a significant dif-ference from the data on unmatched FFF beams.
本研究介绍了 Versa HD 直线加速器（linac）的光子束基本剂量学特性，该加速器能够提供等效于相应平坦化光束的质量的无均整滤过（FFF）光束，这是基于全面的光束数据测量得出的。分析的数据包括 6MV 和 10MV 能量的 PDDs、 profiles、半影、野外剂量、表面剂量、输出因子、头影散射因子和多叶准直器（MLC）传输，这些数据来自 Elekta Versa HD linac 的 FFF 和平坦化光束。6MVFFF 和 10MVFFF 光束的平均能量与平坦化光束相当，且在不同射野尺寸下的 PDD 变化较小。与相应的平坦化光束相比，FFF 光束的 Dmax 在所有射野尺寸下都更深；FFF 光束的电离比随射野尺寸的变化较小；FFF 光束的野外剂量较低，半影较锐利；FFF 光束的离轴轮廓随深度的变化较小。此外，6MVFFF 和 10MVFFF 光束的输出因子随射野尺寸的变化分别减少了 35.7%和 40.9%。FFF 模式下的准直器交换效应减小。头影散射因子的变化，6MVFFF 和 10MVFFF 光束分别平均减少了 59.1%和 73.6%；体模散射因子的变化也较小。所有光束的表面剂量随射野尺寸线性增加。6MVFFF 和 10MVFFF 光束在 10×10cm2 以内的射野尺寸下比相应的平坦化光束有更高的表面剂量，但在较大的射野尺寸下则有更低的表面剂量。与平坦化光束相比，FF 光束的平均 MLC 传输较低。与不匹配的 FFF 光束的数据相比，FFF 光束的等效质量表明存在显著差异。
