Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Phys Med Biol. 2014 Feb 21;59(4):951-60. doi: 10.1088/0031-9155/59/4/951. Epub 2014 Feb 7.
Phantom Scatter Factors, Sp in the Khan formalism (Khan et al 1980 J. Radiat. Oncol. Biol. Phys. 6 745-51) describe medium-induced changes in photon-beam intensity as a function of size of the beam. According to the British Journal of Radiology, Supplement 25, megavoltage phantom scatter factors are invariant as a function of photon-beam energy. However, during the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian TrueBeam(TM) 6-MV FFF and 10-MV FFF), differences were noted in phantom scatter between the filtered beams and FFF-mode beams. The purpose of this work was to evaluate this difference and provide an analytical formalism to explain the phantom scatter differences between FFF-mode and the filtered mode. An analytical formalism was devised to demonstrate the source of phantom scatter differences between the filtered and the FFF-mode beams. The reason for the differences in the phantom scatter factors between the filtered and the FFF-mode beams is hypothesized to be the non-uniform beam profiles of the FFF-mode beams. The analytical formalism proposed here is based on this idea, taking the product of the filtered phantom scatter factors and the ratio of the off-axis ratio between the FFF-mode and the filtered beams. All measurements were performed using a Varian TrueBeam(TM) linear accelerator with photon energies of 6-MV and 10-MV in both filtered and FFF-modes. For all measurements, a PTW Farmer type chamber and a Scanditronix CC04 cylindrical ionization were used. The in-water measurements were made at depth dose maximum and 100 cm source-to-axis distance. The in-air measurements were done at 100 cm source-to-axis distance with appropriate build-up cap. From these measurements, the phantom scatter factors were derived for the filtered beams and the FFF-mode beams for both energies to be evaluated against the phantoms scatter factors calculated using the proposed algorithm. For 6-MV, the difference between the measured and the calculated FFF-mode phantom scatter factors ranged from -0.34% to 0.73%. The average per cent difference was -0.17% (1σ = 0.25%). For 10-MV, the difference ranged from -0.19% to 0.24%. The average per cent difference was -0.17% (1σ = 0.13%). An analytical formalism was presented to calculate the phantom scatter factors for FFF-mode beams using filtered phantom scatter factors as a basis. The overall differences between measurements and calculations were within ± 0.5% for 6-MV and ± 0.25% for 10-MV.
幻影散射因子，Khan 形式主义中的自旋（Khan 等人，1980 年，《放射肿瘤生物学物理杂志》，6745-51）描述了作为光束大小函数的光子束强度的介质诱导变化。根据《英国放射学杂志》增刊 25，兆伏级幻影散射因子作为光子束能量的函数是不变的。然而，在使用带有平坦滤波器的加速器进行调试时（Varian TrueBeam(TM)6-MV FFF 和 10-MV FFF），在过滤光束和 FFF 模式光束之间注意到了幻影散射的差异。这项工作的目的是评估这种差异，并提供一个分析公式来解释 FFF 模式和过滤模式之间的幻影散射差异。设计了一个分析公式来证明过滤模式和 FFF 模式之间的幻影散射差异的来源。过滤模式和 FFF 模式之间的幻影散射因子差异的原因被假设为 FFF 模式光束的非均匀光束轮廓。这里提出的分析公式基于这一想法，取过滤模式的幻影散射因子的乘积和 FFF 模式与过滤模式之间的离轴比的比值。所有测量均使用 Varian TrueBeam(TM)线性加速器在过滤和 FFF 模式下进行，光子能量为 6-MV 和 10-MV。对于所有测量，均使用 PTW Farmer 型室和 Scanditronix CC04 圆柱形电离室进行。在水深剂量最大值和 100cm 源轴距离处进行水中测量。在 100cm 源轴距离处进行空气测量，并使用适当的建堆帽。根据这些测量，为过滤光束和两种能量的 FFF 模式光束导出了幻影散射因子，以便与使用所提出的算法计算的幻影散射因子进行比较。对于 6-MV，测量值与计算值之间的 FFF 模式幻影散射因子差异范围为-0.34%至 0.73%。平均百分比差异为-0.17%（1σ=0.25%）。对于 10-MV，差异范围为-0.19%至 0.24%。平均百分比差异为-0.17%（1σ=0.13%）。提出了一种分析公式，用于使用过滤模式的幻影散射因子作为基础，计算 FFF 模式光束的幻影散射因子。6-MV 的测量值与计算值之间的总体差异在±0.5%以内，10-MV 的测量值与计算值之间的总体差异在±0.25%以内。
