School of Applied Sciences and Health Innovations Research Institute, RMIT University, Melbourne, Australia.
J Appl Clin Med Phys. 2012 Nov 8;13(6):4019. doi: 10.1120/jacmp.v13i6.4019.
The objective of this work is to quantify the systematic errors introduced by the common assumption of invariant secondary electron spectra with changing field sizes, as relevant to stereotactic radiotherapy and other treatment modes incorporating small beam segments delivered with a linac-based stereotactic unit. The EGSnrc/BEAMnrc Monte Carlo radiation transport code was used to construct a dosimetrically-matched model of a Varian 600C linear accelerator with mounted BrainLAB micro-multileaf collimator. Stopping-power ratios were calculated for field sizes ranging from 6 × 6 mm2 up to the maximum (98 × 98 mm2), and differences between these and the reference field were computed. Quantitative stopping power data for the BrainLAB micro-multileaf collimator has been compiled. Field size dependent differences to reference conditions increase with decreasing field size and increasing depth, but remain a fraction of a percent for all field sizes studied. However, for dosimetry outside the primary field, errors induced by the assumption of invariant electron spectra can be greater than 1%, increasing with field size. It is also shown that simplification of the Spencer-Attix formulation by ignoring secondary electrons below the cutoff kinetic energy applied to the integration results in underestimation of stopping-power ratios of about 0.3% (and is independent of field size and depth). This work is the first to quantify stopping powers from a BrainLAB micro-multileaf collimator. Many earlier studies model simplified beams, ignoring collimator scatter, which is shown to significantly influence the spectrum. Importantly, we have confirmed that the assumption of unchanging electron spectra with varying field sizes is justifiable when performing (typical) in-field dosimetry of stereotactic fields. Clinicians and physicists undertaking precise out-of-field measurements for the purposes of risk estimation, ought to be aware that the more pronounced spectral variation results in stopping powers (and hence doses) that differ more than for in-field dosimetry.
这项工作的目的是量化在改变射野大小的情况下，假设次级电子谱不变所引入的系统误差，这与立体定向放射治疗和其他包含使用基于直线加速器的立体定向单元的小射束段的治疗模式有关。使用 EGSnrc/BEAMnrc 蒙特卡罗辐射传输代码构建了配备 BrainLAB 微多叶准直器的瓦里安 600C 直线加速器的剂量匹配模型。计算了从 6×6mm2 到最大（98×98mm2）的射野大小的阻止本领比，并计算了这些值与参考射野的差异。编译了 BrainLAB 微多叶准直器的定量阻止本领数据。与参考条件相比，随射野尺寸减小和深度增加而增大的依赖于射野尺寸的差异，但对于研究的所有射野尺寸，仍保持在百分之几。然而，在主射野外的剂量测定中，假设电子谱不变所引起的误差可能大于 1%，并且随着射野尺寸的增大而增大。还表明，通过忽略应用于积分结果的截止动能以下的次级电子，简化 Spencer-Attix 公式会导致阻止本领比的低估约 0.3%（并且与射野尺寸和深度无关）。这项工作首次对 BrainLAB 微多叶准直器的阻止本领进行了量化。许多早期的研究模型简化了射束，忽略了准直器散射，结果表明这会显著影响谱。重要的是，我们已经证实，在对立体定向场进行（典型）场内剂量测定时，改变射野大小时不改变电子谱的假设是合理的。对于出于风险估计目的进行精确场外测量的临床医生和物理学家，应该意识到，更明显的谱变化会导致阻止本领（因此剂量）的差异大于场内剂量测定。
