Accuray Inc., Madison, WI, USA.
Med Phys. 2011 Nov;38(11):5945-60. doi: 10.1118/1.3644842.
The longitudinal dose ripple on the off-axis caused by helical radiation delivery, such as the TomoTherapy system, has been observed, and its relation with respect to pitch has been studied with empirically found optimal pitches, 0.86∕n, by Kissick et al. [Med. Phys. 32, 1414-1423 (2005)]. This ripple artifact referred to as the thread effect is periodic in nature and is caused by various periodic factors. In this work, the factors that cause the thread effect were unveiled, including jaw profile divergence, the inverse square law, attenuation, and the cone effect, and their impact on the thread effect were studied.
Mathematical formulation for individual and combined factors were set up. Based on theoretical analysis and simulations, optimal pitches that result in local minima of the ripple amplitude with respect to the jaw width and off-axis distance were identified and verified. The effectiveness of optimization in reducing the thread effect were also studied.
Analysis and simulation based on the square-shaped jaw profiles well characterize the thread effect. Simulations based on the real jaw profiles show reduced ripples and very good agreement of optimal pitches compared with those based on the square profiles. The optimal pitches were found to have little jaw width dependence, except for the real jaw profile of small width (1.05 cm). The optimal pitches for the real jaw profile of width 1.05 cm are unidentifiable except for the largest ones, due to the relative smoothness of the jaw profile. With optimized intensity modulation, the thread effect can be largely suppressed. For real jaw profiles, the optimal pitches with or without dose optimization do not change much. The numbers 0.86∕n found by Kissick et al. well approximate the optimal pitches for off-axis distance of 5 cm. However, optimal pitches are not universal for different off-axis distances: they decrease as the off-axis distance increases.
The thread effect can be well explained by the proposed model. Optimization can largely reduce the thread effect. However, an optimal pitch reduces the ripple much easier especially when optimization is limited by many constraints. The optimal pitches predicted by the proposed model could be used as a reference for pitch selection regardless the tumor is at large or small off-axis distance.
在轴外，由于螺旋放射治疗（如 TomoTherapy 系统）会产生纵向剂量纹波，Kissick 等人曾对其与螺距的关系进行过研究，并找到了经验性的最佳螺距，即 0.86∕n。[Med. Phys. 32, 1414-1423 (2005)]。这种周期性的纹波伪影被称为线程效应，是由各种周期性因素引起的。在这项工作中，我们揭示了导致线程效应的因素，包括：机架准直器的发散、平方反比定律、衰减和锥形束效应，并研究了它们对线程效应的影响。
建立了单个和组合因素的数学公式。基于理论分析和模拟，确定并验证了在考虑准直器宽度和轴外距离时，使纹波幅度局部最小的最佳螺距。还研究了优化对减少线程效应的有效性。
基于方形准直器轮廓的分析和模拟很好地描述了线程效应。基于真实准直器轮廓的模拟显示，与基于方形轮廓的模拟相比，纹波减少，最佳螺距非常吻合。除了最小宽度（1.05cm）的真实准直器轮廓外，最佳螺距与准直器宽度几乎没有关系。对于宽度为 1.05cm 的真实准直器轮廓，由于准直器轮廓相对平滑，因此无法确定最佳螺距，除了最大的螺距之外。通过优化强度调制，可以在很大程度上抑制线程效应。对于真实准直器轮廓，无论是否进行剂量优化，最佳螺距都不会有太大变化。Kissick 等人发现的数字 0.86∕n 很好地近似于 5cm 轴外距离的最佳螺距。然而，最佳螺距不是通用的，对于不同的轴外距离：它们随着轴外距离的增加而减小。
所提出的模型可以很好地解释线程效应。优化可以大大减少线程效应。然而，特别是当优化受到许多约束时，一个最佳螺距可以更容易地减少纹波。无论肿瘤在大或小的轴外距离，所提出的模型预测的最佳螺距都可以作为螺距选择的参考。
