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The helical tomotherapy thread effect.

机译：螺旋型tomotherapy线效果。

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Inherent to helical tomotherapy is a dose variation pattern that manifests as a "ripple" (peak-to-trough relative to the average). This ripple is the result of helical beam junctioning, completely unique to helical tomotherapy. Pitch is defined as in helical CT, the couch travel distance for a complete gantry rotation relative to the axial beam width at the axis of rotation. Without scattering or beam divergence, an analytical posing of the problem as a simple integral predicts minima near a pitch of 1 where n is an integer. A convolution-superposition dose calculator (TomoTherapy, Inc.) included all the physics needed to explore the ripple magnitude versus pitch and beam width. The results of the dose calculator and some benchmark measurements demonstrate that the ripple has sharp minima near p=0.86(1). The 0.86 factor is empirical and caused by a beam junctioning of the off-axis dose profiles which differ from the axial profiles as well as a long scatter tail of the profiles at depth. For very strong intensity modulation, the 0.86 factor may vary. The authors propose choosing particular minima pitches or using a second delivery that starts 180 deg off-phase from the first to reduce these ripples: "Double threading." For current typical pitches and beam widths, however, this effect is small and not clinically important for most situations. Certain extremely large field or high pitch cases, however, may benefit from mitigation of this effect.
机译：螺旋断层扫描的内在特点是剂量变化模式，表现为“波纹”（相对于平均值的峰谷）。这种波纹是螺旋束连接的结果，这是螺旋断层扫描完全独有的。螺距的定义如螺旋CT所示，即龙门架完全旋转的相对于旋转轴上的轴向光束宽度的床移动距离。在没有散射或光束发散的情况下，以简单积分的形式对问题的分析表示可以预测接近1 / n的间距处的最小值，其中n是整数。卷积叠加剂量计算器（TomoTherapy，Inc.）包括探索纹波幅度与音高和光束宽度有关的所有物理原理。剂量计算器和一些基准测试的结果表明，纹波在p = 0.86（1 / n）附近具有极小的最小值。 0.86系数是经验值，由离轴剂量分布图的光束交界引起，离轴剂量分布图与轴向分布图不同，并且在深度上分布图的长散射尾部也不同。对于非常强的强度调制，0.86系数可能会有所不同。作者建议选择特定的最小螺距，或使用第二种输送方式，从第一种输送到180度异相，以减少这些波动：“双螺纹”。但是，对于当前的典型音高和波束宽度，这种影响很小，并且在大多数情况下在临床上并不重要。但是，某些极端大的场或高音调的情况可能会受益于这种影响的减轻。

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