Department of Radiation Oncology, Mayo Clinic Rochester, 200 First Street SW, Rochester, MN, 55905, USA.
J Appl Clin Med Phys. 2019 Sep;20(9):69-77. doi: 10.1002/acm2.12702.
To quantify the effects of combining layer-based repainting and respiratory gating as a strategy to mitigate the dosimetric degradation caused by the interplay effect between a moving target and dynamic spot-scanning proton delivery.
An analytic routine modeled three-dimensional dose distributions of pencil-beam proton plans delivered to a moving target. Spot positions and weights were established for a single field to deliver 100 cGy to a static, 15-cm deep, 3-cm radius spherical clinical target volume with a 1-cm isotropic internal target volume expansion. The interplay effect was studied by modeling proton delivery from a clinical synchrotron-based spot scanning system and respiratory target motion, patterned from surrogate patient breathing traces. Motion both parallel and orthogonal to the beam scanning direction was investigated. Repainting was modeled using a layer-based technique. For each of 13 patient breathing traces, the dose from 20 distinct delivery schemes (combinations of four gate window amplitudes and five repainting techniques) was computed. Delivery strategies were inter-compared based on target coverage, dose homogeneity, high dose spillage, and delivery time.
Notable degradation and variability in plan quality were observed for ungated delivery. Decreasing the gate window reduced this variability and improved plan quality at the expense of longer delivery times. Dose deviations were substantially greater for motion orthogonal to the scan direction when compared with parallel motion. Repainting coupled with gating was effective at partially restoring dosimetric coverage at only a fraction of the delivery time increase associated with very small gate windows alone. Trends for orthogonal motion were similar, but more complicated, due to the increased severity of the interplay.
Layer-based repainting helps suppress the interplay effect from intra-gate motion, with only a modest penalty in delivery time. The magnitude of the improvement in target coverage is strongly influenced by individual patient breathing patterns and the tumor motion trajectory.
量化基于层的重绘和呼吸门控相结合的效果，以减轻运动目标和动态点扫描质子输送之间相互作用效应对剂量学的降低。
一种分析例程对递送到运动目标的笔束质子计划的三维剂量分布进行建模。为了将 100cGy 传递到具有 1cm 各向同性内部目标体积扩展的静态、15cm 深、3cm 半径球形临床目标体积，为单个场建立了点位置和权重。通过模拟临床基于同步加速器的点扫描系统的质子输送和呼吸目标运动，从替代患者呼吸轨迹中模拟相互作用效应。研究了与束扫描方向平行和正交的运动。使用基于层的技术对重绘进行建模。对于每个 13 个患者呼吸轨迹，计算了 20 个不同输送方案（四个门控窗口幅度和五种重绘技术的组合）的剂量。基于目标覆盖、剂量均匀性、高剂量溢出和输送时间对输送策略进行了相互比较。
未门控输送时观察到显著的计划质量降低和变异性。减小门控窗口减少了这种变异性并提高了计划质量，但代价是更长的输送时间。与平行运动相比，当运动与扫描方向正交时，剂量偏差要大得多。重绘与门控相结合，仅在与非常小的门控窗口单独使用相关的输送时间增加的一小部分，就可以有效地部分恢复剂量学覆盖。由于相互作用的严重程度增加，对于正交运动的趋势相似，但更为复杂。
基于层的重绘有助于抑制门内运动的相互作用效应，仅对输送时间有适度的惩罚。目标覆盖率的改善幅度强烈受到个体患者呼吸模式和肿瘤运动轨迹的影响。
