Department of Medical Physics, ProCure Proton Therapy Center, Oklahoma City, Oklahoma 73142, USA.
Med Phys. 2013 Sep;40(9):091708. doi: 10.1118/1.4818283.
The main purposes of this study were to (1) investigate the dependency of lateral penumbra (80%-20% distance) of uniform scanning proton beams on various factors such as air gap, proton range, modulation width, compensator thickness, and depth, and (2) compare the lateral penumbra calculated by a treatment planning system (TPS) with measurements.
First, lateral penumbra was measured using solid-water phantom and radiographic films for (a) air gap, ranged from 0 to 35 cm, (b) proton range, ranged from 8 to 30 cm, (c) modulation, ranged from 2 to 10 cm, (d) compensator thickness, ranged from 0 to 20 cm, and (e) depth, ranged from 7 to 15 cm. Second, dose calculations were computed in a virtual water phantom using the XiO TPS with pencil beam algorithm for identical beam conditions and geometrical configurations that were used for the measurements. The calculated lateral penumbra was then compared with the measured one for both the horizontal and vertical scanning magnets of our uniform scanning proton beam delivery system.
The results in the current study showed that the lateral penumbra of horizontal scanning magnet was larger (up to 1.4 mm for measurement and up to 1.0 mm for TPS) compared to that of vertical scanning magnet. Both the TPS and measurements showed an almost linear increase in lateral penumbra with increasing air gap as it produced the greatest effect on lateral penumbra. Lateral penumbra was dependent on the depth and proton range. Specifically, the width of lateral penumbra was found to be always lower at shallower depth than at deeper depth within the spread out Bragg peak (SOBP) region. The lateral penumbra results were less sensitive to the variation in the thickness of compensator, whereas lateral penumbra was independent of modulation. Overall, the comparison between the results of TPS with that of measurements indicates a good agreement for lateral penumbra, with TPS predicting higher values compared to measurements.
Lateral penumbra of uniform scanning proton beams depends on air gap, proton range, compensator thickness, and depth, whereas lateral penumbra is not dependent on modulation. The XiO TPS typically overpredicted lateral penumbra compared to measurements, within 1 mm for most cases, but the difference could be up to 2.5 mm at a deep depth and large air gap.
本研究的主要目的是：（1）研究等角扫描质子束的侧向半影（80%-20%距离）对各种因素的依赖性，如气隙、质子射程、调制宽度、补偿器厚度和深度；（2）比较治疗计划系统（TPS）计算的侧向半影与测量结果。
首先，使用固体水模体和射线照相胶片测量侧向半影，（a）气隙，范围为 0 至 35 cm；（b）质子射程，范围为 8 至 30 cm；（c）调制，范围为 2 至 10 cm；（d）补偿器厚度，范围为 0 至 20 cm；（e）深度，范围为 7 至 15 cm。其次，在虚拟水模体中使用 XiO TPS 进行剂量计算，使用相同的射束条件和几何配置，与测量结果相同。然后，将计算出的侧向半影与我们等角扫描质子束输送系统的水平和垂直扫描磁铁的测量结果进行比较。
本研究结果表明，水平扫描磁铁的侧向半影大于（测量值为 1.4mm，TPS 计算值为 1.0mm），垂直扫描磁铁的侧向半影。TPS 和测量结果均表明，随着气隙的增加，侧向半影几乎呈线性增加，因为气隙对侧向半影的影响最大。侧向半影与深度和质子射程有关。具体而言，在扩展布拉格峰（SOBP）区域内，在较浅的深度处，侧向半影的宽度总是低于较深的深度。补偿器厚度的变化对侧向半影的影响较小，而侧向半影与调制无关。总体而言，TPS 结果与测量结果之间的比较表明，侧向半影的一致性较好，TPS 预测值高于测量值。
等角扫描质子束的侧向半影取决于气隙、质子射程、补偿器厚度和深度，而侧向半影与调制无关。与测量结果相比，XiO TPS 通常会高估侧向半影，大多数情况下在 1mm 以内，但在较深的深度和气隙较大的情况下，差异可能高达 2.5mm。
