Poulsen Per Rugaard, Eley John, Langner Ulrich, Simone Charles B, Langen Katja
Department of Oncology, Aarhus University Hospital, Aarhus, Denmark.
Maryland Proton Treatment Center, University of Maryland School of Medicine, Baltimore, Maryland.
Int J Radiat Oncol Biol Phys. 2018 Jan 1;100(1):226-234. doi: 10.1016/j.ijrobp.2017.09.043. Epub 2017 Oct 4.
To develop and implement a practical repainting method for efficient interplay effect mitigation in proton pencil beam scanning (PBS).
A new flexible repainting scheme with spot-adapted numbers of repainting evenly spread out over the whole breathing cycle (assumed to be 4 seconds) was developed. Twelve fields from 5 thoracic and upper abdominal PBS plans were delivered 3 times using the new repainting scheme to an ion chamber array on a motion stage. One time was static and 2 used 4-second, 3-cm peak-to-peak sinusoidal motion with delivery started at maximum inhalation and maximum exhalation. For comparison, all dose measurements were repeated with no repainting and with 8 repaintings. For each motion experiment, the 3%/3-mm gamma pass rate was calculated using the motion-convolved static dose as the reference. Simulations were first validated with the experiments and then used to extend the study to 0- to 5-cm motion magnitude, 2- to 6-second motion periods, patient-measured liver tumor motion, and 1- to 6-fraction treatments. The effect of the proposed method was evaluated for the 5 clinical cases using 4-dimensional (4D) dose reconstruction in the planning 4D computed tomography scan. The target homogeneity index, HI = (D - D)/D, of a single-fraction delivery is reported, where D and D is the dose delivered to 2% and 98% of the target, respectively, and D is the mean dose.
The gamma pass rates were 59.6% ± 9.7% with no repainting, 76.5% ± 10.8% with 8 repaintings, and 92.4% ± 3.8% with the new repainting scheme. Simulations reproduced the experimental gamma pass rates with a 1.3% root-mean-square error and demonstrated largely improved gamma pass rates with the new repainting scheme for all investigated motion scenarios. One- and two-fraction deliveries with the new repainting scheme had gamma pass rates similar to those of 3-4 and 6-fraction deliveries with 8 repaintings. The mean HI for the 5 clinical cases was 14.2% with no repainting, 13.7% with 8 repaintings, 12.0% with the new repainting scheme, and 11.6% for the 4D dose without interplay effects.
A novel repainting strategy for efficient interplay effect mitigation was proposed, implemented, and shown to outperform conventional repainting in experiments, simulations, and dose reconstructions. This strategy could allow for safe and more optimal clinical delivery of thoracic and abdominal proton PBS and better facilitate hypofractionated and stereotactic treatments.
开发并实施一种实用的重绘方法，以有效减轻质子笔形束扫描（PBS）中的相互作用效应。
开发了一种新的灵活重绘方案，在整个呼吸周期（假定为4秒）内均匀分布适应靶点的重绘次数。使用新的重绘方案，对5个胸部和上腹部PBS计划中的12个射野在运动平台上的电离室阵列上进行了3次照射。一次为静态照射，另外两次使用峰峰值为3 cm、周期为4秒的正弦运动，分别在最大吸气和最大呼气时开始照射。为作比较，在不进行重绘和进行8次重绘的情况下重复所有剂量测量。对于每个运动实验，使用运动卷积静态剂量作为参考计算3%/3 mm伽马通过率。首先通过实验验证模拟结果，然后将研究扩展到0至5 cm的运动幅度、2至6秒的运动周期、患者测量的肝脏肿瘤运动以及1至6次分割治疗。在计划的4D计算机断层扫描中，使用4维（4D）剂量重建对5个临床病例评估了所提出方法的效果。报告了单次分割照射的靶区均匀性指数，HI = (D - D)/D，其中D和D分别是靶区2%和98%处的剂量，D是平均剂量。
不进行重绘时伽马通过率为59.6% ± 9.7%，进行8次重绘时为76.5% ± 10.8%，采用新重绘方案时为92.4% ± 3.8%。模拟结果以1.3%的均方根误差重现了实验伽马通过率，并表明在所有研究的运动场景中，新重绘方案的伽马通过率有显著提高。采用新重绘方案的1次和2次分割照射的伽马通过率与进行8次重绘的3至4次和6次分割照射相似。5个临床病例不进行重绘时的平均HI为14.2%，进行8次重绘时为13.7%，采用新重绘方案时为12.0%，无相互作用效应的4D剂量时为11.6%。
提出并实施了一种有效减轻相互作用效应的新型重绘策略，在实验、模拟和剂量重建中均表现优于传统重绘。该策略可实现胸部和腹部质子PBS的安全且更优化的临床照射，并更好地促进低分割和立体定向治疗。
