Abteilung Biophysik, GSI Helmholtzzentrum für Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany.
Med Phys. 2010 Feb;37(2):449-60. doi: 10.1118/1.3250856.
Clinical outcomes of charged particle therapy are very promising. Currently, several dedicated centers that use scanning beam technology are either close to clinical use or under construction. Since scanned beam treatments of targets that move with respiration most likely result in marked local over- and underdosage due to interplay of target motion and dynamic beam application, dedicated motion mitigation techniques have to be employed. To date, the motion mitigation techniques, rescanning, beam gating, and beam tracking, have been proposed and tested in experimental studies. Rescanning relies on repeated irradiations of the target with the number of particles reduced accordingly per scan to statistically average local misdosage. Specific developments to prohibit temporal correlation between beam scanning and target motion will be required to guarantee adequate averaging. For beam gating, residual target motion within gating windows has to be mitigated in order to avoid local misdosage. Possibly the most promising strategy is to increase the overlap of adjacent particle pencil beams laterally as well as longitudinally to effectively reduce the sensitivity against small residual target motion. The most conformal and potentially most precise motion mitigation technique is beam tracking. Individual particle pencil beams have to be adapted laterally as well as longitudinally according to the target motion. Within the next several years, it can be anticipated that rescanning as well as beam gating will be ready for clinical use. For rescanning, treatment planning margins that incorporate the full extent of target motion as well as motion induced density variations in the beam paths will result in reduced target conformity of the applied dose distributions. Due to the limited precision of motion monitoring devices, it seems likely that beam gating will be used initially to mitigate interplay effects only but not to considerably decrease treatment planning margins. Then, in the next step, beam gating, based on more accurate motion monitoring systems, provides the possibility to restore target conformity as well as steep dose gradients due to reduced treatment planning margins. Accurate motion monitoring systems will be required for beam tracking. Even though beam tracking has already been successfully tested experimentally, full clinical implementation requires direct feedback of the actual target position in quasireal time to the treatment control system and can be anticipated to be several more years ahead.
粒子治疗的临床结果非常有前景。目前，有几个使用扫描束技术的专用中心已经接近临床使用或正在建设中。由于呼吸运动的目标的扫描束治疗最有可能由于目标运动和动态束应用的相互作用导致明显的局部过剂量和欠剂量，因此必须采用专用的运动缓解技术。迄今为止，已经在实验研究中提出并测试了运动缓解技术，包括再扫描、束门控和束跟踪。再扫描依赖于用减少的粒子数重复照射目标，以在统计学上平均局部剂量误差。为了保证充分的平均化，需要专门的开发来禁止束扫描和目标运动之间的时间相关性。对于束门控，必须缓解门控窗口内的残留目标运动，以避免局部剂量误差。最有前途的策略可能是增加相邻粒子铅笔束在横向和纵向的重叠，以有效地降低对小残留目标运动的敏感性。最符合适形和潜在最精确的运动缓解技术是束跟踪。根据目标运动，需要横向和纵向地调整单个粒子铅笔束。在未来几年内，可以预期再扫描和束门控将准备好用于临床使用。对于再扫描，治疗计划边缘包括目标运动的全部范围以及束路径中的运动诱导密度变化，这将导致应用剂量分布的目标适形性降低。由于运动监测设备的精度有限，束门控最初似乎更有可能用于缓解相互作用效应，但不能显著降低治疗计划边缘。然后，在下一个步骤中，基于更精确的运动监测系统的束门控提供了由于治疗计划边缘减少而恢复目标适形性和陡峭剂量梯度的可能性。束跟踪需要精确的运动监测系统。即使束跟踪已经在实验中成功测试，全面的临床实施需要实际目标位置的准实时直接反馈给治疗控制系统，并且可以预期还需要几年时间。
