背景:已知用于移动目标的铅笔束扫描(PBS)质子治疗受到扫描束和器官运动之间的相互作用效应的影响。虽然胸部区域的呼吸运动是器官运动的主要原因,相互作用效应取决于质子加速器的输送特性。
目的:为了评估不同类型的PBS质子加速器和光斑尺寸对相互作用效应的影响,缓解措施,和计划质量的立体定向身体放射治疗(SBRT)治疗非小细胞肺癌(NSCLC)。
方法:选择20例接受光子SBRT治疗的非小细胞肺癌患者进行回顾性研究,以代表不同的肿瘤体积和呼吸运动幅度(中位:0.6cm,腹部压迫)。对于每个病人来说,使用以下方法创建计划:(1)回旋加速器产生的质子束(CPB),光斑尺寸为σ=2.7-7.0mm;(2)线性加速器质子束(LPB)(σ=2.9-5.5mm);(3)线性加速器质子小束(LPMB)(σ=0.9-3.9mm)。CPB的能量切换时间为1秒,LPMB和LPB为0.005s。使用四维计算机断层扫描(4DCT)扫描的每个单独阶段,在总肿瘤体积(GTV)上对计划进行了稳健优化。最初,对单场优化(SFO)计划进行了评估;如果计划质量不符合剂量测定要求,使用多场优化(MFO)。为所有患者创建MFO计划用于比较。对于每个病人来说,所有计划均正常化,99%的GTV接受相同剂量.通过计算10个呼吸阶段的剂量来评估相互作用效应,基于现货分布。每个计划进行2-6次体积重涂(VR)。我们比较了接受100%的GTV处方剂量(V100%RX)的体积,和正常肺V20Gy。
结果:使用SFO可以充分优化20个计划中的12个。就LPB和LPMB的目标覆盖率而言,与MFO计划相比,SFO计划对相互作用效应的敏感性较低。以下比较显示了利用MFO技术的结果。在不重新绘画的相互作用评估中,GTV的平均V100%RX为99.42±0.6%,97.52±3.9%,CPB为94.49±7.3%,LPB,和LPMB计划,分别。在VR之后(CPB为2倍;LPB为3倍;LPMB为5倍),GTV的V100%RX(平均)提高了0.13%,1.84%,和4.63%,分别,达到V100%RX>95%的验收标准。由于线性加速器质子机器的快速能量切换,VR计划的交付时间是LPB计划的最低时间,而LPMB的分娩时间平均比CPB计划长1分钟。小斑点机的优点是在正常肺V20Gy中更好地保留,即使是在VR应用的时候。
结论:在没有重新粉刷的情况下,具有大光斑尺寸的质子机器产生了更强大的对抗相互作用效应的计划。VR的数量随着光斑尺寸的减小而增加,以达到验收标准。对于具有小光斑尺寸和快速能量变化的模态,VR提高了针对相互作用效应的计划鲁棒性。保留LPMB的低剂量保留方面,即使包括运动。
BACKGROUND: Pencil beam scanning (PBS) proton therapy for moving targets is known to be impacted by interplay effects between the scanning beam and organ motion. While respiratory motion in the thoracic region is the major cause for organ motion, interplay effects depend on the delivery characteristics of proton accelerators.
OBJECTIVE: To evaluate the impact of different types of PBS proton accelerators and spot sizes on interplay effects, mitigations, and plan quality for Stereotactic Body Radiation Therapy (SBRT) treatment of non-small cell lung cancer (NSCLC).
METHODS: Twenty NSCLC patients treated with photon SBRT were selected to represent varying tumor volumes and respiratory motion amplitudes (median: 0.6 cm with abdominal compression) for this retrospective study. For each patient, plans were created using: (1) cyclotron-generated proton beams (CPB) with spot sizes of σ = 2.7-7.0 mm; (2) linear accelerator proton beams (LPB) (σ = 2.9-5.5 mm); and (3) linear accelerator proton minibeams (LPMB) (σ = 0.9-3.9 mm). The energy switching time is one second for CPB, and 0.005 s for LPMB and LPB. Plans were robustly optimized on the gross tumor volume (GTV) using each individual phase of four-dimensional computed tomography (4DCT) scans. Initially, single-field optimization (SFO) plans were evaluated; if the plan quality did not meet the dosimetric requirement, multi-field optimization (MFO) was used. MFO plans were created for all patients for comparisons. For each patient, all plans were normalized to have the same dose received by 99% of the GTV. Interplay effects were evaluated by computing the dose on 10 breathing phases, based on the spot distribution. Volumetric repainting (VR) was performed 2-6 times for each plan. We compared volume receiving 100% of the prescribed dose (V100%RX) of the GTV, and normal lung V20Gy.
RESULTS: Twelve of 20 plans can be optimized sufficiently with SFO. SFO plans were less sensitive to the interplay effect compared to MFO plans in terms of target coverage for both LPB and LPMB. The following comparisons showed results utilizing the MFO technique. In the interplay evaluation without repainting, the mean V100%RX of the GTV were 99.42 ± 0.6%, 97.52 ± 3.9%, and 94.49 ± 7.3% for CPB, LPB, and LPMB plans, respectively. Following VR (2 × for CPB; 3 × for LPB; 5 × for LPMB), V100%RX of the GTV were improved (on average) by 0.13%, 1.84%, and 4.63%, respectively, achieving the acceptance criteria of V100%RX > 95%. Because of fast energy switch in linear accelerator proton machines, the delivery time for VR plans was the lowest for LPB plans, while delivery time for LPMB was on average 1 min longer than CPB plans. The advantage of small spot machines was better sparing in normal lung V20Gy, even when VR was applied.
CONCLUSIONS: In the absence of repainting, proton machines with large spot sizes generated more robust plans against interplay effects. The number of VR increased with decreasing spot sizes to achieve the acceptance criteria. VR improved the plan robustness against interplay effects for modalities with small spot sizes and fast energy changes, preserving the low dose sparing aspect of the LPMB, even when motion is included.