Applications of Detectors and Accelerators to Medicine (ADAM) SA, Geneva, Switzerland.
Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Italy.
Med Phys. 2018 Jan;45(1):60-73. doi: 10.1002/mp.12675. Epub 2017 Dec 10.
Spot size σ (in air at isocenter), interspot spacing d, and spot charge q influence dose delivery efficiency and plan quality in Intensity Modulated Proton Therapy (IMPT) treatment planning. The choice and range of parameters varies among different manufacturers. The goal of this work is to demonstrate the influence of the spot parameters on dose quality and delivery in IMPT treatment plans, to show their interdependence, and to make practitioners aware of the spot parameter values for a certain facility. Our study could help as a guideline to make the trade-off between treatment quality and time in existing PBS centers and in future systems.
We created plans for seven patients and a phantom, with different tumor sites and volumes, and compared the effect of small-, medium-, and large-spot widths (σ = 2.5, 5, and 10 mm) and interspot distances (1σ, 1.5σ, and 1.75σ) on dose, spot charge, and treatment time. Moreover, we quantified how postplanning charge threshold cuts affect plan quality and the total number of spots to deliver, for different spot widths and interspot distances. We show the effect of a minimum charge (or MU) cutoff value for a given proton delivery system.
Spot size had a strong influence on dose: larger spots resulted in more protons delivered outside the target region. We observed dose differences of 2-13 Gy (RBE) between 2.5 mm and 10 mm spots, where the amount of extra dose was due to dose penumbra around the target region. Interspot distance had little influence on dose quality for our patient group. Both parameters strongly influence spot charge in the plans and thus the possible impact of postplanning charge threshold cuts. If such charge thresholds are not included in the treatment planning system (TPS), it is important that the practitioner validates that a given combination of lower charge threshold, interspot spacing, and spot size does not result in a plan degradation. Low average spot charge occurs for small spots, small interspot distances, many beam directions, and low fractional dose values.
The choice of spot parameters values is a trade-off between accelerator and beam line design, plan quality, and treatment efficiency. We recommend the use of small spot sizes for better organ-at-risk sparing and lateral interspot distances of 1.5σ to avoid long treatment times. We note that plan quality is influenced by the charge cutoff. Our results show that the charge cutoff can be sufficiently large (i.e., 10 protons) to accommodate limitations on beam delivery systems. It is, therefore, not necessary per se to include the charge cutoff in the treatment planning optimization such that Pareto navigation (e.g., as practiced at our institution) is not excluded and optimal plans can be obtained without, perhaps, a bias from the charge cutoff. We recommend that the impact of a minimum charge cut impact is carefully verified for the spot sizes and spot distances applied or that it is accommodated in the TPS.
在调强质子治疗（IMPT）计划中，光斑大小 σ（在等中心的空气中）、光斑间距 d 和光斑电荷量 q 会影响剂量输送效率和计划质量。不同制造商的参数选择和范围有所不同。本研究的目的是展示光斑参数对 IMPT 治疗计划中剂量质量和输送的影响，展示它们之间的相互依赖性，并使从业者了解特定设施的光斑参数值。我们的研究可以作为在现有质子束治疗中心和未来系统中在治疗质量和时间之间进行权衡的指导。
我们为 7 名患者和一个体模创建了计划，这些患者的肿瘤部位和体积不同，并比较了小、中、大光斑宽度（σ=2.5、5 和 10mm）和光斑间距（1σ、1.5σ 和 1.75σ）对剂量、光斑电荷量和治疗时间的影响。此外，我们还量化了不同光斑宽度和光斑间距下，后处理电荷量阈值切割对计划质量和所需光斑数量的影响。我们展示了给定质子输送系统的最小电荷量（或 MU）截止值的影响。
光斑大小对剂量有很大影响：较大的光斑会导致更多的质子输送到靶区外。我们观察到 2.5mm 和 10mm 光斑之间的剂量差异为 2-13Gy（RBE），额外剂量是由于靶区周围的剂量半影所致。对于我们的患者群体，光斑间距对剂量质量几乎没有影响。这两个参数都会强烈影响计划中的光斑电荷量，从而影响后处理电荷量阈值切割的可能性。如果在后处理计划系统（TPS）中不包括这种电荷量阈值，那么从业者验证给定的低电荷量阈值、光斑间距和光斑大小组合不会导致计划质量下降是很重要的。低平均光斑电荷量出现在小光斑、小光斑间距、多个射束方向和低分数剂量值的情况下。
光斑参数值的选择是在加速器和射束线设计、计划质量和治疗效率之间的权衡。我们建议使用小光斑尺寸以更好地保护器官风险区，并使用 1.5σ 的横向光斑间距，以避免治疗时间过长。我们注意到计划质量会受到电荷量截止值的影响。我们的结果表明，电荷量截止值可以足够大（即 10 个质子），以适应射束输送系统的限制。因此，在治疗计划优化中，不一定需要包括电荷量截止值，从而可以获得帕累托导航（例如，我们机构所采用的），而不会受到电荷量截止值的偏见。我们建议仔细验证最小电荷量切割影响对于应用的光斑尺寸和光斑间距的影响，或者在 TPS 中进行调整。
