Peterson Stephen, Polf Jerimy, Ciangaru George, Frank Steven J, Bues Martin, Smith Al
Department of Radiation Physics, Unit 94, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030, USA.
Med Phys. 2009 Aug;36(8):3693-702. doi: 10.1118/1.3175796.
The purpose of this work was to develop a method to calculate and study the impact of fluctuations in the magnetic field strengths within the steering magnets in a proton scanning beam treatment nozzle on the dose delivered to the patient during a proton therapy treatment. First, an analytical relationship between magnetic field uncertainties in the steering magnets and the resulting lateral displacements in the position of the delivered scanned beam "dose spot" was established. Next, using a simple 3D dose calculation code and data from a validated Monte Carlo model of the proton scanning beam treatment nozzle, the uniform dose delivery to a 3D treatment volume was calculated. The dose distribution was then recalculated using the calculated lateral displacements due to magnetic field fluctuations to the proton pencil beam position. Using these two calculated dose distributions, the clinical effects of the magnetic field fluctuations were determined. A deliberate displacement of four adjacent spots either toward or away from each other was used to determine the "maximum" dose impact, while a random displacement of all spots was used to establish a more realistic clinical dose impact. Changes in the dose volume histogram (DVH) and the presence of hot and cold spots in the treatment volume were used to quantify the impact of dose-spot displacement. A general analytical relationship between magnetic field uncertainty and final dose-spot position is presented. This analytical relationship was developed such that it can be applied to study magnetic beam steering for any scanned beam nozzle design. Using this relationship the authors found for the example beam steering nozzle used in this study that deliberate lateral displacements of 0.5 mm or random lateral displacements of up to 1.0 mm produced a noticeable dose impact (5% hot spot) in the treatment volume. A noticeable impact (3% decrease in treatment volume coverage) on the DVH was observed for random displacements of up to 1.5 mm. For the scanning nozzle studied in this work, these displacement values correlated with an uncertainty value of 2.04% in the magnetic field values of the nozzle steering magnets. The authors conclude that fluctuations in the dose-spot delivery caused by uncertainty in the magnet fields used for beam steering could have clinically significant effects on the delivered dose distribution. Due to differences in the design and implementation of proton beam scanning nozzles at different treatment facilities, the effects of magnetic field fluctuations of dose delivery should be evaluated and understood for each specific nozzle design during clinical commissioning of the treatment nozzle.
这项工作的目的是开发一种方法，用于计算和研究质子扫描束治疗喷嘴中转向磁体内部磁场强度波动对质子治疗过程中输送给患者的剂量的影响。首先，建立了转向磁体中磁场不确定性与所输送扫描束“剂量点”位置的横向位移之间的解析关系。接下来，使用一个简单的三维剂量计算代码和来自经过验证的质子扫描束治疗喷嘴蒙特卡罗模型的数据，计算了向三维治疗体积均匀输送剂量的情况。然后，利用因磁场波动导致的质子笔形束位置的横向位移计算值，重新计算剂量分布。利用这两个计算出的剂量分布，确定了磁场波动的临床影响。通过将四个相邻点故意相互靠近或远离移动来确定“最大”剂量影响，同时通过所有点的随机移动来确定更实际的临床剂量影响。使用剂量体积直方图（DVH）的变化以及治疗体积中热点和冷点的存在来量化剂量点位移的影响。给出了磁场不确定性与最终剂量点位置之间的一般解析关系。开发这种解析关系是为了能够应用于研究任何扫描束喷嘴设计的磁束转向。利用这种关系，作者发现对于本研究中使用的示例束转向喷嘴，故意横向位移0.5毫米或随机横向位移高达1.0毫米会在治疗体积中产生明显的剂量影响（5%热点）。对于高达1.5毫米的随机位移，观察到对DVH有明显影响（治疗体积覆盖率降低3%）。对于本工作中研究的扫描喷嘴，这些位移值与喷嘴转向磁体磁场值2.04%的不确定性值相关。作者得出结论，用于束转向的磁场不确定性引起的剂量点输送波动可能对所输送的剂量分布产生临床上的显著影响。由于不同治疗设施的质子束扫描喷嘴在设计和实施上存在差异，在治疗喷嘴的临床调试期间，应针对每种特定的喷嘴设计评估和了解磁场波动对剂量输送的影响。
