Determination of surface dose in pencil beam scanning proton therapy.
Determination of surface dose in pencil beam scanning proton therapy.
复制标题
笔形束扫描质子治疗中表面剂量的测定。
DOI：
10.1002/mp.14086
复制
发表时间：
2020
期刊：
影响因子：
3.8
通讯作者：
J. Wulff
中科院分区：
文献类型：
--
作者：
A. Kern;C. Bäumer;K. Kröninger;L. Mertens;B. Timmermann;J. Walbersloh;J. Wulff
目的/目标
在最初的几百个水当量μm范围内对表面剂量进行定量是具有挑战性的。然而，质子治疗社区对研究皮肤中的剂量效应非常感兴趣。实验测定受探测器特性的影响，例如探测器体积和材料。ICRU在其报告39中建议评估0.07 mm深度的皮肤剂量。本研究的目的是估计70 µm深度及其附近的吸收剂量。我们使用各种剂量测定方法结合质子笔形束扫描交付，以确定在临床环境中的皮肤剂量。
材料/方法
测试了五种不同的探测器以确定水中的表面剂量：EBT 3和HD-V2 GAFCHROMIC™辐射变色膜、LiF：Mg、Ti热释光剂量计、IBA PPC 05平面平行电离室和PTW 23391外推室。辐照装置由准单能扫描质子笔束组成，动能分别为100 MeV，150 MeV和226.7 MeV。辐射变色胶片以楔形几何形状放置在垂直堆栈中，并使用FilmQA Pro™采用三通道剂量测定法进行分析。外推室PTW 23391在本工作中用作参考，用于具有2 mm固定电极间隙的常规电离室设置中。采用厚度为25 μm、50 μm和75 μm的三个Kapton®入射窗。TLD以粉末形式提供，并压在铝片上。此外，使用版本3.1.p2中的TOPAS对IBA笔形射束扫描喷嘴进行建模，并对水模体中水的剂量进行评分。
结果
将得到的深度剂量曲线标准化为3 cm参考深度处的100%剂量。我们用外推室和TOPAS测量了皮肤剂量。对于实验方法，对于质子能量100 MeV、150 MeV和226.7 MeV，这分别导致79.7± 0.3%、86.0±0.6%和87.1±0.1%。TOPAS的结果分别为80.1±0.2%（100 MeV）、87.1±0.5%（150 MeV）和86.9±0.4%（226.7 MeV）。根据皮肤剂量的实验结果，我们为常用的测量方法提供了一个与临床相关的表面外推因子。这允许探测器的第一测量深度的结果被缩放到趋肤深度处的剂量。最实用的是使用PPC 05腔室的表面外推因子，因为它具有直接阅读、临床广泛可用性和低不确定性。100 MeV、150 MeV和226.7 MeV的计算因子分别为0.986±0.004、0.961±0.008和0.963±0.003。
结论
在这项研究中，不同的实验方法进行了评估，在接近表面的深度测量。实验得到的深度剂量曲线与TOPAS Monte Carlo模拟结果吻合较好。据作者所知，这是第一次根据ICRU 39定义在质子笔形束扫描中对皮肤剂量进行实验测定。
PURPOSE/OBJECTIVE
Quantification of surface dose within the first few hundred water equivalent µm is challenging. Nevertheless, it is of large interest for the proton therapy community to study dose effects in the skin. The experimental determination is affected by the detector properties, such as the detector volume and material. The ICRU in its report 39 recommends assessing the skin dose at a depth of 0.07 mm. The aim of this study is the estimation of the absorbed dose at and around a depth of 70 µm. We used various dosimetric approaches in conjunction with proton pencil beam scanning delivery to determine the skin dose in a clinical setting.
MATERIAL/METHODS
Five different detectors were tested for determining the surface dose in water: EBT3 and HD-V2 GAFCHROMIC™ radiochromic film, LiF:Mg,Ti thermoluminescent dosimeter, IBA PPC05 plane-parallel ionization chamber and PTW 23391 extrapolation chamber. The irradiation setup consisted of quasi-monoenergetic scanned proton pencil beams with kinetic energies of 100 MeV, 150 MeV and 226.7 MeV, respectively. Radiochromic films were placed within a vertical stack and in wedge geometry and were analyzed with FilmQA Pro™ adopting triple channel dosimetry. The extrapolation chamber PTW 23391, which served as a reference in the current work, was used in a conventional ionization chamber setup with a fixed electrode gap of 2 mm. Three Kapton® entrance windows with thicknesses of 25 μm, 50 μm and 75 μm were employed. TLDs were provided as powder and were pressed onto a sheet of aluminum. Furthermore, TOPAS in version 3.1.p2 was used to model an IBA pencil beam scanning nozzle and score dose to water in a water phantom.
RESULTS
The resulting depth dose curves were normalized to their 100% dose at the reference depth of 3 cm. We obtained the skin doses with the extrapolation chamber and with TOPAS. For the experimental approach this resulted in 79.7±0.3%, 86.0±0.6% and 87.1±0.1% for the proton energies 100 MeV, 150 MeV and 226.7 MeV, respectively. The results for TOPAS were 80.1±0.2% (100 MeV), 87.1±0.5% (150 MeV) and 86.9±0.4% (226.7 MeV), respectively. Based on the experimental results of the skin dose, we provided a clinically relevant surface extrapolation factor for the common measurement methods. This allows the result of the first measurement depth of a detector to be scaled to the dose at the skin depth. Most practical would be the use of the surface extrapolation factor for the PPC05 chamber, due to its direct reading, the wide availability in clinics and the low uncertainties. The calculated factors were 0.986±0.004 for 100 MeV, 0.961±0.008 for 150 MeV and 0.963±0.003 for 226.7 MeV.
CONCLUSIONS
In this study, dissimilar experimental approaches were evaluated with respect to measurements at depths close to the surface. The experimental depth dose curves are in good agreement with the simulation with TOPAS Monte Carlo. To the author's knowledge this was the first experimental determination of the skin dose according to the ICRU 39 definition in proton pencil beam scanning.
影响因子：
3.8
作者：
Perl, J.;Shin, J.;Paganetti, H.
通讯作者：
Paganetti, H.