Kern A, Bäumer C, Kröninger K, Wulff J, Timmermann B
West German Proton Therapy Centre Essen (WPE), Essen, 45147, Germany.
University Hospital Essen, Essen, 45147, Germany.
Med Phys. 2021 Feb;48(2):831-840. doi: 10.1002/mp.14626. Epub 2020 Dec 23.
Side effects of radiation therapy may include skin damage. The surface dose is of great interest and contains the buildup effect. In particular, the proton therapy community requires further experimental data to quantify doses in the surface region. This specification includes the skin dose, which is defined according to ICRU Report No. 39 at 70 μm water equivalent depth. The aim of this study is to gather more knowledge of the skin dose by varying key parameters defined by the patient treatment plan. This consists of clinical aspects such as the influence of the air gap, the application of a range shifter (RS), or the proton delivery technique.
MATERIAL/METHODS: Skin doses were determined with a PTW 23391 extrapolation chamber with three thin Kapton® entrance windows operated as a conventional ionization chamber. The impact on the skin dose for quasi-monoenergetic pencil beam scanning (PBS) proton beams was evaluated for clinical air gaps between 3.5 and 51.1 cm. The differences in skin dose were assessed by irradiating equivalent fields with an RS of 51 mm water equivalent thickness (RS51) and without. Furthermore, the delivery techniques PBS, uniform scanning (US), and double scattering (DS) were compared by defining a spread-out Bragg peak (SOBP). TOPAS (V.3.1.2) was used to model an IBA nozzle with PBS and to score dose to water at the surface of a water phantom.
For the monoenergetic fields without the application of the RS the skin dose was constant down to an air gap of 6.2 cm. A lower air gap of 3.5 cm showed a variation in skin dose by up to 2.4% compared to the results obtained with larger air gaps. With the inserted RS51 an increase in the skin dose was found for air gaps smaller than 11.3 cm. Experimentally, a dose difference of 1.4% was recorded for an air gap of 6.2 cm by inserting an RS and none. With the Monte Carlo calculations the largest dose increase was observed at the air gap of 3.5 cm with 1.7% and 4.0% relative to the skin dose results without the RS and to the largest evaluated air gap of 51.1 cm, respectively. The SOBP comparison of the beam modalities at the measuring plane at the isocenter revealed higher skin doses without RS (including RS) by up to +1.9% (+1.5%) for DS and +1.3% (+1.1%) for US compared to PBS. For all three techniques an approx. 2% rise in skin dose was observed for the largest evaluated air gap of 37.7 cm to an air gap of 6.2 cm when using an RS51.
The study investigated aspects of skin dose of a water equivalent phantom by varying key parameters of a proton treatment plan. Parameters like the RS, the air gap, and the delivery modality have an impact on the order of 4.0% for the skin dose at the depth of 70 μm. The increases in skin dose are the effects of the contribution of the increased electron fluence at small air gaps and the emitted hadronic particles produced by the RS.
放射治疗的副作用可能包括皮肤损伤。表面剂量备受关注，且包含剂量建成效应。特别是，质子治疗领域需要更多实验数据来量化表面区域的剂量。本规范包括皮肤剂量，其根据国际辐射单位与测量委员会第39号报告定义为在70μm水等效深度处的剂量。本研究的目的是通过改变患者治疗计划中定义的关键参数，来获取更多关于皮肤剂量的知识。这包括临床方面，如气隙的影响、射程移位器（RS）的应用或质子束传输技术。
材料/方法：使用PTW 23391外推电离室测定皮肤剂量，该电离室带有三个薄的聚酰亚胺（Kapton®）入射窗，可作为传统电离室使用。评估了准单能笔形束扫描（PBS）质子束在3.5至51.1cm临床气隙下对皮肤剂量的影响。通过照射等效野，比较了有51mm水等效厚度的射程移位器（RS51）和没有射程移位器时皮肤剂量的差异。此外，通过定义扩展布拉格峰（SOBP），比较了PBS、均匀扫描（US）和双散射（DS）这几种传输技术。使用TOPAS（V.3.1.2）对带有PBS的IBA喷嘴进行建模，并在水模体表面对水的剂量进行评分。
对于未应用RS的单能野，直至6.2cm的气隙，皮肤剂量保持恒定。与较大气隙的结果相比，3.5cm的小气隙显示皮肤剂量变化高达2.4%。插入RS51后，发现气隙小于11.3cm时皮肤剂量增加。实验上，插入RS和未插入RS时，6.2cm气隙的剂量差异记录为1.4%。通过蒙特卡罗计算，相对于未使用RS时的皮肤剂量结果以及最大评估气隙51.1cm，在3.5cm气隙处观察到最大剂量增加，分别为1.7%和4.0%。在等中心测量平面处对束流模式的SOBP比较显示，与PBS相比，DS无RS（包括RS）时皮肤剂量高至+1.9%（+1.5%），US高至+1.3%（+1.1%）。对于所有三种技术，当使用RS51时，从最大评估气隙37.7cm到6.2cm气隙，观察到皮肤剂量约增加2%。
本研究通过改变质子治疗计划的关键参数，研究了水等效模体的皮肤剂量方面。诸如RS、气隙和传输方式等参数对70μm深度处的皮肤剂量影响约为4.0%。皮肤剂量的增加是小气隙处电子注量增加以及RS产生的发射强子粒子贡献的结果。
