Centre for Medical Radiation Physics, University of Wollongong, Wollongong, Australia.
European Synchrotron Radiation Facility, Biomedical beamline ID17, Grenoble, France.
Med Phys. 2020 Jan;47(1):213-222. doi: 10.1002/mp.13899. Epub 2019 Nov 19.
Microbeam radiation therapy (MRT) is an emerging radiation oncology modality ideal for treating inoperable brain tumors. MRT employs quasi-parallel beams of low-energy x rays produced from modern synchrotrons. A tungsten carbide multislit collimator (MSC) spatially fractionates the broad beam into rectangular beams. In this study, the MSC creates beams 50 μm wide ("peaks") separated by a center-to-center distance of 400 μm ("valleys"). The peak to valley dose ratio (PVDR) is of critical importance to the efficacy of MRT. The underlying radiobiological advantage of MRT relies on high peak dose for tumor control and low valley dose for healthy tissue sparing. Cardio synchronous brain motion of the order 100-200 μm is comparable to microbeam width and spacing. The motion can have a detrimental effect on the PVDR, full width at half maximum (FWHM) of the microbeams, and ultimately the dose distribution. We present the first experimental measurement of the effect of brain motion on MRT dose distribution. Dosimetry in MRT is difficult due to the high dose rate (up to 15-20 kGy/s) and small field sizes.
A real-time dosimetry system based on a single silicon strip detector (SSSD) has been developed with spatial resolution ~10 μm. The SSSD was placed in a water-equivalent phantom and scanned through the microbeam distribution. A monodirectional positioning stage reproduced brain motion during the acquisition. Microbeam profiles were reconstructed from the SSSD and compared with Geant4 simulation and radiochromic HD-V2 film.
The SSSD is able to reconstruct dose profiles within 2 μm compared to film. When brain motion is applied the SSSD shows a two time increase in FWHM of profiles and 50% reduction in PVDR. This is confirmed by Geant4 and film data.
Motion-induced misalignment and distortion of microbeams at treatment delivery will result in a reduced PVDR and increased irradiation of additional healthy tissue compromising the radiobiological effectiveness of MRT. The SSSD was able to reconstruct dose profiles under motion conditions and predict similar effects on FWHM and PVDR as by the simulation. The SSSD is a simple to setup, real-time detector which can provide time-resolved high spatial resolution dosimetry of microbeams in MRT.
微束放射治疗（MRT）是一种新兴的放射肿瘤学治疗模式，非常适合治疗无法手术的脑肿瘤。MRT 采用现代同步加速器产生的低能 X 射线准平行束。碳化钨多狭缝准直器（MSC）将宽束空间分割成矩形束。在这项研究中，MSC 将光束宽度分割成 50μm 宽的“峰”，峰之间的中心到中心距离为 400μm 的“谷”。峰谷剂量比（PVDR）对 MRT 的疗效至关重要。MRT 的基本放射生物学优势依赖于高峰值剂量以控制肿瘤，以及低谷剂量以保护健康组织。心脏同步脑运动约为 100-200μm，与微束宽度和间距相当。这种运动可能会对 PVDR、微束的全宽半最大值（FWHM）以及最终的剂量分布产生不利影响。我们首次实验测量了脑运动对 MRT 剂量分布的影响。由于高剂量率（高达 15-20kGy/s）和小照射野，MRT 中的剂量测定非常困难。
我们开发了一种基于单硅条探测器（SSSD）的实时剂量测定系统，其空间分辨率约为 10μm。SSSD 被放置在水等效体模中，并在微束分布中进行扫描。单向定位台在采集过程中模拟脑运动。从 SSSD 重建微束轮廓，并与 Geant4 模拟和放射性化学 HD-V2 胶片进行比较。
与胶片相比，SSSD 能够在 2μm 内重建剂量分布。当施加脑运动时，SSSD 显示出轮廓 FWHM 增加两倍，PVDR 降低 50%。这得到了 Geant4 和胶片数据的证实。
在治疗过程中，运动引起的微束错位和变形会降低 PVDR，并增加对额外健康组织的照射，从而降低 MRT 的放射生物学疗效。SSSD 能够在运动条件下重建剂量分布，并预测与模拟相似的 FWHM 和 PVDR 效果。SSSD 是一种简单易用的实时探测器，可提供 MRT 中微束的时间分辨高空间分辨率剂量测定。
