Gaa T, Reinhart M, Hartmann B, Jakubek J, Soukup P, Jäkel O, Martišíková M
Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany.
Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany; Department of Radiation Oncology, Heidelberg University Hospital, Im Neuenheimer Feld 400, 69120 Heidelberg, Germany.
Phys Med. 2017 Jun;38:140-147. doi: 10.1016/j.ejmp.2017.05.055. Epub 2017 May 30.
Non-invasive methods for monitoring of the therapeutic ion beam extension in the patient are desired in order to handle deteriorations of the dose distribution related to changes of the patient geometry. In carbon ion radiotherapy, secondary light ions represent one of potential sources of information about the dose distribution in the irradiated target. The capability to detect range-changing inhomogeneities inside of an otherwise homogeneous phantom, based on single track measurements, is addressed in this paper.
Air and stainless steel inhomogeneities, with PMMA equivalent thickness of 10mm and 4.8mm respectively, were inserted into a PMMA-phantom at different positions in depth. Irradiations of the phantom with therapeutic carbon ion pencil beams were performed at the Heidelberg Ion Beam Therapy Center. Tracks of single secondary ions escaping the phantom under irradiation were detected with a pixelized semiconductor detector Timepix. The statistical relevance of the found differences between the track distributions with and without inhomogeneities was evaluated.
Measured shifts of the distal edge and changes in the fragmentation probability make the presence of inhomogeneities inserted into the traversed medium detectable for both, 10mm air cavities and 1mm thick stainless steel. Moreover, the method was shown to be sensitive also on their position in the observed body, even when localized behind the Bragg-peak.
The presented results demonstrate experimentally, that the method using distributions of single secondary ion tracks is sensitive to the changes of homogeneity of the traversed material for the studied geometries of the target.
为应对与患者几何形状变化相关的剂量分布恶化问题，需要采用非侵入性方法来监测患者体内治疗性离子束的扩展情况。在碳离子放射治疗中，次级轻离子是有关被照射靶区剂量分布的潜在信息源之一。本文探讨了基于单径迹测量来检测原本均匀体模内部射程变化不均匀性的能力。
将等效厚度分别为10mm空气和4.8mm不锈钢的不均匀性物质插入到聚甲基丙烯酸甲酯（PMMA）体模不同深度位置。在海德堡离子束治疗中心用治疗性碳离子笔形束对体模进行照射。用像素化半导体探测器Timepix检测照射下体模逸出的单个次级离子的径迹。评估了有无不均匀性情况下径迹分布差异的统计相关性。
测量得到的远端边缘位移和碎片概率变化使得无论是10mm气腔还是1mm厚不锈钢插入到被穿过介质中时，不均匀性的存在都可被检测到。此外，该方法对于不均匀性在被观察物体中的位置也很敏感，即使其位于布拉格峰之后。
所呈现的结果通过实验证明，对于所研究的靶区几何形状，利用单个次级离子径迹分布的方法对被穿过物质的均匀性变化敏感。
