Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, TX, 77030, USA.
Department of Imaging Physics, The University of Texas M. D. Anderson Cancer Center, Houston, TX, 77030, USA.
Med Phys. 2018 Feb;45(2):773-782. doi: 10.1002/mp.12700. Epub 2017 Dec 21.
Synthetic tissue equivalent (STE) materials currently used to simulate tumor and surrounding tissues for IROC-Houston's anthropomorphic head and thorax QA phantoms cannot be visualized using magnetic resonance (MR) imaging. The purpose of this study was to characterize dual MR/CT-visible STE materials that can be used in an end-to-end QA phantom for MR-guided radiotherapy (MRgRT) modalities.
Over 80 materials' MR, CT, and dosimetric STE properties were investigated for use in MRgRT QA phantoms. The materials tested included homogeneous and heterogeneous materials to simulate soft tissue/tumor and lung tissues. Materials were scanned on a Siemens' Magnetom Espree 1.5 T using four sequences, which showed the materials visual contrast between T1- and T2-weighted images. Each material's Hounsfield number and electron density data was collected using a GE's CT Lightspeed Simulator. Dosimetric properties were examined by constructing a 10 × 10 × 20 cm phantom of the selected STE materials that was divided into three sections: anterior, middle, and posterior. Anterior and posterior pieces were composed of polystyrene, whereas the middle section was substituted with the selected STE materials. EBT3 film was inserted into the phantom's midline and was irradiated using an Elekta's Versa 6 MV beam with a prescription of 6 Gy at 1.5 cm and varying field size of: 10 × 10 cm , 6 × 6 cm , and 3 × 3 cm . Measured film PDD curves were compared to planning system calculations and conventional STE materials' percent depth dose (PDD) curves.
The majority of the tested materials showed comparable CT attenuation properties to their respective organ site; however, most of the tested materials were not visible on either T1- or T2-weighted MR images. Silicone, hydrocarbon, synthetic gelatin, and liquid PVC plastic-based materials showed good MR image contrast. In-house lung equivalent materials made with either silicone- or hydrocarbon-based materials had HUs ranging from: -978 to -117 and -667 to -593, respectively. Synthetic gelatin and PVC plastic-based materials resembled soft tissue/tumor equivalent materials and had HUs of: -175 to -170 and -29 to 32, respectively. PDD curves of the selected MR/CT-visible materials were comparable to IROC-Houston's conventional phantom STE materials. The smallest field size showed the largest disagreements, where the average discrepancies between calculated and measured PDD curves were 1.8% and 5.9% for homogeneous and heterogeneous testing materials, respectively.
Gelatin, liquid plastic, and hydrocarbon-based materials were determined as alternative STE substitutes for MRgRT QA phantoms.
目前用于模拟 IROC-Houston 人体头颈部 QA 体模中肿瘤和周围组织的合成组织等效物（STE）材料不能通过磁共振成像（MR）进行可视化。本研究的目的是描述可用于 MR 引导放疗（MRgRT）模式的端到端 QA 体模的双 MR/CT 可见 STE 材料。
研究了 80 多种材料的 MR、CT 和剂量学 STE 特性，以用于 MRgRT QA 体模。测试的材料包括均质和非均质材料，以模拟软组织/肿瘤和肺组织。使用西门子的 Magnetom Espree 1.5T 对材料进行扫描，使用四个序列显示材料在 T1 和 T2 加权图像之间的视觉对比度。使用通用电气的 CT Lightspeed Simulator 收集每个材料的亨氏数和电子密度数据。通过构建一个 10×10×20cm 的选定 STE 材料的体模来检查剂量学特性，该体模分为三个部分：前部、中部和后部。前、后两部分由聚苯乙烯组成，而中间部分则用选定的 STE 材料替代。EBT3 胶片插入体模的中线，并使用 Elekta 的 Versa 6MV 射线以 6Gy 的处方在 1.5cm 和不同射野大小下进行照射：10×10cm、6×6cm 和 3×3cm。测量的胶片 PDD 曲线与计划系统计算和传统 STE 材料的百分深度剂量（PDD）曲线进行比较。
大多数测试材料的 CT 衰减特性与相应的器官部位相当；然而，大多数测试材料在 T1 或 T2 加权 MR 图像上均不可见。硅酮、碳氢化合物、合成明胶和液体 PVC 塑料基材料具有良好的 MR 图像对比度。由硅酮或碳氢化合物基材料制成的内部肺等效材料的 HU 值范围分别为：-978 至-117 和-667 至-593。合成明胶和 PVC 塑料基材料类似于软组织/肿瘤等效材料，HU 值分别为：-175 至-170 和-29 至 32。选定的 MR/CT 可见材料的 PDD 曲线与 IROC-Houston 的传统体模 STE 材料相当。最小射野大小显示出最大的差异，均匀和非均匀测试材料的计算和测量 PDD 曲线之间的平均差异分别为 1.8%和 5.9%。
明胶、液体塑料和碳氢化合物基材料被确定为用于 MRgRT QA 体模的替代 STE 替代品。
