Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Waehringer Guertel 18-20/4L; A-1090 Wien, Austria. High-field MR Centre, Medical University of Vienna/AKH, Lazarettg. 14, A-1090 Vienna, Austria.
Phys Med Biol. 2018 Mar 12;63(6):06NT01. doi: 10.1088/1361-6560/aab00b.
Recent developments in radiation therapy aimed at more precise dose delivery along with higher dose gradients (dose painting) and more efficient dose delivery with higher dose rates e.g. flattening filter free (FFF) irradiation. Magnetic-resonance-imaging based polymer gel dosimetry offers 3D information for precise dose delivery techniques. Many of the proposed polymer gels have been reported to exhibit a dose response, measured as relaxation rate ΔR2, which is dose rate dependent. A lack of or a reduced dose-rate sensitivity is very important for dosimetric accuracy, especially with regard to the increasing clinical use of FFF irradiation protocols with LINACs at high dose rates. Some commonly used polymer gels are based on Methacrylic-Acid-Gel-Initiated-by-Copper (MAGIC). Here, we report on the dose sensitivity (ΔR2/ΔD) of MAGIC-type gels with different oxygen scavenger concentration for their specific dependence on the applied dose rate in order to improve the dosimetric performance, especially for high dose rates. A preclinical x-ray machine ('Yxlon', E = 200 kV) was used for irradiation to cover a range of dose rates from low [Formula: see text] = 0.6 Gy min to high [Formula: see text] = 18 Gy min. The dose response was evaluated using R2-imaging of the gel on a human high-field (7T) MR-scanner. The results indicate that all of the investigated dose rates had an impact on the dose response in polymer gel dosimeters, being strongest in the high dose region and less effective for low dose levels. The absolute dose rate dependence [Formula: see text] of the dose response in MAGIC-type gel is significantly reduced using higher concentrations of oxygen scavenger at the expense of reduced dose sensitivity. For quantitative dose evaluations the relative dose rate dependence of a polymer gel, normalized to its sensitivity is important. Based on this normalized sensitivity the dose rate sensitivity was reduced distinctly using an increased oxygen scavenger concentration with reference to standard MAGIC-type gel formulation at high dose rate levels. The proposed gel composition with high oxygen scavenger concentration exhibits a larger linear active dose response and might be used especially in FFF-radiation applications and preclinical dosimetry at high dose rates. We propose in general to use high dose rates for calibration and evaluation as the change in relative dose sensitivity is reduced at higher dose rates in all of the investigated gel types.
最近，放射治疗领域的发展旨在实现更精确的剂量输送，同时实现更高的剂量梯度（剂量绘制）和更高的剂量率效率，例如使用无均整过滤器（FFF）照射。基于磁共振成像的聚合物凝胶剂量测定法可提供用于精确剂量输送技术的 3D 信息。许多提出的聚合物凝胶已被报道表现出剂量响应，以弛豫率 ΔR2 测量，该响应与剂量率有关。缺乏或降低剂量率灵敏度对于剂量准确性非常重要，尤其是考虑到随着 LINAC 以高剂量率使用越来越多的 FFF 照射方案，这一点更为重要。一些常用的聚合物凝胶基于甲基丙烯酸凝胶引发铜（MAGIC）。在这里，我们报告了不同氧清除剂浓度的 MAGIC 型凝胶的剂量敏感性（ΔR2/ΔD），以了解其对应用剂量率的特定依赖性，以便提高剂量性能，特别是对于高剂量率。使用临床前 X 射线机（'Yxlon'，E = 200 kV）进行照射，以覆盖从低[公式：见文本] [Formula: see text] = 0.6 Gy min 到高[公式：见文本] [Formula: see text] = 18 Gy min 的剂量率范围。使用凝胶在人体高场（7T）MR 扫描仪上的 R2 成像评估剂量响应。结果表明，所有研究的剂量率都对聚合物凝胶剂量计的剂量响应产生了影响，在高剂量区域的影响最大，而对低剂量水平的影响较小。使用较高浓度的氧清除剂，MAGIC 型凝胶的剂量响应的绝对剂量率依赖性[公式：见文本]显著降低，但剂量敏感性降低。对于定量剂量评估，归一化为灵敏度的聚合物凝胶的相对剂量率依赖性很重要。基于此归一化灵敏度，与标准 MAGIC 型凝胶配方相比，在高剂量率水平下，使用较高的氧清除剂浓度可明显降低剂量率敏感性。具有高氧清除剂浓度的建议凝胶组成具有更大的线性有效剂量响应，可能特别用于 FFF 辐射应用和高剂量率的临床前剂量测定。我们一般建议使用高剂量率进行校准和评估，因为在所研究的所有凝胶类型中，相对剂量灵敏度的变化在较高剂量率下会降低。
