Three-dimensional visualization and measurement of conformal dose distributions using magnetic resonance imaging of BANG polymer gel dosimeters
Three-dimensional visualization and measurement of conformal dose distributions using magnetic resonance imaging of BANG polymer gel dosimeters
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DOI：
10.1016/s0360-3016(97)00146-6
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发表时间：
1997-07-15
影响因子：
7
通讯作者：
Gore, JC
中科院分区：
文献类型：
--
作者：
Ibbott, GS;Maryanski, MJ;Gore, JC
目的/目标：测量复杂的剂量分布（通过多个光束、多个源或多源驻留位置照射产生的剂量分布）需要一个能够在完整治疗过程中整合剂量的剂量计。集成剂量计设备通常只能测量点（离子室、二极管、TLD）或平面（薄膜）上的剂量。随着需要成形、非共面光束的共形剂量分布的使用越来越多，对能够记录和显示 3D 剂量分布的剂量计的需求也会增加。需要使用 3D 剂量计来确认当前一代 3D 治疗计划计算机生成的治疗计划的准确性。方法和材料：已经证明使用 Fricke 注入凝胶和磁共振成像 (MRI) 来演示立体定向束的定位 (11)。最近开发的 BANG 聚合物凝胶剂量测定系统（MGS Research, Inc.，吉尔福德，康涅狄格州）基于分散在组织等效凝胶中的丙烯酸单体的辐射诱导链聚合，通过提供不随时间恶化的准确、定量剂量分布数据，超越了 Fricke-gel 方法 (6, 9)。改进的BANG2配方含有3%N,N'-亚甲基双丙烯酰胺、3%丙烯酸、1%氢氧化钠、5%明胶和88%水，其中所有百分比均按重量计。将凝胶倒入容量瓶中，容量瓶的尺寸与人头相当。凝胶采用复杂的光束布置进行照射，类似于适形放射治疗所使用的光束布置。使用西门子 1.5T 成像仪和 Hahn 自旋回波脉冲序列获取凝胶图像（90 度-tau-180 度-tau 获取，针对不同的 tau 值）。这些图像通过网络传输到一台编写了数据分析和显示程序的麦金塔计算机。该程序使用基于 Levenberg-Marquardt 算法的单指数非线性最小二乘拟合，根据多个 TE 图像计算 R2 图。该程序还通过将多项式拟合到一组剂量和 R2 数据点（从在试管中照射已知剂量的凝胶获得）来创建剂量到 R2 校准函数。然后可以将此函数应用于任何其他 R2 图，以便可以计算和显示剂量图。结果：通过暴露于已知剂量的辐射，凝胶已被证明在剂量范围为 0 至 10 Gy 时呈线性响应，并且其响应与束能量或模式无关。剂量分布已在正交平面上成像，并且可以以方便的形式显示，以便与等剂量计划进行比较。凝胶响应稳定；凝胶制成后可随时进行辐照，辐照后短暂间隔后可随时进行成像。结论：聚合物凝胶剂量计已被证明是一种显示三维剂量分布的有价值的装置。成像的剂量分布可以轻松地与计算的剂量分布进行比较，以验证治疗计划系统。将来，可以在拟人模型中制备凝胶，以确认独特的患者剂量分布。 (C) 1997 爱思唯尔科学公司。
Purpose/Objective: The measurement of complex dose distributions (those created by irradiation through multiple beams, multiple sources, or multiple source dwell positions) requires a dosimeter that can integrate the dose during a complete treatment. Integrating dosimeter devices generally are capable of measuring only dose at a point (ion chamber, diode, TLD) or in a plane (film). With increasing use of conformal dose distributions requiring shaped, noncoplanar beams, there will be an increased requirement for a dosimeter that can record and display a 3D dose distribution. The use of a 3D dosimeter will be required to confirm the accuracy of treatment plans produced by the current generation of 3D treatment-planning computers.Methods and Materials: The use of a Fricke-infused gel and magnetic resonance imaging (MRI) to demonstrate the localization of stereotactic beams has been demonstrated (11). The recently developed BANG polymer gel dosimetry system (MGS Research, Inc., Guilford, CT), based on radiation-induced chain polymerization of acrylic monomers dispersed in a tissue-equivalent gel, surpasses the Fricke-gel method by providing accurate, quantitative dose distribution data that do not deteriorate with time (6, 9). The improved BANG2 formulation contains 3% N,N'-methylene-bisacrylamide, 3% acrylic acid, 1% sodium hydroxide, 5% gelatin, and 88% water, where all percentages are by weight. The gel was poured into volumetric flasks, of dimensions comparable to a human head. The gels were irradiated with complex beam arrangements, similar to those used for conformal radiation therapy. Images of the gels were acquired using a Siemens 1.5T imager and a Hahn spin-echo pulse sequence (90 degrees-tau-180 degrees-tau-acquire, for different values of tau). The images were transferred via network to a Macintosh computer for which a data analysis and display program was written. The program calculates R2 maps on the basis of multiple TE images, using a monoexponential nonlinear least-squares fit based on the Levenberg-Marquardt algorithm. The program also creates a dose-to-R2 calibration function by fitting a polynomial to a set of dose and R2 data points, obtained from gels irradiated in test tubes to known doses. This function can then be applied to any other R2 map, so that a dose map can be computed and displayed.Results: Through exposure to known doses of radiation, the gel has been shown to respond linearly,vith dose in the range of 0 to 10 Gy, and its response is independent of the beam energy or modality. Dose distributions have been imaged in orthogonal planes, and can be displayed in a convenient form for comparison with isodose plans. The response of the gel is stable; the gel can be irradiated at any time after its manufacture, and imaging can be conducted any time following a brief interval after irradiation.Conclusion: The polymer gel dosimeter has been shown to be a valuable device for displaying three-dimensional dose distributions. The imaged dose distribution can be compared easily with calculated dose distributions, to validate a treatment planning system. In the future, gels may be prepared in anthropomorphic phantoms, to confirm unique patient dose distributions. (C) 1997 Elsevier Science Inc.