Technical Note: A direct ray-tracing method to compute integral depth dose in pencil beam proton radiography with a multilayer ionization chamber
Technical Note: A direct ray-tracing method to compute integral depth dose in pencil beam proton radiography with a multilayer ionization chamber
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DOI：
10.1118/1.4966703
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发表时间：
2016-12-01
期刊：
影响因子：
3.8
通讯作者：
Vander Stappen, Francois
中科院分区：
文献类型：
--
作者：
Farace, Paolo;Righetto, Roberto;Vander Stappen, Francois
目的：介绍一种在多层电离室（MLIC）铅笔质子放射照相（PR）中进行体内距离误差定位的快速射线追踪算法。方法：铅笔束PR是通过均匀放置在一个正方形（45x45mm(2)视场）的9x9个点上获得的，这些点能够穿过幻影（210 MeV）。出口光束通过MLIC采集，以采样积分深度剂量（IDDMLIC）。通过移动沙发以获得多个45x45mm2帧，获得电子密度和头部幻影的pr。为了映射相应的距离误差，将IDDMLIC的二维集与(i)在CT处模拟MLIC的水体积中由处理计划系统（TPS）通过解析（IDDTPS）和蒙特卡罗（IDDMC）算法计算的积分深度剂量，以及（ii）通过相同CT数据由简单射线追踪算法（IDDdirect）直接计算的积分深度剂量进行比较。对光斑图的精确空间位置进行了数值调整，测试了不同的平面内位置，并选择了IDDdirect和IDDMLIC之间距离差最小的位置。结果：TPS和射线追踪方法均可实现距离误差映射，但对微小的误差非常敏感。在均匀区域，直接光线追踪算法计算的距离误差与解析算法和蒙特卡罗算法计算的结果相匹配。在这两个模型中，射线追踪和蒙特卡罗算法比解析TPS计算更好地模拟了横向非均匀性。因此，当铅笔束穿过横向异质性时，直接算法映射的距离误差比解析算法得到的距离误差更符合蒙特卡罗图。最后，光线追踪算法的简单性允许实现自动空间对准的原型程序。结论：射线追踪算法可可靠地替代MLIC PR中的TPS方法进行体内距离验证，可作为开发CT标定空间对准和校正软件工具的关键组成部分。(C) 2016年美国医学物理学家协会。
Purpose: To introduce a fast ray-tracing algorithm in pencil proton radiography (PR) with a multilayer ionization chamber (MLIC) for in vivo range error mapping.Methods: Pencil beam PR was obtained by delivering spots uniformly positioned in a square (45x45 mm(2) field-of-view) of 9x9 spots capable of crossing the phantoms (210 MeV). The exit beam was collected by a MLIC to sample the integral depth dose (IDDMLIC). PRs of an electron-density and of a head phantom were acquired by moving the couch to obtain multiple 45x45 mm2 frames. To map the corresponding range errors, the two-dimensional set of IDDMLIC was compared with (i) the integral depth dose computed by the treatment planning system (TPS) by both analytic (IDDTPS) and Monte Carlo (IDDMC) algorithms in a volume of water simulating the MLIC at the CT, and (ii) the integral depth dose directly computed by a simple ray-tracing algorithm (IDDdirect) through the same CT data. The exact spatial position of the spot pattern was numerically adjusted testing different in-plane positions and selecting the one that minimized the range differences between IDDdirect and IDDMLIC.Results: Range error mapping was feasible by both the TPS and the ray-tracing methods, but very sensitive to even small misalignments. In homogeneous regions, the range errors computed by the direct ray-tracing algorithm matched the results obtained by both the analytic and the Monte Carlo algorithms. In both phantoms, lateral heterogeneities were better modeled by the ray-tracing and the Monte Carlo algorithms than by the analytic TPS computation. Accordingly, when the pencil beam crossed lateral heterogeneities, the range errors mapped by the direct algorithm matched better the Monte Carlo maps than those obtained by the analytic algorithm. Finally, the simplicity of the ray-tracing algorithm allowed to implement a prototype procedure for automated spatial alignment.Conclusions: The ray-tracing algorithm can reliably replace the TPS method in MLIC PR for in vivo range verification and it can be a key component to develop software tools for spatial alignment and correction of CT calibration. (C) 2016 American Association of Physicists in Medicine.