Farace Paolo, Righetto Roberto, Deffet Sylvain, Meijers Arturs, Vander Stappen Francois
Proton Therapy Unit, Hospital of Trento, Trento 38100, Italy.
Institute of Information and Communication Technologies, Université Catholique de Louvain (UCL), Louvain-La-Neuve, 1348, Belgium.
Med Phys. 2016 Dec;43(12):6405. doi: 10.1118/1.4966703.
To introduce a fast ray-tracing algorithm in pencil proton radiography (PR) with a multilayer ionization chamber (MLIC) for in vivo range error mapping.
Pencil beam PR was obtained by delivering spots uniformly positioned in a square (45 × 45 mm field-of-view) of 9 × 9 spots capable of crossing the phantoms (210 MeV). The exit beam was collected by a MLIC to sample the integral depth dose (IDD). PRs of an electron-density and of a head phantom were acquired by moving the couch to obtain multiple 45 × 45 mm frames. To map the corresponding range errors, the two-dimensional set of IDD was compared with (i) the integral depth dose computed by the treatment planning system (TPS) by both analytic (IDD) and Monte Carlo (IDD) 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 (IDD) 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 IDD and IDD.
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.
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.
介绍一种用于铅笔束质子射线照相术（PR）的快速射线追踪算法，该算法采用多层电离室（MLIC）进行体内射程误差映射。
通过在能够穿过模体（210 MeV）的9×9个点的正方形（45×45 mm视野）中均匀分布点来获得铅笔束PR。出射束由MLIC收集以采样积分深度剂量（IDD）。通过移动治疗床以获取多个45×45 mm帧，获得电子密度模体和头部模体的PR。为了映射相应的射程误差，将二维IDD集与以下两者进行比较：（i）在模拟CT处MLIC的水体积中，通过解析算法（IDD）和蒙特卡罗算法（IDD）由治疗计划系统（TPS）计算的积分深度剂量；（ii）通过简单射线追踪算法（IDD）直接通过相同CT数据计算的积分深度剂量。通过测试不同的平面内位置并选择使IDD与IDD之间的射程差异最小的位置，对光斑图案的精确空间位置进行数值调整。
TPS和射线追踪方法都可行进行射程误差映射，但对即使很小的未对准也非常敏感。在均匀区域中，直接射线追踪算法计算的射程误差与解析算法和蒙特卡罗算法获得的结果相匹配。在两个模体中，射线追踪和蒙特卡罗算法比解析TPS计算能更好地模拟横向不均匀性。因此，当铅笔束穿过横向不均匀性时，直接算法映射的射程误差比解析算法获得的射程误差更符合蒙特卡罗映射。最后，射线追踪算法的简单性允许实现用于自动空间对准的原型程序。
射线追踪算法可以在MLIC PR中可靠地替代TPS方法进行体内射程验证，并且它可以成为开发用于空间对准和CT校准校正的软件工具的关键组件。
