Chow James C L, Wong Eugene, Chen Jeff Z, Van Dyk Jake
Department of Physics, London Regional Cancer Center, 790 Commissioners Road East, London, ON N6A 4L6, Canada.
Med Phys. 2003 Oct;30(10):2686-94. doi: 10.1118/1.1601331.
The objective of this study is to seek an accurate and efficient method to calculate the dose distribution of a photon arc. The algorithms tested include Monte Carlo, pencil beam kernel (PK), and collapsed cone convolution (CCC). For the Monte Carlo dose calculation, EGS4/DOSXYZ was used. The SRCXYZ source code associated with the DOSXYZ was modified so that the gantry angle of a photon beam would be sampled uniformly within the arc range about an isocenter to simulate a photon arc. Specifically, photon beams (6/18 MV, 4 x 4 and 10 x 10 cm2) described by a phase space file generated by BEAM (MCPHS), or by two point sources with different photon energy spectra (MCDIV) were used. These methods were used to calculate three-dimensional (3-D) distributions in a PMMA phantom, a cylindrical water phantom, and a phantom with lung inhomogeneity. A commercial treatment planning system was also used to calculate dose distributions in these phantoms using equivalent tissue air ratio (ETAR), PK and CCC algorithms for inhomogeneity corrections. Dose distributions for a photon arc in these phantoms were measured using a RK ion chamber and radiographic films. For homogeneous phantoms, the measured results agreed well (approximately 2% error) with predictions by the Monte Carlo simulations (MCPHS and MCDIV) and the treatment planning system for the 180 degrees and 360 degrees photon arcs. For the dose distribution in the phantom with lung inhomogeneity with a 90 degrees photon arc, the Monte Carlo calculations agreed with the measurements within 2%, while the treatment planning system using ETAR, PK and CCC underestimated or overestimated the dose inside the lung inhomogeneity from 6% to 12%.
本研究的目的是寻求一种准确且高效的方法来计算光子弧形照射的剂量分布。所测试的算法包括蒙特卡罗算法、笔形束核算法（PK）和坍缩圆锥卷积算法（CCC）。对于蒙特卡罗剂量计算，使用了EGS4/DOSXYZ。对与DOSXYZ相关联的SRCXYZ源代码进行了修改，以便在围绕等中心的弧形范围内均匀采样光子束的机架角度，从而模拟光子弧形照射。具体而言，使用了由BEAM（MCPHS）生成的相空间文件描述的光子束（6/18 MV，4×4和10×10 cm²），或具有不同光子能谱的两个点源（MCDIV）。这些方法用于计算在有机玻璃模体、圆柱形水体模和具有肺部不均匀性的模体中的三维（3-D）分布。还使用了一种商业治疗计划系统，使用等效组织空气比（ETAR）、PK和CCC算法进行不均匀性校正，来计算这些模体中的剂量分布。使用RK电离室和射线照相胶片测量了这些模体中光子弧形照射的剂量分布。对于均匀模体，测量结果与蒙特卡罗模拟（MCPHS和MCDIV）以及治疗计划系统对180度和360度光子弧形照射的预测结果吻合良好（误差约为2%）。对于具有90度光子弧形照射的肺部不均匀性模体中的剂量分布，蒙特卡罗计算结果与测量值的误差在2%以内，而使用ETAR、PK和CCC的治疗计划系统对肺部不均匀性内部剂量的低估或高估幅度为6%至12%。
