Pflugfelder D, Wilkens J J, Szymanowski H, Oelfke U
Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany.
Med Phys. 2007 Apr;34(4):1506-13. doi: 10.1118/1.2710329.
In radiotherapy with scanned particle beams, tissue heterogeneities lateral to the beam direction are problematic in two ways: they pose a challenge to dose calculation algorithms, and they lead to a high sensitivity to setup errors. In order to quantify and avoid these problems, a heterogeneity number H(i) as a method to quantify lateral tissue heterogeneities of single beam spot i is introduced. To evaluate this new concept, two kinds of potential errors were investigated for single beam spots: First, the dose calculation error has been obtained by comparing the dose distribution computed by a simple pencil beam algorithm to more accurate Monte Carlo simulations. The resulting error is clearly correlated with H(i). Second, the analysis of the sensitivity to setup errors of single beam spots also showed a dependence on H(i). From this data it is concluded that H(i) can be used as a criterion to assess the risks of a compromised delivered dose due to lateral tissue heterogeneities. Furthermore, a method how to incorporate this information into the inverse planning process for intensity modulated proton therapy is presented. By suppressing beam spots with a high value of H(i), the unfavorable impact of lateral tissue heterogeneities can be reduced, leading to treatment plans which are more robust to dose calculation errors of the pencil beam algorithm. Additional possibilities to use the information of H(i) are outlined in the discussion.
在扫描粒子束放射治疗中，射束方向横向的组织不均匀性在两个方面存在问题：它们对剂量计算算法构成挑战，并且导致对摆位误差高度敏感。为了量化并避免这些问题，引入了不均匀性数值H(i)作为量化单个射束点i横向组织不均匀性的一种方法。为了评估这个新概念，针对单个射束点研究了两种潜在误差：第一，通过将简单笔形束算法计算的剂量分布与更精确的蒙特卡罗模拟进行比较，获得了剂量计算误差。所得误差与H(i)明显相关。第二，对单个射束点摆位误差敏感性的分析也显示出对H(i)的依赖性。从这些数据得出结论，H(i)可用作评估由于横向组织不均匀性导致所交付剂量受损风险的一个标准。此外，还提出了一种将此信息纳入调强质子治疗逆向计划过程的方法。通过抑制具有高H(i)值的射束点，可以降低横向组织不均匀性的不利影响，从而得到对笔形束算法剂量计算误差更稳健的治疗计划。讨论中概述了使用H(i)信息的其他可能性。
