Vanderstraeten Barbara, Olteanu Ana Maria Luiza, Reynaert Nick, Leal Antonio, De Neve Wilfried, Thierens Hubert
Department of Medical Physics, Ghent University, Gent, Belgium.
Int J Radiat Oncol Biol Phys. 2007 Oct 1;69(2):628-37. doi: 10.1016/j.ijrobp.2007.06.036.
To formulate uncertainty-based stopping criteria for Monte Carlo (MC) calculations of intensity-modulated radiotherapy and intensity-modulated arc therapy patient dose distributions and evaluate their influence on MC simulation times and dose characteristics.
For each structure of interest, stopping criteria were formulated as follows: sigma(rel) <or=sigma(rel,tol) or Dsigma(rel) <or=D(lim)sigma(rel,tol) within >or=95% of the voxels, where sigma(rel) represents the relative statistical uncertainty on the estimated dose, D. The tolerated uncertainty (sigma(rel,tol)) was 2%. The dose limit (D(lim)) equaled the planning target volume (PTV) prescription dose or a dose value related to the organ at risk (OAR) planning constraints. An intensity-modulated radiotherapy-lung, intensity-modulated radiotherapy-ethmoid sinus, and intensity-modulated arc therapy-rectum patient case were studied. The PTV-stopping criteria-based calculations were compared with the PTV+OAR-stopping criteria-based calculations.
The MC dose distributions complied with the PTV-stopping criteria after 14% (lung), 21% (ethmoid), and 12% (rectum) of the simulation times of a 100 million histories reference calculation, and increased to 29%, 44%, and 51%, respectively, by the addition of the OAR-stopping criteria. Dose-volume histograms corresponding to the PTV-stopping criteria, PTV+OAR-stopping criteria, and reference dose calculations were indiscernible. The median local dose differences between the PTV-stopping criteria and the reference calculations amounted to 1.4% (lung), 2.1% (ethmoid), and 2.5% (rectum).
For the patient cases studied, the MC calculations using PTV-stopping criteria only allowed accurate treatment plan evaluation. The proposed stopping criteria provided a flexible tool to assist MC patient dose calculations. The structures of interest and appropriate values of sigma(rel,tol) and D(lim) should be selected for each patient individually according to the clinical treatment planning goals.
制定基于不确定性的停止标准，用于调强放射治疗和调强弧形治疗患者剂量分布的蒙特卡罗（MC）计算，并评估其对MC模拟时间和剂量特征的影响。
对于每个感兴趣的结构，停止标准制定如下：在≥95%的体素内，σ(rel)≤σ(rel,tol) 或 Δσ(rel)≤D(lim)σ(rel,tol)，其中σ(rel)表示估计剂量D的相对统计不确定性。可容忍的不确定性（σ(rel,tol)）为2%。剂量限值（D(lim)）等于计划靶体积（PTV）处方剂量或与危及器官（OAR）计划约束相关的剂量值。研究了调强放射治疗-肺部、调强放射治疗-筛窦和调强弧形治疗-直肠患者病例。将基于PTV停止标准的计算与基于PTV+OAR停止标准的计算进行比较。
在进行1亿次历史参考计算的模拟时间的14%（肺部）、21%（筛窦）和12%（直肠）后，MC剂量分布符合PTV停止标准，通过添加OAR停止标准，分别增加到29%、44%和51%。对应于PTV停止标准、PTV+OAR停止标准和参考剂量计算的剂量体积直方图难以区分。PTV停止标准与参考计算之间的中位局部剂量差异分别为1.4%（肺部）、2.1%（筛窦）和2.5%（直肠）。
对于所研究的患者病例，仅使用PTV停止标准的MC计算就能进行准确的治疗计划评估。所提出的停止标准为辅助MC患者剂量计算提供了一个灵活的工具。应根据临床治疗计划目标，为每个患者单独选择感兴趣的结构以及σ(rel,tol)和D(lim)的适当值。
