视网膜母细胞瘤外照射小野D形准直器的蒙特卡罗绝对剂量学公式的检验
Testing Monte Carlo absolute dosimetry formalisms for a small field `D'-shaped collimator used in retinoblastoma external beam radiotherapy
摘要：
目的：探讨两种蒙特卡罗模拟绝对剂量学方法在小野专用“D”形准直器用于视网膜母细胞瘤体外光子束放射治疗中的有效性。方法：采用蒙特卡罗程序{ \sc penelope}对直线加速器、专用准直器和水模进行模拟。在Popescu { \it et al.}的方法中获得所考虑射野大小的绝对剂量（单位：Gy/监视器单元）。其中计及监视室中的计数后向散射剂量。实验数据的结果进行了比较，发现与一个简单的Monte Carlo近似计算的绝对剂量和分析各向异性算法所提供的。我们的分析允许仿真跟踪参数的研究。两组参数已被认为是模拟粒子输运的直线加速器目标。结果如下：跟踪参数的变化在参考条件下估计的剂量中产生了约10%或更大的不可忽略的差异。对于所分析的准直器几何形状的两个参数组，绝对剂量的蒙特卡罗结果与实验结果相差2.6%和1.7%。对于所研究的射野，更简单的方法产生的绝对剂量在统计学上与Popescu { \it et al.}的方法获得的绝对剂量一致。分析各向异性算法低估了实验的绝对剂量的差异大于蒙特卡罗结果。结论：研究的方法可以被认为是绝对剂量测定的情况下，小，“D”形和离轴辐射场。然而，在直线加速器目标的辐射输运的详细描述是强制性的一个准确的绝对剂量。
英文摘要：
Purpose: To investigate the validity of two Monte Carlo simulation absolute dosimetry approaches in the case of a small field dedicated `D'-shaped collimator used for the retinoblastoma treatment with external photon beam radiotherapy. Methods: The Monte Carlo code {\sc penelope} is used to simulate the linac, the dedicated collimator and a water phantom. The absolute doses (in Gy per monitor unit) for the field sizes considered are obtained within the approach of Popescu {\it et al.} in which the tallied backscattered dose in the monitor chamber is accounted for. The results are compared to experimental data, to those found with a simpler Monte Carlo approximation for the calculation of absolute doses and to those provided by the analytical anisotropic algorithm. Our analysis allows for the study of the simulation tracking parameters. Two sets of parameters have been considered for the simulation of the particle transport in the linac target. Results: The change in the tracking parameters produced non-negligible differences, of about 10\% or larger, in the doses estimated in reference conditions. The Monte Carlo results for the absolute doses differ from the experimental ones by 2.6\% and 1.7\% for the two parameter sets for the collimator geometries analyzed. For the studied fields, the simpler approach produces absolute doses that are statistically compatible with those obtained with the approach of Popescu {\it et al.} The analytical anisotropic algorithm underestimates the experimental absolute doses with discrepancies larger than those found for Monte Carlo results. Conclusions: The approach studied can be considered for absolute dosimetry in the case of small, `D'-shaped and off-axis radiation fields. However, a detailed description of the radiation transport in the linac target is mandatory for an accurate absolute dosimetry.
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