Paganetti Harald
Department of Radiation Oncology, Massachusetts General Hospital & Harvard Medical School, Boston, MA 02114, USA.
Phys Med Biol. 2009 Jul 21;54(14):4399-421. doi: 10.1088/0031-9155/54/14/004. Epub 2009 Jun 23.
Dose in radiation therapy is traditionally reported as the water-equivalent dose, or dose to water. Monte Carlo dose calculations report dose to medium and thus a methodology is needed to convert dose to medium into dose to water (or vice versa) for comparison of Monte Carlo results with results from planning systems. This paper describes the development of a formalism to convert dose to medium into dose to water for proton fields when simulating the dose with Monte Carlo techniques. The conversion is based on relative stopping power but also considers energy transferred via nuclear interactions. The influence of different interaction mechanisms of proton beams (electromagnetic versus nuclear) is demonstrated. Further, an approximate method for converting doses retroactively is presented. Based on the outlined formalism, five proton therapy patients with a total of 33 fields were analyzed. Dose distributions, dose volume histograms and absolute doses to assess the clinical significance of differences between dose to medium and dose to water are presented. We found that the difference between the two dose reporting definitions can be up to 10% for high CT numbers if analyzing the mean dose to the target. The difference is clinically insignificant for soft tissues. For the structures analyzed, the mean dose to water could be converted to dose to medium by applying a correction factor increasing linearly with increasing average CT number in the volume. We determined that an approximate conversion method, done retroactively with an energy-independent stopping power ratio and without considering nuclear interaction events separately (as compared to on-the-fly conversion during simulation), is sufficiently accurate to compute mean doses. It is insufficient, however, when analyzing the beam range. For proton beams stopping in bony anatomy, the predicted beam range can differ by 2-3 mm when comparing dose to tissue and dose to water.
放射治疗中的剂量传统上报告为水等效剂量，即对水的剂量。蒙特卡罗剂量计算报告的是对介质的剂量，因此需要一种方法将对介质的剂量转换为对水的剂量（反之亦然），以便将蒙特卡罗结果与计划系统的结果进行比较。本文描述了一种形式主义的发展，用于在使用蒙特卡罗技术模拟质子场剂量时，将对介质的剂量转换为对水的剂量。这种转换基于相对阻止本领，但也考虑了通过核相互作用传递的能量。展示了质子束不同相互作用机制（电磁与核）的影响。此外，还提出了一种追溯转换剂量的近似方法。基于所概述的形式主义，分析了5例质子治疗患者共33个射野。给出了剂量分布、剂量体积直方图和绝对剂量，以评估对介质的剂量和对水的剂量之间差异的临床意义。我们发现，如果分析靶区的平均剂量，对于高CT值，这两种剂量报告定义之间的差异可达10%。对于软组织，这种差异在临床上无显著意义。对于所分析的结构，通过应用一个随体积内平均CT值增加而线性增加的校正因子，可以将对水的平均剂量转换为对介质的剂量。我们确定，一种近似转换方法，即使用与能量无关的阻止本领比进行追溯转换，且不单独考虑核相互作用事件（与模拟期间的实时转换相比），对于计算平均剂量足够准确。然而，在分析射野范围时，它是不够的。对于在骨质结构中停止的质子束，当比较对组织的剂量和对水的剂量时，预测的射野范围可能相差2 - 3毫米。
