Dose to 'water-like' media or dose to tissue in MV photons radiotherapy treatment planning: still a matter of debate
Dose to 'water-like' media or dose to tissue in MV photons radiotherapy treatment planning: still a matter of debate
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DOI：
10.1088/0031-9155/60/1/309
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发表时间：
2015-01-07
影响因子：
3.5
通讯作者：
Andreo, Pedro
中科院分区：
文献类型：
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作者：
Andreo, Pedro
蒙特卡罗治疗计划 (MCTP) 之间的差异已得到研究，该假设基于“类水”组织（其密度由 CT 程序获得）或基于由 CT 确定的密度得出的组织成分。已计算了 6 MV 光子束的一系列介质和身体组织的阻止本领和电子注量，包括其物理数据（密度和阻止本领）的变化。这些量已用于利用空腔理论确定吸收剂量。需要强调的是，ICRU 或 ICRP 报告中给出的组织成分不应被视为物理常数，因为它们对应于有限数量的人体样本获得的平均值。已经表明，与水的质量阻止本领比更依赖于患者与患者之间的成分差异，因此依赖于其平均激发能（I 值），而不是质量密度。不同介质中的电子注量也更依赖于介质成分（及其 I 值）而不是密度。然而，由于注量和阻止本领之间的平衡，从它们的乘积计算出的剂量比独立阻止本领和注量变化所暗示的更恒定。此外，剂量比的取消最大限度地减少了“类水”和“组织”方法之间的差异，产生几乎相同的结果，除了骨骼之外，对于脂肪组织也有较小程度的影响。考虑到整个治疗计划组织分割和剂量计算过程中涉及的大量近似值和不确定性，从一种方法改变到另一种方法似乎是不合理的。关键问题仍然是组织的组成及其 I 值，并且由于无法获得个体患者的这些信息，因此无论选择何种方法都不会导致与水参考剂量的显着差异，对于骨组织来说，这些差异的最大值约为 5%。然而，考虑到先进剂量计算方法的当前发展，剂量到组织的规划应该是首选，因为预计该领域的进展将逐渐改进MCTP和数值传输方法中包含的一些粗略近似。所获得的微小差异还表明，基于广泛使用的方法从剂量到组织到剂量到水的回顾性转换将在很大程度上增加治疗计划过程的最终不确定性。事实证明，由于水和身体组织中的电子注量分布之间的差异，转换需要额外的注量校正，而迄今为止该校正被忽略。提供了改进的转换表达式和注量校正因子的数据。即使在剂量到组织的环境中，这些也是必要的，以便将治疗计划标准化为治疗单元的参考剂量测定，始终根据水吸收剂量进行校准。
The difference between Monte Carlo Treatment Planning (MCTP) based on the assumption of 'water-like' tissues with densities obtained from CT procedures, or on tissue compositions derived from CT-determined densities, have been investigated. Stopping powers and electron fluences have been calculated for a range of media and body tissues for 6 MV photon beams, including changes in their physical data (density and stopping powers). These quantities have been used to determine absorbed doses using cavity theory. It is emphasized that tissue compositions given in ICRU or ICRP reports should not be given the standing of physical constants as they correspond to average values obtained for a limited number of human-body samples.It has been shown that mass stopping-power ratios to water are more dependent on patient-to-patient composition differences, and therefore on their mean excitation energies (I-values), than on mass density. Electron fluence in different media are also more dependent on media composition (and their I-values) than on density. However, as a consequence of the balance between fluence and stopping powers, doses calculated from their product are more constant than what the independent stopping powers and fluence variations suggest.Additionally, cancelations in dose ratios minimize the differences between the 'water-like' and 'tissue' approaches, yielding practically identical results except for bone, and to a lesser extent for adipose tissue. A priori, changing from one approach to another does not seem to be justified considering the large number of approximations and uncertainties involved throughout the treatment planning tissue segmentation and dose calculation procedures. The key issue continues to be the composition of tissues and their I-values, and as these cannot be obtained for individual patients, whatever approach is selected does not lead to significant differences from a water reference dose, the maximum of these being of the order of 5% for bone tissues. Considering, however, current developments in advanced dose calculation methods, planning in terms of dose-to-tissue should be the preferred choice, under the expectancy that progress in the field will gradually improve some of the crude approximations included in MCTP and numerical transport methods.The small differences obtained also show that a retrospective conversion from dose-to-tissue to dose-to-water, based on a widely used approach, would mostly increase the final uncertainty of the treatment planning process. It is demonstrated that, due to the difference between electron fluence distributions in water and in body tissues, the conversion requires an additional fluence correction that has so far been neglected. An improved expression for the conversion and data for the fluence correction factor are provided. These will be necessary even in a dose-to-tissue environment, for the normalization of the treatment plan to the reference dosimetry of the treatment unit, always calibrated in terms of absorbed dose to water.