Department of Medical Physics, Wisconsin Institute for Medical Research, University of Wisconsin-Madison, Madison, WI, USA.
Phys Med Biol. 2013 Feb 21;58(4):887-902. doi: 10.1088/0031-9155/58/4/887. Epub 2013 Jan 21.
Uncertainties in the estimated mean excitation energies (I-values) needed for calculating proton stopping powers can be in the order of 10-15%, which introduces a fundamental limitation in the accuracy of proton range determination. Previous efforts have quantified shifts in proton depth dose distributions due to I-value uncertainties in water and homogenous tissue phantoms. This study is the first to quantify the clinical impact of I-value uncertainties on proton dose distributions within patient geometries. A previously developed Geant4 based Monte Carlo code was used to simulate a proton treatment plan for three patients (prostate, pancreases, and liver) with varying tissue I-values. A uniform variation study was conducted in which the tissue I-values were varied by ±5% and ±10% of the nominal values as well as a probabilistic variation study in which the I-values were randomly sampled according to a normal distribution with the mean equal to the nominal I-value and a standard deviation of 5 and 10% of the nominal values. Modification of tissue I-values impacted both the proton range and SOBP width. R(90) range shifts up to 7.7 mm (4.4.%) and R(80) range shifts up to 4.8 mm (1.9%) from the nominal range were recorded. Modulating the tissue I-values by 10% the nominal value resulted in up to a 3.5% difference mean dose in the target volumes and organs at risk compared to the nominal case. The range and dose differences were the largest for the deeper-seated prostate and pancreas cases. The treatments that were simulated with randomly sampled I-values resulted in range and dose differences that were generally within the upper and lower bounds set by the 10% uniform variations. This study demonstrated the impact of I-value uncertainties on patient dose distributions. Clearly, sub-millimeter precision in proton therapy would necessitate a reduction in I-value uncertainties to ensure an efficacious clinical outcome.
估算质子阻止本领所需的平均激发能 (I 值) 的不确定性可能在 10-15%的范围内，这给质子射程确定的准确性带来了根本性的限制。以前的研究已经量化了由于水和同质组织体模中的 I 值不确定性引起的质子深度剂量分布的偏移。本研究首次量化了 I 值不确定性对患者几何体内质子剂量分布的临床影响。先前开发的基于 Geant4 的蒙特卡罗代码用于模拟三个患者（前列腺、胰腺和肝脏）的质子治疗计划，这些患者的组织 I 值有所不同。进行了均匀变化研究，其中组织 I 值相对于名义值变化了±5%和±10%，以及概率变化研究，其中 I 值根据正态分布随机抽样，平均值等于名义 I 值，标准差为名义值的 5%和 10%。组织 I 值的变化既影响质子射程又影响 SOBP 宽度。从名义射程记录到 R(90) 射程的变化高达 7.7 毫米 (4.4.%)，R(80) 射程的变化高达 4.8 毫米 (1.9%)。将组织 I 值调制为名义值的 10%，与名义情况相比，靶区和危及器官的平均剂量差异高达 3.5%。对于更深的前列腺和胰腺病例，射程和剂量差异最大。用随机抽样的 I 值模拟的治疗导致射程和剂量差异通常在 10%均匀变化设定的上限和下限范围内。本研究证明了 I 值不确定性对患者剂量分布的影响。显然，质子治疗的亚毫米精度将需要降低 I 值的不确定性，以确保有效的临床结果。
