Department of Radiation Oncology, Mayo Clinic, Phoenix, AZ, USA.
Department of Radiology, Mayo Clinic, Rochester, MN, USA.
J Appl Clin Med Phys. 2021 Mar;22(3):131-140. doi: 10.1002/acm2.13191. Epub 2021 Feb 20.
To demonstrate that variation in chemical composition has a negligible effect on the mapping curve from relative electron density (RED) to proton stopping power ratio (SPR), and to establish the theoretical framework of using Megavoltage (MV) computed tomography (CT), instead of kilovoltage (kV) dual energy CT, to accurately estimate proton SPR.
A simulation study was performed to evaluate the effect of chemical composition variation on kVCT number and proton SPR. The simulation study involved both reference and simulated human tissues. The reference human tissues, together with their physical densities and chemical compositions, came from the ICRP publication 23. The simulated human tissues were created from the reference human tissues assuming that elemental percentage weight followed a Gaussian distribution. For all tissues, kVCT number and proton SPR were obtained through (a) theoretical calculation from tissue's physical density and chemical composition which served as the ground truth, and (b) estimation from RED using the calibration curves established from the stoichiometric method. Deviations of the estimated values from the calculated values were quantified as errors in using RED to estimate kVCT number and proton SPR.
Given a chemical composition variation of 5% (1σ) of the nominal percentage weights, the total estimation error of using RED to estimate kVCT number was 0.34%, 0.62%, and 0.77% and the total estimation error of using RED to estimate proton SPR was 0.30%, 0.22%, and 0.16% for fat tissues, non-fat soft tissues and bone tissues, respectively.
Chemical composition had a negligible effect on the method of using RED to determine proton SPR. RED itself is sufficient to accurately determine proton SPR. MVCT number maintains a superb linear relationship with RED because it is highly dominated by Compton scattering. Therefore, MVCT has great potential in reducing the proton range uncertainty.
证明化学成分的变化对从相对电子密度（RED）到质子阻止本领比（SPR）的映射曲线的影响可以忽略不计，并建立使用兆伏（MV）计算机断层扫描（CT）而不是千伏（kV）双能 CT 来准确估计质子 SPR 的理论框架。
进行了一项模拟研究，以评估化学成分变化对 kVCT 数和质子 SPR 的影响。该模拟研究涉及参考和模拟人体组织。参考人体组织及其物理密度和化学成分来自 ICRP 出版物 23。模拟人体组织是从参考人体组织创建的，假设元素百分比重量遵循高斯分布。对于所有组织，kVCT 数和质子 SPR 通过以下两种方式获得：(a) 从组织的物理密度和化学成分理论计算，这些数据作为真实值；(b) 使用从化学计量法建立的校准曲线从 RED 进行估计。通过 RED 估计 kVCT 数和质子 SPR 的估算值与计算值之间的偏差被量化为使用 RED 估计的误差。
给定名义百分比重量的 5%（1σ）的化学成分变化，使用 RED 估计 kVCT 数的总估计误差为 0.34%、0.62%和 0.77%，使用 RED 估计质子 SPR 的总估计误差为 0.30%、0.22%和 0.16%，分别为脂肪组织、非脂肪软组织和骨骼组织。
化学成分对使用 RED 确定质子 SPR 的方法影响可以忽略不计。RED 本身足以准确确定质子 SPR。MVCT 数与 RED 保持极好的线性关系，因为它主要由康普顿散射决定。因此，MVCT 具有降低质子射程不确定性的巨大潜力。
