Cherenkov emission-based external radiotherapy dosimetry: II. Electron beam quality specification and uncertainties
Cherenkov emission-based external radiotherapy dosimetry: II. Electron beam quality specification and uncertainties
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DOI：
10.1002/mp.13413
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发表时间：
2019-05-01
期刊：
影响因子：
3.8
通讯作者：
El Naqa, Issam
中科院分区：
文献类型：
--
作者：
Zlateva, Yana;Muir, Bryan R.;El Naqa, Issam
目的切伦科夫辐射（CE）是体外放射治疗中普遍存在的辐射。它的独特之处还在于它具有3D、微米分辨率、无扰动、水中剂量测定的承诺，并具有与光束质量无关的探测器响应校准。我们的目标是将基于CE的剂量测定带入临床，我们在这里用电子束激发这一点。我们蒙特卡罗（MC）计算和表征宽束CE剂量转换因子在水中的临床代表性库的电子束质量，解决束质量规范和参考深度的选择，并制定了初步的不确定性预算的基础上，我们的MC结果和相关的实验工作的同伴研究（论文I）。方法宽电子束CE-剂量转换因子kC θ +/-Δ θ包括在水中以束轴上的极角θ +/-Δ θ产生的CE。通过修改EGSnrc代码SPRRZnrc，计算了来自四个BEAMnrc模型（Varian Clinac 2100 C/D、Clinac 21 EX、TrueBeam和Elekta Precise）的总共20种电子束质量的kC θ +/-delta θ因子。我们检查了θ +/-Δ θ =90圆+/-90圆（4 π检测）、90圆+/-5圆、45圆+/-45圆和90圆+/-45圆的光束质量、深度和检测角度依赖性。如论文I中所讨论的，4 π检测提供了最强的CE-剂量相关性，并且具有小Δ θ的θ =90圆是最实用的。这两个额外的配置被认为是这两个极端之间的折衷。我们根据电子束质量说明符R50（从50% CE C50的深度获得）来解决束质量规范和参考深度选择，并推导出论文I中提出的基于CE的剂量测定形式主义的最佳情况不确定度预算。在每个检测配置。结果kC θ +/-Δ θ因子被证明可以捕获低于CE阈值（类似于可见光中的260 keV）的光束光谱、角度、光子污染和电子注量的变化，与理论一致。根据C50，模拟R50值的二阶多项式拟合的均方根偏差和最大偏差分别为0.05和0.11 mm（4 pi）以及0.20和0.33 mm（90圈+/-5圈检测）。论文I中实验数据的拟合性能在实验不确定度（+/- 1.5 mm，95% CI）内与这些值一致。在参考深度dref= aR 50 +B，kC θ +/-delta θ的两项幂函数拟合在4 pi和90圆+/- 5圆检测时，总dref相关剂量不确定度贡献估计值分别为0.8%和1.1%，组合标准剂量不确定度的初步最佳情况估计值分别为1.1%和1.3%。两个中间孔径的结果和相应的不确定性一般与4 π情况相同。此外，R50和C50之间的差异导致的深度CE（PDC）的理论上一致的下游偏移将4 π转换的深度依赖性提高了一个数量级（+/- 2.8%）。因此，一个大的孔径为中心的θ值之间的45圈和90圈结合下游PDC的转变，可推荐为束轴CE为基础的电子束剂量测定在water.Conclusions通过提供R50为基础的CE剂量转换数据，并展示了潜在的剂量不确定性的顺序为1%，我们使CE为基础的电子束剂量测定更接近临床实现。
Purpose Cherenkov emission (CE) is ubiquitous in external radiotherapy. It is also unique in that it carries the promise of 3D, micrometer-resolution, perturbation-free, in-water dosimetry with a beam quality-independent detector response calibration. Our aim is to bring CE-based dosimetry into the clinic and we motivate this here with electron beams. We Monte Carlo (MC) calculate and characterize broad-beam CE-to-dose conversion factors in water for a clinically representative library of electron beam qualities, address beam quality specification and reference depth selection, and develop a preliminary uncertainty budget based on our MC results and relative experimental work of a companion study (Paper I). Methods Broad electron beam CE-to-dose conversion factors kC theta +/-delta theta include CE generated at polar angles theta +/- delta theta on beam axis in water. With modifications to the EGSnrc code SPRRZnrc, kC theta +/-delta theta factors are calculated for a total of 20 electron beam qualities from four BEAMnrc models (Varian Clinac 2100C/D, Clinac 21EX, TrueBeam, and Elekta Precise). We examine beam quality, depth, and detection angle dependence for theta +/-delta theta=90 circle +/- 90 circle (4 pi detection), 90 circle +/- 5 circle, 45 circle +/- 45 circle, and 90 circle +/- 45 circle. As discussed in Paper I, 4 pi detection offers the strongest CE-dose correlation and theta=90 circle with small delta theta is most practical. The two additional configurations are considered as a compromise between these two extremes. We address beam quality specification and reference depth selection in terms of the electron beam quality specifier R50, obtained from the depth of 50% CE C50, and derive a best-case uncertainty budget for the CE-based dosimetry formalism proposed in Paper I at each detection configuration. Results The kC theta +/-delta theta factor was demonstrated to capture variations in the beam spectrum, angle, photon contamination, and electron fluence below the CE threshold (similar to 260 keV in the visible) in accordance with theory. The root-mean-square deviation and maximum deviation of a second-order polynomial fit of simulated R50 values in terms of C50 were 0.05 and 0.11 mm at 4 pi and 0.20 and 0.33 mm at 90 circle +/- 5 circle detection, respectively. The fit performance on experimental data in Paper I was in agreement with these values within experimental uncertainties (+/- 1.5 mm, 95% CI). A two-term power function fit of kC theta +/-delta theta in terms of R50 at a reference depth dref=aR50+b resulted in total dref-dependent dose uncertainty contribution estimate of 0.8% and 1.1% and preliminary best-case estimate of the combined standard dose uncertainty of 1.1% and 1.3% at 4 pi and 90 circle +/- 5 circle detection, respectively. The results and corresponding uncertainties with the two intermediate apertures were generally of the same order as the 4 pi case. In addition, a theoretically consistent downstream shift of the percent-depth CE (PDC) by the difference between R50 and C50 improved the depth dependence of the 4 pi conversion by an order of magnitude (+/- 2.8%). Therefore, a large aperture centered on a theta value between 45 circle and 90 circle combined with a downstream PDC shift may be recommended for beam-axis CE-based electron beam dosimetry in water.Conclusions By delivering R50-based CE-to-dose conversion data and demonstrating the potential for dosimetric uncertainty on the order of 1%, we bring CE-based electron beam dosimetry closer to clinical realization.