Carleton Laboratory for Radiotherapy Physics, Carleton University, Ottawa K1S 5B6, Canada.
Med Phys. 2013 Jan;40(1):011713. doi: 10.1118/1.4770284.
To investigate several aspects of the Chen and Nath spectroscopic method of determining the dose rate constants of (125)I and (103)Pd seeds [Z. Chen and R. Nath, Phys. Med. Biol. 55, 6089-6104 (2010)] including the accuracy of using a line or dual-point source approximation as done in their method, and the accuracy of ignoring the effects of the scattered photons in the spectra. Additionally, the authors investigate the accuracy of the literature's many different spectra for bare, i.e., unencapsulated (125)I and (103)Pd sources.
Spectra generated by 14 (125)I and 6 (103)Pd seeds were calculated in vacuo at 10 cm from the source in a 2.7 × 2.7 × 0.05 cm(3) voxel using the EGSnrc BrachyDose Monte Carlo code. Calculated spectra used the initial photon spectra recommended by AAPM's TG-43U1 and NCRP (National Council of Radiation Protection and Measurements) Report 58 for the (125)I seeds, or TG-43U1 and NNDC(2000) (National Nuclear Data Center, 2000) for (103)Pd seeds. The emitted spectra were treated as coming from a line or dual-point source in a Monte Carlo simulation to calculate the dose rate constant. The TG-43U1 definition of the dose rate constant was used. These calculations were performed using the full spectrum including scattered photons or using only the main peaks in the spectrum as done experimentally. Statistical uncertainties on the air kerma/history and the dose rate/history were ≤0.2%. The dose rate constants were also calculated using Monte Carlo simulations of the full seed model.
The ratio of the intensity of the 31 keV line relative to that of the main peak in (125)I spectra is, on average, 6.8% higher when calculated with the NCRP Report 58 initial spectrum vs that calculated with TG-43U1 initial spectrum. The (103)Pd spectra exhibit an average 6.2% decrease in the 22.9 keV line relative to the main peak when calculated with the TG-43U1 rather than the NNDC(2000) initial spectrum. The measured values from three different investigations are in much better agreement with the calculations using the NCRP Report 58 and NNDC(2000) initial spectra with average discrepancies of 0.9% and 1.7% for the (125)I and (103)Pd seeds, respectively. However, there are no differences in the calculated TG-43U1 brachytherapy parameters using either initial spectrum in both cases. Similarly, there were no differences outside the statistical uncertainties of 0.1% or 0.2%, in the average energy, air kerma/history, dose rate/history, and dose rate constant when calculated using either the full photon spectrum or the main-peaks-only spectrum.
Our calculated dose rate constants based on using the calculated on-axis spectrum and a line or dual-point source model are in excellent agreement (0.5% on average) with the values of Chen and Nath, verifying the accuracy of their more approximate method of going from the spectrum to the dose rate constant. However, the dose rate constants based on full seed models differ by between +4.6% and -1.5% from those based on the line or dual-point source approximations. These results suggest that the main value of spectroscopic measurements is to verify full Monte Carlo models of the seeds by comparison to the calculated spectra.
研究 Chen 和 Nath 光谱法确定 (125)I 和 (103)Pd 种子剂量率常数的几个方面[Z. Chen 和 R. Nath, Phys. Med. Biol. 55, 6089-6104 (2010)]，包括在他们的方法中使用线或双点源近似的准确性，以及忽略光谱中散射光子影响的准确性。此外，作者还研究了文献中许多不同的裸 (即未封装的 (125)I 和 (103)Pd 源)的光谱的准确性。
在距离源 10 厘米处，在一个 2.7 × 2.7 × 0.05 cm3 的体素中，使用 EGSnrc BrachyDose 蒙特卡罗代码在真空中计算 14 个 (125)I 和 6 个 (103)Pd 种子的光谱。计算光谱使用 AAPM 的 TG-43U1 和 NCRP（国家辐射防护和测量委员会）报告 58 为 (125)I 种子推荐的初始光子光谱，或 TG-43U1 和 NNDC(2000)（国家核数据中心，2000）为 (103)Pd 种子。发射光谱被视为来自蒙特卡罗模拟中的线或双点源，以计算剂量率常数。使用 TG-43U1 定义的剂量率常数。这些计算使用包括散射光子的全光谱或实验中使用的全谱中的主峰来进行。空气比释动能/历史和剂量率/历史的统计不确定度≤0.2%。还使用全种子模型的蒙特卡罗模拟计算了剂量率常数。
当使用 NCRP 报告 58 初始光谱计算时，(125)I 光谱中 31 keV 线的强度与主峰的强度之比平均比使用 TG-43U1 初始光谱计算时高 6.8%。当使用 TG-43U1 而不是 NNDC(2000)初始光谱计算时，(103)Pd 光谱中 22.9 keV 线相对于主峰的强度平均下降 6.2%。三个不同研究的测量值与使用 NCRP 报告 58 和 NNDC(2000)初始光谱的计算值更吻合，平均差异分别为 (125)I 和 (103)Pd 种子的 0.9%和 1.7%。然而，在这两种情况下，使用任何一种初始光谱，计算的 TG-43U1 近距离放射治疗参数都没有差异。同样，当使用全光子谱或仅主峰谱进行计算时，平均能量、空气比释动能/历史、剂量率/历史和剂量率常数的差异在统计不确定度 0.1%或 0.2%以内。
我们基于使用计算的轴上光谱和线或双点源模型计算的剂量率常数与 Chen 和 Nath 的值非常吻合（平均为 0.5%），验证了他们从光谱到剂量率常数的更近似方法的准确性。然而，基于全种子模型的剂量率常数与基于线或双点源近似的剂量率常数相差+4.6%至-1.5%。这些结果表明，光谱测量的主要价值是通过与计算光谱进行比较来验证种子的全蒙特卡罗模型。
