McLaughlin David J, Hogstrom Kenneth R, Neck Daniel W, Gibbons John P
Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA, USA.
Mary Bird Perkins Cancer Center, Baton Rouge, LA, USA.
J Appl Clin Med Phys. 2018 May;19(3):183-192. doi: 10.1002/acm2.12317. Epub 2018 Mar 30.
This study compares energy spectra of the multiple electron beams of individual radiotherapy machines, as well as the sets of spectra across multiple matched machines. Also, energy spectrum metrics are compared with central-axis percent depth-dose (PDD) metrics.
A lightweight, permanent magnet spectrometer was used to measure energy spectra for seven electron beams (7-20 MeV) on six matched Elekta Infinity accelerators with the MLCi2 treatment head. PDD measurements in the distal falloff region provided R and R metrics in Plastic Water , which correlated with energy spectrum metrics, peak mean energy (PME) and full-width at half maximum (FWHM).
Visual inspection of energy spectra and their metrics showed whether beams on single machines were properly tuned, i.e., FWHM is expected to increase and peak height decrease monotonically with increased PME. Also, PME spacings are expected to be approximately equal for 7-13 MeV beams (0.5-cm R spacing) and for 13-16 MeV beams (1.0-cm R spacing). Most machines failed these expectations, presumably due to tolerances for initial beam matching (0.05 cm in R ; 0.10 cm in R ) and ongoing quality assurance (0.2 cm in R ). Also, comparison of energy spectra or metrics for a single beam energy (six machines) showed outlying spectra. These variations in energy spectra provided ample data spread for correlating PME and FWHM with PDD metrics. Least-squares fits showed that R and R varied linearly and supralinearly with PME, respectively; however, both suggested a secondary dependence on FWHM. Hence, PME and FWHM could serve as surrogates for R and R for beam tuning by the accelerator engineer, possibly being more sensitive (e.g., 0.1 cm in R corresponded to 2.0 MeV in FWHM).
Results of this study suggest a lightweight, permanent magnet spectrometer could be a useful beam-tuning instrument for the accelerator engineer to (a) match electron beams prior to beam commissioning, (b) tune electron beams for the duration of their clinical use, and (c) provide estimates of PDD metrics following machine maintenance. However, a real-time version of the spectrometer is needed to be practical.
本研究比较了各放射治疗机器的多电子束能谱，以及多台匹配机器的能谱集。此外，还将能谱指标与中心轴百分深度剂量（PDD）指标进行了比较。
使用一台轻型永磁光谱仪，在配备MLCi2治疗头的六台匹配的医科达Infinity加速器上，测量了七束电子束（7 - 20 MeV）的能谱。在远端剂量跌落区域进行的PDD测量，得出了水模体中的R和R指标，这些指标与能谱指标、峰值平均能量（PME）和半高宽（FWHM）相关。
对能谱及其指标的目视检查表明，单台机器上的电子束是否已正确调谐，即随着PME的增加，FWHM预计会增大，峰值高度预计会单调降低。此外，对于7 - 13 MeV电子束（0.5 cm的R间距）和13 - 16 MeV电子束（1.0 cm的R间距），PME间距预计大致相等。大多数机器未达到这些预期，可能是由于初始束匹配的公差（R为0.05 cm；R为0.10 cm）和持续质量保证（R为0.2 cm）。此外，对单束能量（六台机器）的能谱或指标进行比较时，发现了异常能谱。这些能谱变化为将PME和FWHM与PDD指标相关联提供了充足的数据分布。最小二乘法拟合表明，R和R分别与PME呈线性和超线性变化；然而，两者均表明对FWHM存在二次依赖性。因此，PME和FWHM可作为加速器工程师进行束调谐时R和R的替代指标，可能更为敏感（例如，R中0.1 cm对应FWHM中2.0 MeV）。
本研究结果表明，一台轻型永磁光谱仪可能是加速器工程师的一种有用的束调谐仪器，可用于（a）在束调试前匹配电子束，（b）在临床使用期间调谐电子束，以及（c）在机器维护后提供PDD指标的估计值。然而，需要一个实时版本的光谱仪才具有实用性。
