使用硅半导体探测器采集FFF小野光子束基础数据的扫描参数优选研究

Scanning Parameter Optimization of Silicon Detector for Flattening Filter-Free Model Photon Beam Basic Data Measurement

目的探讨使用硅半导体探测器采集非均整模式小尺寸辐射野兆伏级光子束的百分深度剂量(Percentile Depth Dose,PDD)、射野离轴比(Field Off Axis Ratio,OAR)等基础数据的扫描参数优选问题。方法以蒙卡仿真结果为参考,使用PTW60016硅半导体探测器,采用均值±标准差、Gamma1D分析等方法,定量评价不同采样时间与采样步长对小野射线质及半影区宽度测量偏差的影响。结果不同测量参数组合条件下,四个典型小野(1cm×1cm^4cm×4cm)射线质的标准差均在0.005以内;采样时间的增加对PDD和OAR测量精度影响较小,采样步长的减小有利于提高OAR半影区和PDD建成区测量精度,对大深度处PDD测量精度的提高无意义;采样时间对半影区宽度影响较小(标准差均小于0.05mm),采样步长的增大导致半影区展宽。结论射线质对采样时间和步长不敏感;PDD测量精度对采样时间不敏感,4~50mm区域与采样步长成负相关,51~300mm深度不敏感;OAR半影区宽度对采样时间不敏感,与采样步长正相关;综合测量精度和效率,本研究推荐PDD数据采用0.5s、0.5mm(50mm以内)和1mm(51~300mm),OAR数据0.5s、0.5mm作为PTW60016的小野基础数据扫描参数。

Objective To optimize the scanning parameters of silicone detector used for radiotherapy Flattening filter-free (FFF) model photon beam basic data measurement,such as the percentage depth dose (PDD) and off-axis ratio (OAR).Methods We obtained measurements of basic beam data from a 6 FFF photo radiotherapy linac using the PTW-60016 diode-p detector.The measurements were performed with various scanning parameter sets and were compared with Monte Carlo simulation as reference. The differences in were quantitatively analyzed with respect to the dependence on sampling steps and times.Results The standard deviation in the PDD quality index of each small field (1 cm×1 cm,2 cm×2 cm,3 cm×3 cm,4 cm×4 cm) was within 0.005. The increase of sampling time had little influence on the measurement accuracy of PDD and OAR.The decrease of sampling step size was conducive to improving the measurement accuracy of OAR penumbra area and PDD built-up area,but it was meaningless to improve the measurement accuracy of PDD at large depth.In the meantime,we found that while penumbra measurements were insensitive to sampling time,the penumbra width broadened with increased sampling step.Conclusion PDD quality index was not sensitive to sampling time and step.The OAR penumbra was not sensitive to sampling time.However,penumbra width was positively correlated with sampling step.As a trade-off between measurement accuracy and efficiency,two scanning parameter sets with (0.5 s,1.0 mm) and (0.5 s,0.5 mm) was recommended as optimized protocols using PTW-60016 silicon detector for FFF photo beam basic data measurement.