两种胶片分析方法比较多叶光栅到位精确度:多中心测量结果的初步研究

【摘 要】 目的:应用两种胶片分析方法分析调强治疗多叶光栅（MLC）到位精确度。 方法:选择4个省共15家医院，其中8家为Varian加速器，MLC型号均为Millenium 120；7家为Elekta加速器，MLC型号为MLCi或MLCi2。胶片放在固体水模体30 cm×30 cm，dmax点处（水下1.5 cm），SAD=100 cm，6 MV照射，250 MU（监督系数）/栅栏野，应用计划系统，在EBT3胶片上形成5条MLC栅栏野，每条栅栏野射野宽度为6 mm，5条栅栏野射野中心位置相对于中间栅栏野射野中心的位置距离分别为-6、-3、0、3、6 cm。将照射后的胶片用Epson Expression 10000XL扫描，应用Film QATM Pro软件得到栅栏野剂量曲线（profile），并用两种归一方法即截断部分光密度值区域后归一和归一到局部位置区的光密度值，从射野位置及中心位置偏差、射野宽度及偏差4个方面分析比较MLC到位精确及多中心测量结果。 结果:两种分析方法比较，5条栅栏野实际射野位置相对于计划射野位置偏差，均测得9家医院位置偏差超过国际原子能机构（IAEA）规定偏差限值±0.5 mm；分析每条栅栏野射野中心位置的偏差，分析结果均符合IAEA规定限值±0.5 mm；分析5条栅栏野宽度，并与计划设定宽度6 mm相比较，偏差均符合IAEA规定不超过±1 mm；分析射野宽度最大最小值偏差及标准差，分析结果均符合IAEA规定偏差不超过±0.75 mm，标准差不超过0.30 mm。 结论:两种胶片分析方法测量MLC叶片到位精确度，结果相近，差别较小，在此实验中两种归一方法均可被用。

Multi-leaf collimator positioning accuracy analyzed with two film analysis methods: a preliminary multi-center measurement

Abstract: Objective To assess a picket fence test using two methods of film analysis for verifying the positioning accuracy of multi-leaf collimator (MLC). Methods Among selected 15 hospitals in 4 provinces, 8 of them applied Varian accelerator using MLC (Millenium 120), and the other 7 adopted Elekta accelerator using MLC (MLCi or MLCi2). The film was placed at dmax (1.5 cm underwater) in a solid water phantom of 30 cm×30 cm, with a source axis distance of 100 cm. The film received an irradiation of 6 MV and 250 MU per MLC strip. Five MLC strips were generated on EBT3 film, and the width of each strip was 6 mm. The centers of 5 MLC strips were located at -6, -3, 0, 3 and 6 cm from the center of the middle MLC strip. The irradiated films were scanned with Epson Expression 10000XL scanner, and the dose curve of irradiation field (profile) was analyzed and calculated by Film QATM Pro software. Two normalization methods, namely truncating at the smallest local min and max values and normalizing to 0 and 1 (TLN) and equalizing all local min and max values to 0 and 1 (ELN), were used to analyze the field location and deviation of the center, field width and deviation for comparing the MLC positioning accuracy and multi-center measured results. Results The deviation between measured field positions of 5 strips and planned strip positions in 9 hospitals exceeded ±0.5 mm which was recommended by International Atomic Energy Agency (IAEA). The deviation of the center of each irradiation field was up to standard which is ±0.5 mm recommended by IAEA. The deviation between the width of 5 fence fields and planned width (6 mm) was less than ±1 mm, conforming to IAEA standard. Moreover, the deviation and standard deviation of maximum-minimum widths met IAEA standards (no more than 0.75 mm and 0.30 mm, respectively). Conclusion The MLC positioning accuracy measured by two normalization methods of film analysis is similar. Both TLN and ELN are reliable in the verification of MLC positioning accuracy.