不同三维探测器对鼻咽癌VMAT计划MLC位置误差探测灵敏度的研究

Study on the sensitivity of different 3D detectors to MLC positioning errors in volumetric modulated arc therapy (VMAT) plan for nasopharyngeal carcinoma

目的研究鼻咽癌容积旋转调强(VMAT)计划剂量验证中,Delta^4和ArcCHECK两种三维探测器对多叶准直器(MLC)叶片位置误差检测的灵敏度。方法选取10例鼻咽癌VMAT计划,对原始文件中每个MLC子野的叶片分别引入0.5~4.0mm的位置误差,使子野整体扩大、缩小或偏向一侧平移,模拟VMAT治疗中MLC可能出现的位置误差。分别用Delta^4和ArcCHECK进行验证测量,比较计划系统计算值与测量结果的γ通过率并行配对t检验。结果当评价标准取3mm/3%时,两种探测器所有患者的原计划验证绝对剂量γ通过率均>95%,Delta^4和ArcCHECK可以检测出的MLC外扩、内收以及平移误差分别是1.5、1.0、2.0mm和3.0、1.0、3.0mm;而取2mm/2%评价标准时,患者原计划验证绝对剂量γ通过率有较大幅度下降,此时Delta^4和ArcCHECK能检出的MLC外扩、内收和平移误差分别是1.0、1.0、2.0mm和1.5、0.5、2.0mm。结论Delta4和ArcCHECK鼻咽癌VMAT计划的剂量验证可以检测出不同类型和大小的MLC位置误差,但两者的检测灵敏性略有差异,而对<1.0mm微小误差的检测都不够敏感,日常工作中仍需加强MLC的质量保证。

Objective To compare the sensitivity of Delta^4 and ArcCHECK 3D detectors to detect the multi-leaf collimator (MLC) positioning errors in the dose verification of volumetric modulated arc therapy (VMAT) plans for nasopharyngeal carcinoma (NPC). Methods Ten NPC patients receiving VMAT plans were selected. The positioning error of 0.5-4.0 mm was introduced into the leaves of each MLC segment in the original file to expand, contract or shift the whole segments. The possible positioning errors of MLC in the treatment of VMAT were simulated. The Delta^4 and ArcCHECK were utilized to verify the measurements. The absolute gamma passing rate was compared between the calculated dose and the measured dose of the VMAT plan by using the paired t-test. Results When the evaluation criterion was taken as 3 mm/3%, the absolute passing rate verified by the original plans of two detectors was greater than 95%. The positioning errors of MLC expansion, contraction and shifting detected by Delta^4 and ArcCHECK were 1.5 mm, 1.0 mm, 2.0 mm and 3.0 mm, 1.0 mm, and 3.0 mm, respectively. When taking 2 mm/2% as the evaluation criterion, the absolute passing rate verified by the original plan was decreased significantly. Delta^4 and ArcCHECK detected that the positioning errors of MLC expansion, contraction and shifting were 1.0 mm, 1.0 mm, 2.0 mm and 1.5 mm, 0.5 mm, and 2.0 mm, respectively. Conclusions The dose verification of the VMAT plan for NPC by Delta^4 and ArcCHECK can detect different types and sizes of MLC positioning errors, whereas the detection sensitivity slightly differs between Delta^4 and ArcCHECK. Both of them are not sensitive to detect the MLC positioning errors less than 1.0 mm. It is fairly necessary to strengthen the quality assurance of MLC in the daily work.