二维碳离子放疗计划中摆位误差对靶区剂量的影响

EFFECTS OF POSITIONING ERROR ON DOSE TO TARGET VOLUME IN TWO-DIMENSIONAL CARBON ION RADIOTHERAPY PLANNING

目的探讨二维碳离子放疗计划中摆位误差对靶区剂量的影响。方法基于现有的放疗计划系统ciPlan,分别在水膜体和人体CT影像中建立一个6 cm×6 cm×6 cm大小的规则计划靶区,采用碳离子二维均匀扫描,模拟靶区中心在左-右(L-R)、前-后(A-P)和头-脚(H-F)三个维度中单个方向、2个方向和3个方向分别偏离0.5、1.0、2.0、3.0和5.0 mm时,靶区90%和95%体积所受剂量的变化。结果在水模体当中,靶区中心偏移量≤1 mm时,靶区90%和95%体积所受剂量的变化量很小;当靶区中心偏移量>1 mm时,靶区90%和95%体积所受剂量的变化量存在不同程度的增大,但靶区中心偏移量<2 mm时,靶区90%和95%体积所受剂量的偏差低于5%;当靶区中心偏移量>3 mm时,靶区90%和95%体积所受剂量变化剧烈,偏差甚至达到近50%。在人体CT影像中,靶区中心偏移量≤1 mm时,靶区90%和95%体积所受剂量的变化量很小;当靶区中心偏移量>1 mm时,靶区90%和95%体积所受剂量就会出现比较复杂的变化。另外,不管是在水膜体中还是在人体CT影像中,剂量偏差均具有方向性,且靶区95%体积所受剂量相对于靶区90%体积所受剂量对位置精确度表现出更加明显的相关性。结论在碳离子二维均匀扫描过程中,为了保证靶区的剂量,摆位误差应≤1 mm;靶区95%体积所受剂量相对于靶区90%体积所受剂量对摆位精确度的依赖更强;在非均匀模体CT影像中,位置的偏移对靶区剂量的影响更为复杂。

Objective To investigate the influence of positioning error on the dose to the target volume in two-dimensio-nal(2D)carbon ion radiotherapy planning.Methods Using the existing ciPlan planning system,a regular planning target vo-lume 6 cm×6 cm×6 cm in size was established in a water phantom and human CT images.With 2D uniform carbon ion scanning,we simulated the changes in doses to 90%and 95%of the target volume when the center of the target volume was deviated by 0.5,1.0,2.0,3.0,or 5.0 mm in single,two,or three directions of left-right,anterior-posterior,and head-foot directions.Results In the water phantom,when the offset of the target center was≤1 mm,the changes in doses to 90%and 95%of the target volume were both very small.When the offset was>1 mm,the changes in doses to 90%and 95%of the target volume showed different degrees of increases,but smaller than 5%if the offset was<2 mm.When the deviation was>3 mm,the doses to 90%and 95%of the target volume showed dramatic changes,which could reach nearly 50%.In human CT images,when the center offset was≤1 mm,the changes in doses to 90%and 95%of the target volume were very small.When the center offset was>1 mm,the doses to 90%and 95%of the target volume showed more complex changes.In addition,dose changes showed directio-nality in both the water phantom and human CT images,and the dose to 95%of the target volume was more dependent on the accuracy of position than that to 90%of the target volume.Conclusion In 2D uniform carbon ion scanning,the positioning error should be≤1 mm to ensure the dose to the target volume.The dose to 95%of the target volume is more dependent on positioning accuracy than the dose to 90%of the target volume.The influence of positioning error on the target dose is more complex in the inhomogeneous CT image phantom.