碳素纤维床对宫颈癌容积弧形旋转调强放射治疗计划的剂量影响

Dosimetirc effects of carbon fiber couch on volumetric modulated arc therapy of cervical cancer

目的:研究碳素纤维床对宫颈癌容积弧形旋转调强放射治疗(VMAT)计划的剂量影响及其修正方法。方法:使用CT电子密度模体校准大孔径CT的CT值,在Eclipse 10.0计划系统中建立相对电子密度-CT值曲线。将医用电子直线加速器上的碳素纤维床在CT下进行扫描,图像传输至Eclipse计划系统并测量碳素纤维床的CT值。以此CT值为基础在计划系统中建立碳素纤维床的模型,测量碳素纤维床在计划系统中和实际情况下对剂量的衰减系数并进行比较,两者测量条件保持一致。选取宫颈癌ⅠA、ⅡB期共5例患者,使用Eclipse计划系统设计无碳素纤维床计划。之后在患者定位图像上分别建立Thin、Medium和Thick这3种厚度的碳素纤维床模型,将无碳素纤维床治疗计划直接移植到3种不同厚度碳素纤维床图像上,治疗中心不变,机器条数不变,进行剂量计算。最终比较有和无碳素纤维床之间、3种不同厚度碳素纤维床之间计划靶区(PTV)与危及器官的剂量差异。结果:Thin和Thick厚度碳素纤维床对实际测量和计划系统计算的剂量衰减系数相差均不超过±1%;不论是PTV还是危及器官,无碳素纤维床与3种不同厚度碳素纤维床的剂量参数结果比较均具有统计学意义(P<0.05);不同厚度碳素纤维床对PTV和危及器官剂量评价的准确性有一定影响。结论:加速器碳素纤维床对宫颈癌VMAT计划的剂量分布有一定影响;在进行VMAT治疗时,应准确建立碳素纤维床的模型参与剂量运算;并且根据靶区与碳素纤维床之间的位置关系选择添加相应厚度的碳素纤维床模型。

Objective To study the dosimetric effects of carbon fiber couch on the volumetric modulated arc therapy(VMAT)of cervical cancer and it's correction methods.Methods Computed tomography(CT)electron density phantom was used to calibrate the CT value of large aperture CT,and the relative electron density-CT value curve was established with Eclipse 10.0 treatment planning system(TPS).After the carbon fiber couch on the medical electron linear accelerator was scanned with CT,the obtained CT image was transmitted to the Eclipse TPS and the CT value of the carbon fiber couch was measured.Base on the CT value,a carbon fiber couch model was established in the TPS.Under the same measurement conditions,the attenuation coefficients of the carbon fiber couch in the TPS and in the actual condition were measured and analysis.A total of 5 patients with stage IA or IIB cervical cancer were selected,and plans with no carbon fiber couch were designed using Eclipse TPS.Then,3 models of carbon fiber couch of different thicknesses(Thin,Med and Thick)were established on the positioning image,and the plan with no couch was directly transplanted to the images of 3 different couches.Dose calculation was performed based on that the treatment center and machine unit remained unchanged.Finally,the dosimetric differences in planning target volume(PTV)and organs-at-risk(OAR)were compared between plan with and without carbon fiber couch and among plans with 3 different couches.Results For Thin and Thick carbon fiber couches,the deviation of attenuation coefficient between the actual measurement and TPS calculated results was not more than±1%.There were statistical differences in the dose indexes of PTV and OAR between plans with no couch and those with couch(P<0.05).However,the thickness of carbon fiber couch has a certain effect on the accuracy of PTV and OAR dose evaluations.Conclusion Carbon fiber couch affects the dose distribution of VMAT plan for cervical cancer.Therefore,an appropriate model of carbon fiber couch should be established and participated in the dose calculation when VMAT is carried out.Moreover,the thickness of carbon fiber couch model should be determined according to the position relationship between PTV and carbon fiber couch.