碳素纤维床CT值对头部肿瘤放疗计划剂量分布的影响

Effects of CT value of carbon fiber couch on dose distribution in radiotherapy plan for intracranial tumor

目的:定量分析碳素纤维床CT值对头部肿瘤放疗计划剂量分布的影响。方法:在Varian Eclipse 13.6计划系统中建立9种不同CT值的碳素纤维床和均匀圆柱水模体模型,并将圆柱水模体放置于碳素纤维床中间,在10 cm×10 cm射野下,采用6 MV X射线机架在0°~180°之间以10°为间隔行等中心照射,计算不同CT值的碳素纤维床的吸收剂量差异系数。选取头部肿瘤患者15例,以Eclipse计划系统提供的默认CT值碳素纤维床为基础,设计放疗计划,并将该计划保存为模板计划。随后将模板计划移植至其余8种不同CT值的碳素纤维床图像中,不进行通量优化,重新计算剂量分布。记录9种不同CT值碳素纤维床计划的D2%、D50%、D98%、CI、HI以及GI。结果:在-700 HU至-300 HU内,随着Panel Surface CT值的增加,吸收剂量差异系数逐渐减小;在-1000 HU至-900 HU内,随着Panel Interior CT值的增加,吸收剂量差异系数逐渐增大。默认CT值的碳素纤维床计划与其他8种碳素纤维床计划的D2%、D50%、D98%,以及GI差异具有统计意义(P<0.05),而HI则无统计学差异(P>0.05)。结论:物理师在设计放疗计划时,应根据实测治疗床的CT值构建碳素纤维床模型。

Objective To quantitatively analyze the effects of the CT value of carbon fiber couch on the dose distribution in radiotherapy plan for intracranial tumor.Methods The carbon fiber couch with 9 difference CT values and an uniform cylindrical water phantom model were established in Varian Eclipse 13.6 treatment planning system(TPS),and the phantom was positioned at the center of the couch.Isocentric irradiation in the field of 10 cm×10 cm was realized by 6 MV X ray and at gantry angles from 0°to 180°,with an interval of 10°,and the absorbed dose difference coefficientδfor the carbon fiber couch with difference CT values was calculated.A total of 15 patients with intracranial tumor were enrolled in the study.The plan which was designed by Eclipse TPS and based on carbon fiber couch with default CT value was taken as template plan.The template plan was directly transplanted to the images of 8 different couches with different CT values and no flux optimization for the recalculation of dose distribution.The D2%,D50%,D98%,conformity index,homogeneity index and gradient index in 9 plans based on different couches were recorded and analyzed.Results In the range of-700 HU to-300 HU,δwas gradually decreased with the increase of Panel Surface CT value,while in the range of-1000 HU to-900 HU,δwas gradually increased with the increase of Panel Interior CT value.There were significant differences in D2%,D50%,D98%and gradient index between the plan based on carbon fiber couch with default CT value and the other 8 plans(P<0.05),but no significant difference was found in homogeneity index(P>0.05).Conclusion During radiotherapy planning,the carbon fiber couch model should be constructed according to the measured CT value of carbon fiber couch.