不同CT值赋值法对脑转移瘤放疗剂量计算影响的研究

Study on the effects of different CT values assignment methods on dose calculation of brain metastases radiotherapy

目的研究不同CT值赋值法对脑转移瘤放疗计划剂量计算的影响,为基于磁共振(MR)图像进行放疗计划设计提供基础。方法选取35例接受放疗的脑转移瘤患者,每位患者在放疗前同一天分别进行CT和MR模拟定位,基于CT图像制定三维适形放射治疗(3D-CRT)或调强放射治疗(IMRT)计划为原计划Plan1。将CT图像和MR图像刚性配准,在CT和MR图像上勾画主要的组织和器官,计算各组织器官的群体化CT值。基于CT图像,采用3种CT值赋值法生成3组伪CT,分别为:全组织赋予140 HU;空腔、骨骼和软组织分别赋予-700、700和20 HU;不同组织器官分别赋予群体化的CT值。Plan1在3组伪CT上重新计算剂量分布,分别获得Plan2、Plan3、Plan4,然后比较这3组计划和Plan1的剂量学差异。结果骨骼、空腔平均CT值分别为(735.3±68.0)、(-723.9±27.0)HU,软组织的平均CT值基本分布在-70~70 HU。Plan2、Plan3、Plan4相比Plan1的剂量差异依次减小,在剂量指标比较中,眼晶状体最大剂量差异最大,分别可达5.0%以上、1.5%~2.0%、1.0%~1.5%,其余剂量指标差异的95%置信区间上限基本不超过2.0%、1.2%、0.8%。在像素点剂量比较中,局部靶区病例中差异>1%的区域主要分布在靠近射野的皮肤处,而全脑靶区病例中主要分布在骨骼与空腔、软组织交界处,以及靠近射野的皮肤处。此外,CT值赋值法在3D-CRT的剂量学差异大于IMRT,在全脑靶区病例大于局部靶区病例。结论不同CT值赋值法对脑转移瘤放疗计划剂量计算的影响显著,对骨骼、空腔和软组织赋予合适CT值,剂量计算偏差可基本控制于1.2%以内,而对各组织器官赋予群体化的CT值,可进一步将偏差控制于0.8%以内,满足临床要求。

Objective To study the effects of different CT values assignment methods on the dose calculation of radiotherapy plan for brain metastases,which will provide a reference for radiotherapy treatment planning based on MR images.Methods A total of 35 patients treated with radiotherapy for brain metastases were selected,with pre-treatment CT and MR simulated positioning performed at the same day.Based on the simulation CT images,three dimensional conformal radiation therapy(3D-CRT)or intensity modulated radiation therapy(IMRT)plans were calculated as the original plan(Plan1).The CT and MR images were rigidly registered and then the main tissues and organs were delineated on CT and MR images.The average CT values of each tissue and organ were calculated.Three groups of pseudo CT were generated by three CT values assignment methods based on the CT images:whole tissue was assigned 140 HU;cavity,bone and other tissues were assigned-700 HU,700 HU and 20 HU,respectively;different tissues and organs were assigned corresponding CT values.The dose distribution of Plan1 was recalculated on three groups of pseudo-CT to obtain Plan2,Plan3 and Plan4,respectively.Finally,the dosimetric difference between Plan1 and other plans(including Plan2,Plan3 and Plan4)were compared.Results The average CT values of bone and cavity were(735.3±68.0)HU and(-723.9±27.0)HU,respectively.The average CT values of soft tissues was mostly distributed from-70 to 70 HU.The dosimetric differences between Plan2,Plan3,Plan4,and Plan1 decreased in turn.The differences of maximum dose of lens were the biggest,which can reach more than 5.0%,1.5%-2.0%and 1.0%-1.5%,respectively,and the differences of other dose parameters were basically less than 2.0%,1.2%and 0.8%,respectively.In the pixelwise dosimetric comparison,the areas with more than 1%difference in the local target cases were mainly distributed in the skin near the field.On the other hand,those in the whole brain target cases were mainly distributed at the bone,cavity,bone and soft tissues junction,and the skin near the field.In addition,the dose calculation error of CT value assignment methods in 3D-CRT plan was slightly larger than that in IMRT plan,and that in whole brain target cases were significantly larger than that in local target cases.Conclusions Different CT value assignment methods have a significant effect on the dose calculation of radiotherapy for brain metastases.When appropriate CT values are given to bone,air cavity and soft tissue,respectively,the deviation of dose calculation can be basically controlled within 1.2%.And by assigning mass CT values to various tissues and organs,the deviation can be further controlled within 0.8%,which can meet the clinical requirements.