CT-电子密度转换曲线误差对IMRT剂量计算结果的影响

Effect of the error of CT-electron density conversion curve on IMRT dose calculation

目的:研究CT-相对电子密度转换曲线误差对调强放射治疗(IMRT)计划剂量计算结果的影响。方法:随机选取南方医科大学顺德医院IMRT治疗的宫颈癌患者10例,在Eclipse治疗计划系统中对IMRT计划引入CT-电子密度转换曲线误差(±0.5%、±1.0%、±1.5%、±2.0%和±3.0%),重新计算剂量分布,每例患者得到10个带有误差的新计划并与原计划进行比较。分析转换曲线误差对剂量计算结果的影响,包括靶区相关剂量参数、适形度指数(Conformal Index,CI)、均匀性指数(Homogeneity Index,HI)和脊髓、肾脏、小肠、膀胱、直肠等危及器官体积剂量参数。分析不同转换曲线误差和各剂量参数偏差值之间的关系。结果:转换曲线引入正误差时计划剂量参数降低,引入负误差时计划剂量参数升高,引入的误差越大剂量变化越大。当转换曲线误差为1.5%时,靶区平均覆盖率为94.73%±1.86%,误差继续增大,带来的影响超出临床可接受范围。CI和HI在不同误差的计划之间没有统计学意义(P>0.05)。不同转换曲线误差和各剂量参数偏差值之间存在显著性负相关性(P均<0.001),并利用Matlab得到不同转换曲线误差和各剂量参数偏差值之间的相关公式。结论:当转换曲线误差大于1.5%时,剂量偏差无法满足临床要求。计划系统建模时需建立正确的CT-相对电子密度转换曲线,对CT模拟机要定期QA,以保证治疗计划剂量计算的精度。

Objective To investigate the effect of the error of CT-electron density conversion curve on the calculation of dose distribution of intensity-modulated radiotherapy(IMRT)plan.Methods Ten patients undergoing IMRT for cervical cancer in Shunde Hospital of Southern Medical University were enrolled.Different CT-electron density conversion curve errors(±0.5%,±1.0%,±1.5%,±2.0%and±3.0%)were introduced into IMRT plans in Eclipse treatment planning system for recalculating dose distribution and obtaining 10 new plans for every patient which were then compared with the original plan without conversion curve errors.The effect of conversion curve errors on the dose calculation was discussed by analyzing the related dosimetric parameters,conformity index,homogeneity index of target areas,and the volume-dose parameters of organs-at-risk such as spinal cord,kidney,small intestine,bladder,rectum and so on.The relationships between different conversion curve errors and the deviation value of each dosimetric parameter were analyzed.Results The dosimetric parameters were decreased when positive errors were introduced into IMRT plan while increased when negative errors were introduced.The greater the conversion curve error was,the greater the effects on dosimetric parameters were.When the conversion curve error was 1.5%,the average target coverage was 94.73%+1.86%;and the dosimetric effect coming from larger conversion curve errors(higher than 1.5%)was out of the acceptable range in clinic.No statistical difference was found among plans with different conversion curve errors in conformity index and homogeneity index(P>0.05).There were significant negative correlations between conversion curve errors and the deviation values of different dosimetric parameters(all P<0.001),and the corresponding correlation formulas were obtained by Matlab software.Conclusion With conversion curve error higher than 1.5%,the dosimetric deviation is out of the acceptable range and the dose distribution cannot meet the clinical requirements.An accurate CT-electron density conversion curve should be created when building treatment model by planning system,and the quality assurance of CT-sim should be performed regularly,thereby ensuring the accurate calculation of dose calculation of treatment plan.