等能量层重扫描技术在肺癌质子调强治疗中对相互影响效应的改善

Mitigation of interplay effects with layer repainting techniques in intensity-modulated proton therapy for early-stage non-small cell lung cancer

目的评估肺癌质子调强治疗(IMPT)中肿瘤运动所致的相互影响(Interplay)效应,并比较不同次数的等能量层重扫描技术对Interplay效应的缓解效果。方法选取8例肺癌患者进行回顾性分析。所有患者行4DCT扫描,生成10个时相。基于商业化质子治疗计划系统,采用稳健性优化算法,在4DCT图像的参考时相设计临床靶区(CTV)概念的IMPT计划。稳健性优化过程中不确定性因素包括:摆位不确定性±5 mm,射程不确定性为±3.5%,CTV在4DCT中各时相变化不确定性。采用蒙特卡洛算法计算IMPT计划的最终剂量分布。评估Interplay效应时,基于混合形变算法及质子治疗模拟程序,计算患者4D静态剂量和4D动态剂量分布在感兴趣器官(ROI)的DVH差异,定义Interplay效应的评价指标[ΔI(ROI,DVH)],利用该指标定量评估IMPT计划的Interplay效应;采用等能量层重扫描技术对IMPT计划分别进行3、4、5、6、7次重扫描,并利用ΔI(ROI,DVH)标评估不同次重扫描对Interplay的缓解效果。结果由Interplay效应导致IMPT计划中靶区CTV的D95%、CI、HI的均值分别降低了13.7%、12.7%、24.6%,危及器官中双肺V5Gy、V20Gy、V30Gy分别提高了0.8%、3.4%、2.6%。与未使用重扫描技术相比,不同次数重扫描技术使得CTV靶区覆盖D95%的均值分别提高了4.5%、3.8%、3.8%、3.6%、5.7%。危及器官中双肺平均V20Gy分别减小了1.5%、1.8%、1.7%、1.6%、1.9%。结论Interplay效应导致IMPT计划剂量分布变差,等能量层重扫描技术的应用可以改善靶区剂量分布,建议结合患者自身特征选择最佳重扫描次数。

Objective The purpose of current study was to evaluate the interplay effects in intensity-modulated proton therapy(IMPT)for lung cancer and compare the results of different Iso-energy layer repainting techniques in the mitigation of interplay effects.Methods Eight patients with lung cancer who underwent 4DCT were retrospectively selected.A robust CTV-based IMPT plan was generated for each based on commercial TPS,considering patient setup errors±5 mm,range uncertainties±3.5%,and CTV time structure motion in 4DCT image.Monte Carlo dose engines were used for all IMPT plans in the final dose calculation.The 4D static dose(4DSD)and 4D dynamic dose(4DDD)were calculated using a hybrid deformable algorithm and simulated proton delivery system for interplay effects.An index[ΔI(ROI,DVH)]was developed to quantitatively evaluate the interplay effects.We applied Iso-energy layer repainting techniques with different numbers of repainting(3,4,5,6,7)to the robust IMPT plans and evaluated the difference in the mitigation of interplay effects based on theΔI(ROI,DVH)index.Results Due to interplay effects,the mean values of target coverage,conformity and homogeneity index reduced by 13.7%,12.7%and 24.6%,respectively.The mean values of lung V5Gy and V20Gy improved by 0.8%,3.4%and 2.6%.Compared to the IMPT plans without layer repainting,Multiple iso-energy layers repainting techniques improved the mean values of CTV coverage by 4.5%,3.8%,3.8%,3.6%and 5.7%,respectively.The average values of lung V20Gy reduced by 1.5%,1.8%,1.7%,1.6%and 1.9%,respectively.Conclusions In the robust CTV-based IMPT plans,the interplay effects degraded the target dose distribution but were mitigated using iso-energy layer repainting techniques.We recommended to use the layer repainting technique according to the characteristics of the patient.