用锥形束CT图像引导技术分析肺癌放疗摆位误差及其对剂量分布影响

目的 研究千伏级锥形束CT (CBCT)引导下肺癌放疗摆位误差及其对靶区和正常组织受量影响。方法 采用瓦里安IX直线加速器机载影像系统对 23例肺癌患者三维放疗前行CBCT扫描,并与计划CT图像匹配获得左右、头脚、前后方向摆位误差。在CMS计划系统中将治疗中心移至扫描中心,模拟未移床时剂量分布,研究摆位误差对计划靶体积(PTV)、大体肿瘤体积(GTV)及正常组织受量影响,并行F检验。结果 左右、头脚、前后方向误差分别为 －0.18、－0.43、－2.64 mm,其绝对值最大分别为13、 21、8 mm。5 mm内误差在3个方向上分别占93.9%、71.4%、81.9%。模拟未移床时的剂量分布显示95%PTV剂量、PTV平均剂量、95%GTV剂量、GTV平均剂量分别为原计划的89.7%(F=7.04,P=0.011)、97.9%、95.5%、98.8%(F=0.32,1.68,0.11;P=0.572,0.201,0.740)。肺接受≥5、10、20、30 Gy体积占总体积百分比、肺平均受量、脊髓1 cm³体积受量分别为原计划的102.7%、103.1%、105.0%、110.6%、103.0%、98.1%。任一方向误差>5 mm对95%PTV受照剂量的影响有统计学意义(F=14.58,P=0.001)。结论 肺癌放疗摆位误差大多在5 mm内,以头脚方向误差最大,任一方向误差>5 mm将对PTV受量产生明显影响。

Analysis of setup errors and its impact on dosimetric distribution for thoracic carcinoma radiotherapy with cone-beam CT-based image guidance

Objective To study the role of kilovolt cone beam CT (CBCT) on the geometrical accuracy of three-dimensional conformal radiotherapy (3DCRT) and intensity modulated radiotherapy (IMRT) and to analyze the effect of the setup error on the dosimetric distribution of targets and peripheral organs at risk for thoracic carcinoma. Methods 23 patients with thoracic carcinoma were collected. Varian-IX linear accelerator on-board imaging system was used to acquire CBCT scans in 3DCRT or IMRT before delivery. The right-left (x), superior-inferior (y), anterior-posterior (z) setup errors can be obtained from the images automatically restructured by the system. And then CMS treatment planning system was used to analyze the impact of setup errors on dosimetric distribution of targets and peripheral organs at risk if the isocenter was replaced by the actual location which obtained from the CBCT scan. Results The setup error were －0.181 mm, －0.429 mm, －2.637 mm respectively and the absolute maximum was 13 mm, 21 mm, 8 mm respectively according to 182 CBCT scans on x, y, z directions. Within 5 mm of which accounted for 93.9%, 71.4%, 81.9% respectively. Target dose reduced with respect to treatment plan, mean value were 89.7%(F=7.04,P=0.011), 97.9%, 95.5% and 98.8%(F=0.32,1.68,0.11;P=0.572,0.201,0.740) for PTV D95%, PTV D_mean, GTV D95% and GTV D_mean. Mean value of the difference in normal organ dose were 102.7%, 103.1%, 105.0%, 110.6%, 103.0%, 98.1% for the volume of lung accept 5 Gy, 10 Gy, 20 Gy, 30 Gy, the mean dose of lung, the maximum dose of 1 cm³ spinal cord. The difference of PTV D95% was significant if setup errors more than 5 mm in any direction (F=14.58,P=0.001). Conclusions Setup errors of this group mostly within 5 mm. It was more obvious in the y directions. There was significantly difference if setup errors more than 5 mm in any direction.