三种阈值下勾画非小细胞肺癌PET图像靶区及影响的研究

摘要

目的 比较不同阈值对18FDG PET-CT图像中非小细胞肺癌靶区勾画及放疗计划可能产生的影响.方法 选择CT图像上原发灶边界清楚的、呼吸动度≤5 mm的非小细胞肺癌8例,注射18FDG后1 h行PET扫描并以CT图像作衰减校正.以CT图像勾画的大体肿瘤体积(GTVCT)为标准,比较PET图像上用3种阈值条件[即肿瘤内最大像素值的42%(42%Imax(total))、本底平均像素值+肿瘤内最大像素值与本底平均像素值的差值的20%(Iback+20%Imax-back(max))和本底平均像素值+肿瘤内每层最大像素值与本底平均像素值的差值的20%(Iback+20%Imax-back(slice))]勾画的GTV(计为GTV42%、GTV20%max和GTV20%slice)与GTVCT差异及对GTVCT覆盖率的差异.以GTVCT、GTV42%、GTV20%max、GTV20%slice三维外放1 cm为计划靶体积,分别计为PTVCT、PTV42%、PTV20%max、PTV20%slice.对不同PTV设计三维适形放疗计划,并均给予靶区剂量66 Gy分33次6.6周完成.比较以不同PTV设计的计划中,PTVCT内接受＜95%处方剂量的体积(VPTV)及肺V20,并推算可能产生的TCP和肺NTCP的差异.结果 GTV42%、GTV20%max、GTV20%slice与GTVCT的中位体积差分别为-54.1%,-21.5%和5.3%,三者对GTVCT的覆盖率中位数分别为45.9%、78.0%和95.3%(F=57.50,P＜0.01).以不同PTV设计放疗计划时,PTV42%的中位VPTV为7.5%,由此可能导致TCP中位下降1%.PTV20%max和PTV20%slice的中位VPTV分别为1.3%和0.0%,其TCP与PTVCT的相似,与PTV42%的不同.三者的肺V20和肺NTCP与PTVCT的相似.结论 层面化阈值条件Iback+20%Imax-back(slice)可能是PET图像用于肺癌靶区勾画的较准确阈值,该阈值不依赖于预先由CT提供的肿瘤体积信息,可望用于伴有肺不张的非小细胞肺癌的靶区勾画.%Objective To evaluate the accuracy, of different threshold segmentation of 18FDG PET for target volume delineation of non-small cell lung cancer(NSCLC) and the potential influence on radiotherapy treatment planning. Methods Eight NSCLC patients who had tumor with clear margin on CT scan and the amplitude of tumor movements not more than 5 mm were enrolled. PET scans were carried out at 1 h after intravenous injection of 18FDG with CT image for attenuation revisement. Gross target volume (GTV) delineated on CT image ( GTVCT ) was used as the standard. Then, GTVs were delineated on PET image with three different threshold segmentation of 42% Imax(total) (42% of maximum voxel intensity within the tumor) ,Iback + 20% Imax-back(max) (mean background intensity + 20% of normalized background-subtracted maximum voxel intensity within the tumor) and Iback -20% Imax-back(slice) (mean background intensity + 20% of normalized background-subtracted maximum voxel intensity of each slice within the tumor) ,the corresponding GTV was named as GTV42%, GTV20%max and GTV20%slice. Both the size of GTV42%, GTV20%max, GTV20%slice and GTVCT,and the coverage over GTVCT for each GTV were compared. A three dimensional margin of 1 cm were added to GTVCT, GTV42%, GTV20%max and GTV20%slice to form corresponding PTVCT, PTV42%, PTV20%max and PTV20%slic e. Three dimensional conformal radiotherapy treatment plans were designed based on PTVCT,PTV42% , PTV20%max and PTV20%slice respectively for each patient. The prescription dose of all PTVs was 66 Gy in 33 fractions in 6.6 weeks. Both the volume accepting dose less than 95% of prescription dose within PTVCT ( VPTV ) and the lung V20 were compared among the four plans based on different PTVs. Tumor control probability(TCP) as well as lung normal tissue complication probability (NTCP) were also compared. Resuits Eight patients were enrolled in this study. Median deviation of volume between GTVPET and GTVCT were -54.1% , -21.5 % and 5.3 % for GTV42% , GTV20%max and GTV20%slice, respectively. Median coverage over GTVCT of GTV42% , GTV20%max and GTV20%slice was 45.9% ,78.0% and 95.3% respectively( F = 57.50,P＜0.01). Median 7.5% of VPTV was observed for radiotherapy treatment plan based on PTV42% ,which meant that it might induce median 1% decrease of TCP comparing with that of radiotherapy treatment plan based on PTVCT. Whereas,there were only 1.3% and 0.0% of VPTV for treatment plans based on PTV20%max and PTV20%slice respectively. As far as TCP was concemed, both PTV20%max group and PTV20%slice group were superior to PTV42% group,there was no significant difference among PTV20%max group, PTV20%slice group and PTVCT group. Lung V20 and lung NTCP showed no significant difference among all groups. Conclusions The threshold segmentation of Iback + 20% Imax-back(slice) , being slice specialized, might be an optimal threshold segmentation for target volume delineation of lung caner. Independent of information of target volume provided by CT scan in advance,it is recommended to use for the target volume delineation of NSCLC with atelectasis.