磁共振成像定位在低位直肠癌术前放疗大体肿瘤靶区勾画中的应用

目的 探讨磁共振成像（MRI）定位在直肠癌术前放疗中的应用价值。方法 选择经肠镜下病理活检和全身分期检查明确诊断为局部晚期低位直肠癌的患者40例，男性22例，女性18例，年龄31~80岁，中位年龄58岁。先后在相同的体位和固定装置下行CT和MRI定位扫描。由两名直肠癌放疗专业的医生分别在两种定位图像上进行大体肿瘤靶区（GTV）勾画。通过放疗计划系统（TPS）计算出CT和MRI定位图像上勾画的直肠原发肿瘤GTV（GTV_CT、GTV_MRI）的长度、体积及GTV下界距肛缘的距离，并比较这两组数据的差异。结果 肛门指诊提示肿瘤下界距肛缘均≤ 5 cm。GTV_CT的平均长度为（5.21±1.65）cm，长于GTV_MRI的平均长度（4.46±1.51）cm（t=5.059，P<0.05）。GTV_CT的平均体积为（55.71±31.57）cm³，大于GTV_MRI的平均体积（44.02 ±25.11）cm³（t=6.977，P<0.05）。通过肛门指诊判断的肿瘤下界距肛缘的距离为（3.72±0.93）cm，长于GTV_CT下界距肛缘的距离（t=-5.503，P<0.05），但与GTV_MRI有较高的一致性（P>0.05）。应用CT-MRI融合定位制定调强放疗计划，40例患者中无3、4级放疗不良反应发生。术后pCR率为32.5%。结论 MRI定位勾画出的GTV范围更小，对于肿瘤的下界的确定也更为准确。MRI定位的应用可能会通过提高靶区勾画的精确性，从而提高放射治疗的疗效，减少不良反应的发生率。

Application of MRI simulation in delineation of gross tumor volume in pre-operative radiotherapy for low rectal carcinoma

Objective To explore the value of MRI simulation in the pre-operative radiotherapy for locally advanced low rectal carcinoma. Methods A total of 40 patients diagnosed with locally advanced low rectal carcinoma by endoscopic biopsy and radiological staging examinations were included in this study. There were 22 male and 18 female with median age 58 years (range 31-80). Patients underwent CT and MRI simulation scanning in the same position and fixing device. GTV under CT images and MRI images were delineated respectively by two experienced radiologists. Primary tumor length, tumor volume and distance of distal tumor from the anal verge were calculated by treatment planning system(TPS). The two groups of data were compared. Results The distance of distal tumor to the anal verge were all no more than 5 cm on digital examination. The mean length of GTV_CT was remarkably longer than that of GTV_MRI(5.21±1.65) cm vs. (4.46±1.51) cm, t=5.059, P<0.05]. The mean volume of GTV_CT was significantly larger than that of GTV_MRI(55.71±31.57) cm³ vs. (44.02 ±25.11) cm³, t=6.977, P<0.05)]. The mean distance of distal tumor to the anal verge was (3.72±0.93) cm, significantly longer than that of lower bounds of GTV_CT to the anal verge, which had a high consistency with GTV_MRI. The IMRT plan was based on CT-MRI fusion images. There were no 3-4 grade adverse effects of radiotherapy. The overall pCR rate was 32.5%. Conclusions MRI simulation could define smaller GTV and more precise lower bounds than CT. With improved accuracy of target volumes contours, the application of MRI simulation may promote the efficacy of radiotherapy and result in a reduction in the incidence of toxicities.