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目的:直肠癌骶前复发患者由于既往直肠癌根治手术，不同程度地改变了原有盆腔器官毗邻结构，复发肿瘤组织与盆腔内结构分界欠清，常规CT检查由于其软组织分辨率较差，对于实施放疗前准确的靶区勾画存在一定困难。故本研究探索磁共振成像（MRI）在直肠癌根治术后骶前复发病灶靶区勾画的指导作用。方法:采用描述性病例系列研究方法。收集2014年5月至2019年5月期间，于北京大学人民医院收治、经病理证实、或经多学科团队（MDT）讨论、MRI和CT及病例信息完整的直肠癌根治术后骶前复发30例患者的临床资料。根据CT和MRI图像中勾画出的患者骶前复发的肿瘤区域（GTV），包括骶前复发病灶（GTVT）和转移淋巴结（GTVN），计算出GTV体积，测量肿瘤边界及径线等指标，比较在MRI和CT两种影像方法中的差异。结果:30例患者在CT中勾画测量所得GTV体积均大于MRI，其中GTVT-CT中位体积为67.86（范围5.12~234.10）cmn 3，明显大于GTVT-MR的43.02（范围3.42~142.50）cmn 3，差异有统计学意义（n Z=-4.288，n P<0.001）。CT与MRI勾画的GTVN值分别为（0.43±0.11）cmn 3和（0.40±0.10）cmn 3，差异无统计学意义（n t=1.550，n P=0.132）。在CT图像中勾画的骶前复发病灶的各边界及径线的均值均较MRI图像中所勾画的边界大，其中GTVT的上下径CT和MRI分别为（6.66±2.92）cm比（5.17±2.40）cm（n t=5.466，n P<0.001），前界分别为（3.24±2.51）cm比（2.69±2.48）cm（n t=4.685，n P<0.001），前后径分别为（4.92±2.02）cm比（4.04±1.57）cm（n t=6.210，n P<0.001），左界分别为（3.05±1.00）cm比（2.64±0.78）cm（n t=2.561，n P=0.016），右界分别为2.66（0.00~4.23）cm比1.82（-1.10~3.59）cm（n Z=-3.950，n P<0.001），横径分别为（5.01±1.78）cm比（3.82±1.29）cm（n t=4.648，n P<0.001），差异均具有统计学意义。在前向脏器受累如肠管、前列腺、膀胱以及后向骶骨受累范围的判断方面，MRI显示优于CT。15例患者根据MRI所指导的靶区制定放疗计划并行放射治疗，10例达临床症状缓解。n 结论:MRI图像中所勾画的直肠癌术后骶前复发病灶的GTV较CT小，MRI对肿瘤与周围正常组织边界的判定较CT显示更清晰，因而能够更加精确地显示肿瘤的侵及范围，从而指导放疗的准确实施。“,”Objective:Presacral recurrence of rectal cancer have altered the adjacent structures of original pelvic organs due to the previous radical surgery of rectal cancer, and the boundary between recurrent tumor tissues and pelvic internal structures is not clear. Conventional CT examination has poor soft tissue resolution, which makes it difficult to accurately delineate the target area of radiotherapy. This study aimed to explore the guiding role of magnetic resonance imaging (MRI) in delineating the target area of presacral recurrence after radical resection of rectal cancer.Methods:A descriptive case series research method was adopted. From May 2014 to May 2019, the clinical data of 30 patients with presacral recurrence after radical resection of rectal cancer were collected, who were admitted to Peking University People's Hospital, confirmed by pathology or discussed by multidisciplinary team (MDT), with complete MRI, CT and case information. According to the gross tumor volume (GTV) with presacral recurrence outlined in CT and MRI images, including presacral recurrent lesions (GTVT) and metastatic lymph nodes (GTVN), the GTV volume was calculated, and the tumor boundary and diameter were measured. The differences between MRI and CT were compared.Results:The volume of GTVT-CT was larger than that of GTVT-MR in all the 30 patients. The median volume of GTVT-CT was 67.86 (range 5.12-234.10) cmn 3, which was significantly larger than 43.02 (range 3.42-142.50) cmn 3 of GTVT-MR with statistically significant difference (n Z=-4.288, n P<0.001). The mean volume of GTVN outlined by CT and MRI was (0.43±0.11) cmn 3 and (0.40±0.10) cmn 3 respectively without statistically significant difference (n t=1.550, n P=0.132). The mean values of boundary and radial line of the presacral lesions on CT images were all longer than those on MRI images. The vertical diameter of GTVT on CT and MRI images was (6.66±2.92) cm and (5.17±2.40) cm (n t=5.466, n P<0.001); the anterior boundary was (3.24±2.51) cm and (2.69±2.48) cm (n t=4.685, n P<0.001); the anteroposterior diameter was (4.92±2.02) cm and (4.04±1.57) cm (n t=6.210, n P<0.001); the left boundary was (3.05±1.00) cm and (2.64±0.78) cm (n t=2.561, n P=0.016); the right boundary was 2.66 (0.00-4.23) cm and 1.82 (-1.10-3.59) cm (n Z=-3.950, n P<0.001); the transverse diameter was (5.01±1.78) cm and (3.82±1.29) cm (n t=4.648, n P<0.001), respectively, whose differences were all statistically significant. MRI was superior to CT in judging the involvement of anterior organs, such as intestine, prostate, bladder and the posterior sacrum. Fifteen patients received radiotherapy according to the target area guided by MRI and 10 patients obtained clinical symptom relief.n Conclusion:Compared with CT, the GTV of postoperative presacral recurrence of rectal cancer outlined in MRI images is smaller, and MRI can determine the boundary between tumor and surrounding normal tissues more precisely, so it can show the invasion range of tumor more accurately and guide the accurate implementation of radiotherapy.