Guidance of magnetic resonance imaging for target area delineation of postoperative presacral recurrence of rectal cancer

Abstract

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) cm(3), which was significantly larger than 43.02 (range 3.42-142.50) cm(3) of GTVT-MR with statistically significant difference (Z=-4.288, P<0.001). The mean volume of GTVN outlined by CT and MRI was (0.43±0.11) cm(3) and (0.40±0.10) cm(3) respectively without statistically significant difference (t=1.550, 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 (t=5.466, P<0.001); the anterior boundary was (3.24±2.51) cm and (2.69±2.48) cm (t=4.685, P<0.001); the anteroposterior diameter was (4.92±2.02) cm and (4.04±1.57) cm (t=6.210, P<0.001); the left boundary was (3.05±1.00) cm and (2.64±0.78) cm (t=2.561, P=0.016); the right boundary was 2.66 (0.00-4.23) cm and 1.82 (-1.10-3.59) cm (Z=-3.950, P<0.001); the transverse diameter was (5.01±1.78) cm and (3.82±1.29) cm (t=4.648, 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. 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.