Aim: The aim of this study is to evaluate the accuracy of preoperative magnetic resonance imaging (MRI) in the prediction of circumferential resection margin (CRM) and to determine whether each different MRI scan plane provides an accurate CRM assessment. Method: Fifty-seven consecutive patients with mid-rectal cancer were enrolled prospectively. The CRM measurement from each MRI plane according to tumor location was compared with CRM measurement on whole-mount sections with the definition of threatened CRM as 2 mm in distance. The difference in performance among the sagittal, axial and oblique MR images was analyzed by using receiver operating characteristic (ROC) curves (Az). Results: For anterior tumors (n = 17), the Az of the sagittal, axial and oblique MR planes were 0.66, 0.83 and 0.79, respectively. For lateral tumors (n = 17), the Az of the sagittal, axial and oblique MR planes were 0.53, 0.66 and 0.78, respectively. For posterior tumors (n = 23), the Az of the sagittal, axial and oblique MR planes were 0.76, 0.82 and 0.97, respectively. Conclusions: MRI provides an accurate prediction of preoperative CRM. There exist differences in diagnostic accuracy according to each different scan plane of MRI and tumor location within the rectum.
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