Background: This study was designed to identify factors related to inaccurate prediction of circumferential resection margin (CRM) and the extent of mesorectal invasion (EMI) in T3 tumors by preoperative magnetic resonance imaging (MRI) in rectal cancer. Methods: A total of 66 patients with rectal cancer were enrolled prospectively in this study. CRM was defined as the distance from the outer tumor margin to the mesorectal fascia, and EMI was defined as the distance from the outer surface of the muscularis propria to the outermost tumor margin. CRM and EMI measurements on MRI were compared with corresponding measurements from whole-mount sections using 1-mm and 5-mm reference values, respectively, as prognostic indicators. The following variables were analyzed for relevance to preoperative staging: tumor distance from the anal verge (lower and middle), tumor location (anterior and posterior), tumor (T) and nodal (N) stage, and the thickness of the perirectal fat (anterior, left, right, and posterior positions). Results: MRI correctly predicted CRM status in 57 of the 66 tumors and EMI status in 51 of the 58 tumors (eight T1 tumors were excluded from EMI analysis). Univariate analysis showed that T3 stage, lymph node involvement, anterior tumor location, and thin perirectal fat thickness at the anterior portion were related to incorrect prediction of CRM. Anterior tumor location and thin perirectal fat thickness at the anterior portion were related to incorrect prediction of EMI. Conclusions: Preoperative chemoradiation can be accurately guided by preoperative MRI staging, but CRM and EMI in anterior rectal tumor should be interpreted with caution.
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