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Low rectal cancers (<5 cm from the anal verge), compared with all others, have greater positive resection margin rates, attributed to mesorectal tapering and higher perforation risk. The aim of this study was to assess positive resection margin prediction by using magnetic resonance imaging staging.The following features were analyzed by using preoperative magnetic resonance imaging from 101 consecutive patients with low rectal tumors: tumor location (posterior/anterior) and magnetic resonance stage (Stage 1-2, tumor within the intersphincteric plane; Stage 3-4 tumor extending into the intersphincteric plane). Magnetic resonance imaging tumor regression grade was measured where posttreatment magnetic resonance imaging was available and compared with histopathologic findings.Seventy of 101 patients had abdominoperineal excisions, and 31 of 101 had low anterior resections. Using logistic regression, positive resection margin odds were higher for magnetic resonance Stages 3 to 4 than Stages 1 to 2 by a factor of 17.7 (P < 0.001), and positive resection margin odds were higher by a factor of 2.8 for anterior vs. posterior tumors (P = 0.026). Magnetic resonance imaging tumor regression grade strongly predicted for positive resection margins; 11 of 15 patients with little treatment response had positive resection margins, compared with 2 of 15 with >50 percent complete treatment response on magnetic resonance imaging (P < 0.001).Significant magnetic resonance imaging positive resection margin predictors are tumor into or beyond the intersphincteric plane and magnetic resonance imaging tumor regression grade.