Predicting Positive Radial Margin on Restaging MRI of Patients with Low Rectal Cancer: Can We Do Better?

Objective  The aim of this study was to characterize the tissue involving the margin and study if this information will affect margin prediction on restaging magnetic resonance imaging (MRI) in low rectal adenocarcinoma (LRC) patients treated with neoadjuvant long-course chemoradiotherapy (LCCRT). Methods  In this retrospective study of nonmetastatic LRC (distal margin <5 cm from the anal verge) treated with LCCRT followed by surgery, a radiologist blinded to outcome reread the restaging MRI and documented if the radial margin was involved by tumor, fibrosis, or mucin reaction using T2 high-resolution (HR) and diffusion-weighted imaging (DWI). The diagnostic performance of tumor-involving margin on restaging MRI was assessed using surgical histopathology as a reference. Interobserver agreement between three independent radiologists was assessed in a subset. Results  We included 133 patients (80 males and 53 females) with a mean (range) age of 44.7 (21-86) years and 82% of them had well or moderately differentiated adenocarcinoma. Baseline MRI showed T3 ( n  = 58) or T4 ( n  = 60) disease in 89% of the patients. The pathological margin was positive in 21% ( n  = 28) cases. In restaging MRI, the circumferential resection margin (CRM) ≤1 mm in 75.1% ( n  = 100) cases and MRI predicted tumor, fibrosis, and mucin reaction at the margin in 60, 34, and 6%, respectively, and histopathology showed tumor cells in 33, 14.7, and 16.6% of them, respectively. LRC with tumor-involving margin and bad response (MR tumor regression grade [mr-TRG] 3-5) on restaging MRI had fourfold increased risk of positive pathological circumferential resection margin (pCRM). There was moderate and fair inter-reader agreement for the tissue type involving the CRM ( κ  = 0.471) and mr-TRG ( κ  = 0.266), p  < 0.05. The use of both distance criteria and tumor-involving margins improved the diagnostic accuracy for margin prediction from 39 to 66% on restaging MRI. Conclusions  Margin prediction on restaging MRI can be improved by characterizing the tissue type involving the margin in low rectal cancer patients. The inter-reader agreement was moderate for determining the tissue type.