Magnetic Resonance Imaging Features as Predictors of Response and Prognosis Following Neoadjuvant Therapy in Rectal Cancer: A Retrospective Cohort Study.

Background: MRI-detected circumferential resection margin (CRM) involvement, extramural venous invasion (EMVI), and tumor deposits (TDs) are established high-risk features associated with poor prognosis in rectal cancer. These features may help identify patients who are more likely to benefit from neoadjuvant therapy (NAT). However, the prognostic significance of changes in these MRI high-risk features after NAT remains unclear. The objective of this study was to determine whether baseline MRI high-risk features can identify patients likely to benefit from NAT and whether their changes after NAT predict treatment response and prognosis in rectal cancer. Methods: This retrospective single-center cohort study included patients who underwent curative resection for rectal adenocarcinoma and received neoadjuvant therapy (NAT) at Sun Yat-sen University, Sixth Affiliated Hospital, between 2017 and 2019. Patients with MRI-detected CRM involvement, EMVI, or TDs were classified as MRI high-risk. Those who showed a shift from MRI high-risk to MRI low-risk-defined by the absence of CRM involvement, EMVI, or TDs-on post-treatment scans were considered good responders. The survival outcomes of good responders were then compared to those of poor responders, who remained persistently MRI high-risk. Additional analyses were performed within the baseline MRI high-risk subgroup to determine which MRI high-risk features were most strongly associated with prognosis. Results: The study included 302 patients in total. Of these, 146 (48.3%) were classified as MRI high-risk and 156 (51.7%) as MRI low-risk based on pre-treatment imaging. The high-risk group had significantly worse outcomes: three-year disease-free survival (DFS) was 54.8% compared with 93.6% in the low-risk group (p < 0.001), three-year overall survival (OS) was 76% versus 98.1% (p < 0.001), and the rate of local recurrence (LR) at three years was 10.9% compared to 1.3% (p = 0.002). Among the 146 patients initially identified as MRI high-risk, those who converted to low-risk status after treatment showed improved outcomes, with a three-year DFS of 82.9%, OS of 94.3%, and LR rate of 2.9%. Within the baseline MRI high-risk subgroup, baseline mrEMVI and mrTD, as well as post-treatment ymrMRF, ymrEMVI, and ymrTD, were associated with worse DFS. Conclusions: Baseline MRI high-risk features and their persistence after NAT were associated with poor prognosis in rectal cancer. MRI risk conversion after NAT may help identify a subgroup of initially high-risk patients with more favorable outcomes.