Overstaging of the mesorectal fascia following neoadjuvant therapy and its impact on therapeutic management: a single-center retrospective cohort study of 506 mesorectal fascia positive patients

Background: Accurate assessment of mesorectal fascia (MRF) involvement status after neoadjuvant therapy (NAT) is critical for guiding post-NAT treatment. However, the discordance between magnetic resonance imaging (MRI)-based evaluations and histopathological results may drive overtreatment and complicate organ-preservation strategies. This study aimed to evaluate the association between post-NAT MRF involvement and pathological circumferential resection margin (CRM) positivity. Methods: This retrospective cohort study included treatment-naïve rectal cancer patients with MRI-confirmed MRF involvement between January 2014 and January 2024. All patients underwent MRI restaging after the NAT. The diagnostic performance, including sensitivity and specificity, of MRI-assessed MRF status was assessed to determine its efficacy in predicting pathological CRM positivity. Logistic regression and mixed-effects models were used to quantify the association between MRF status and CRM positivity. Cox regression analysis was used to assess the effect of MRF positivity on survival outcomes. Results: Among 506 enrolled patients, restaging MRI showed persistent MRF involvement in 50.2% (254/506). The CRM-positive rate was 10.2% in the MRF-positive group, compared to 1.6% in the MRF-negative group. Concordance between MRI and pathological assessment was poor (sensitivity: 0.867, specificity: 0.521, Kappa: 0.086). Nevertheless, MRF positivity independently predicted CRM positivity [odds ratio (OR): 6.228, 95% confidence interval (CI): 2.349–21.507, P