MRI-Based Structured Assessment of Treatment Response in Rectal Cancer After Total Neoadjuvant Therapy: Correlation With Surgical and Pathological Outcomes

Background This study aimed to evaluate the utility of structured post-treatment magnetic resonance imaging (MRI) assessment for response evaluation in rectal cancer patients treated with total neoadjuvant therapy, with emphasis on surgical planning, histopathological correlation, and cautious multidisciplinary assessment of potential organ-preservation eligibility. Methodology This retrospective, observational cohort study included 100 patients with biopsy-proven rectal cancer who underwent baseline and post-treatment rectal MRI after total neoadjuvant therapy. Post-treatment imaging was reviewed using a structured response assessment template incorporating T2-weighted tumor-bed morphology, magnetic resonance tumor regression grade, diffusion restriction, mesorectal fascia status, extramural vascular invasion, nodal response, sphincter complex involvement, and pelvic sidewall disease. Imaging-based response categories were compared with operative and histopathological outcomes where available. Results Structured MRI assessment was performed in all 100 patients. Complete or near-complete imaging response was identified in 32 (32.0%) patients, incomplete response in 58 (58.0%) patients, and indeterminate response in 10 (10.0%) patients. Histopathological correlation was available in 84 surgically treated patients, of whom 21 (25.0%) patients had pathological complete response and 63 (75.0%) patients had residual viable tumor. When incomplete or indeterminate response was considered positive for residual disease, structured post-treatment MRI showed sensitivity of 88.9%, specificity of 81.0%, positive predictive value of 93.3%, negative predictive value of 70.8%, and accuracy of 86.9% for detecting residual viable tumor. Persistent intermediate T2 signal, focal diffusion restriction, residual extramural vascular invasion, and threatened mesorectal fascia were significantly associated with residual viable tumor. Exploratory predictive analysis showed strong associations of persistent intermediate T2 signal and focal diffusion restriction with residual viable tumor. Receiver operating characteristic analysis of the ordered structured MRI response category showed good discriminatory performance, with an area under the curve of 0.860. Sensitivity analyses showed that diagnostic performance varied according to the handling of indeterminate response, supporting cautious interpretation of equivocal post-treatment findings. Conclusions Structured post-treatment rectal MRI provides a practical framework for documenting response after total neoadjuvant therapy and communicating residual tumor morphology, diffusion restriction, mesorectal fascia status, extramural vascular invasion, nodal response, and sphincter or pelvic sidewall involvement to the multidisciplinary team. Although diagnostic performance was good in the surgically treated subgroup, findings should be interpreted cautiously because histopathological confirmation was unavailable in all patients, the watch-and-wait subgroup was small, follow-up was limited, and external validation was not performed. Structured MRI may support, but should not replace, integrated clinical, endoscopic, pathological, biochemical, and multidisciplinary assessment when considering organ-preserving management.