Conclusions

Structured post-treatment rectal MRI provides a practical framework for documenting treatment response after total neoadjuvant therapy and for communicating residual tumor morphology, diffusion restriction, magnetic resonance tumor regression pattern, mesorectal fascia status, extramural vascular invasion response, nodal response, and sphincter or pelvic sidewall involvement to the multidisciplinary team. In this retrospective cohort, structured MRI showed good diagnostic performance for detecting residual viable tumor in the surgically treated subgroup, and persistent intermediate T2 signal, focal diffusion restriction, residual extramural vascular invasion, and threatened mesorectal fascia were strongly associated with residual viable tumor. Exploratory receiver operating characteristic analysis demonstrated good discriminatory performance of the ordered structured MRI response category; however, these findings should be interpreted cautiously because histopathological confirmation was not available in all patients, the watch-and-wait subgroup was small, follow-up was limited, and external validation was not performed. Structured MRI may support multidisciplinary assessment of potential organ-preservation eligibility, but it should not replace integrated clinical, endoscopic, pathological, biochemical, surgical, and multidisciplinary evaluation. Larger prospective multicenter studies with standardized MRI protocols, patient-level predictive modeling, longer follow-up, and external validation are required before routine use as a validated independent predictive framework.