Introduction

The management of locally advanced rectal cancer has changed substantially with the increasing use of total neoadjuvant therapy. Traditionally, magnetic resonance imaging (MRI) was used mainly for baseline local staging, assessment of mesorectal fascia involvement, extramural vascular invasion, nodal disease, sphincter complex involvement, and surgical planning. With total neoadjuvant therapy, MRI has acquired an additional role in post-treatment response assessment, because treatment decisions now extend beyond routine total mesorectal excision to include local excision and non-operative organ-preserving strategies in carefully selected patients [1-6].

Organ-preserving management has become increasingly relevant in patients who show clinical complete response or near-complete response after neoadjuvant treatment. The watch-and-wait approach requires careful patient selection and close surveillance, because local regrowth may occur and timely salvage treatment remains essential. Long-term observational data have shown that non-operative management can be feasible in selected complete responders when strict clinical, endoscopic, and imaging follow-up protocols are used [7,8].

Accurate assessment of treatment response after neoadjuvant therapy remains challenging. Post-treatment fibrosis, edema, mucinous degeneration, desmoplastic change, and inflammatory wall thickening may mimic residual tumor, while small foci of viable tumor may be difficult to identify within a fibrotic tumor bed. T2-weighted imaging provides anatomical detail and allows assessment of tumor regression morphology, mesorectal fascia clearance, sphincter complex involvement, and pelvic sidewall extension. MRI guidelines and previous rectal cancer imaging studies have emphasized the importance of standardized assessment of mesorectal fascia, extramural tumor spread, extramural vascular invasion, and tumor regression. Diffusion-weighted imaging (DWI) adds functional information and may improve detection of residual viable tumor, particularly when interpreted together with high-resolution T2-weighted images and apparent diffusion coefficient maps [9-16].

The emergence of watch-and-wait management has made radiologic response assessment more clinically relevant and more demanding. A complete or near-complete clinical response cannot be determined by MRI alone and requires integration with digital rectal examination, endoscopy, carcinoembryonic antigen level, and multidisciplinary review. However, MRI remains essential for identifying adverse residual features such as persistent intermediate T2 signal, focal diffusion restriction, residual extramural vascular invasion, suspicious mesorectal or lateral pelvic nodes, threatened mesorectal fascia, and persistent sphincter or levator involvement. Recent literature on rectal MRI response assessment has highlighted the need for careful interpretation after neoadjuvant therapy because post-treatment pitfalls may affect selection for surgery, local excision, or organ-preserving surveillance [17-20]. Studies evaluating DWI, combined T2-weighted and diffusion-weighted imaging interpretation, and standardized assessment of complete or near-complete response have shown that MRI can improve response assessment but cannot reliably confirm complete response in isolation. Reader variability, overlap between fibrosis and residual viable tumor, equivocal diffusion signal, and absence of uniform long-term outcome validation remain important limitations. Therefore, post-treatment MRI is best interpreted as one component of integrated multidisciplinary response assessment rather than as an independent determinant of watch-and-wait eligibility.

Despite the central role of MRI, post-treatment rectal cancer reports may vary considerably in terminology, completeness, and clinical usefulness. A structured approach can reduce reporting variability by ensuring that key response parameters are evaluated consistently and communicated clearly to the multidisciplinary team. Structured reporting is particularly important in the era of total neoadjuvant therapy, where the radiologist is expected not only to describe residual disease but also to help guide individualized management.

Although several established MRI-based frameworks, including European Society of Gastrointestinal and Abdominal Radiology (ESGAR) recommendations, Magnetic Resonance Imaging and Rectal Cancer European Equivalence (MERCURY)-based assessment, MRI-based tumor regression grade (mrTRG) grading, and DWI response assessment, provide important guidance for rectal cancer staging and restaging, post-treatment reporting after total neoadjuvant therapy remains variable in routine clinical practice. Existing frameworks often emphasize selected components such as tumor regression, mesorectal fascia status, diffusion restriction, or nodal response, but a practical consolidated template that systematically documents all key post-treatment findings relevant to surgery and organ-preservation decisions is still needed. This study addresses this gap by evaluating a structured institutional MRI response assessment template that integrates T2-weighted tumor-bed morphology, DWI, mesorectal fascia clearance, extramural vascular invasion response, nodal response, sphincter and levator involvement, pelvic sidewall disease, and management-relevant response categories in a single reporting framework.

This study aimed to evaluate the utility of a structured post-treatment MRI assessment template for rectal cancer patients treated with total neoadjuvant therapy. The objectives were to assess MRI-based treatment response using T2-weighted morphology, DWI, mesorectal fascia status, extramural vascular invasion, nodal response, and sphincter or pelvic sidewall involvement, and to correlate structured MRI response categories with surgical and histopathological outcomes.