Optimizing preoperative magnetic resonance imaging assessment for low rectal cancer patients: Insights from a multicenter retrospective study in Dutch dedicated centers.

The treatment of rectal cancer is inherently complex—a multimodal pathway involving surgical resection according to the principles of total mesorectal excision (TME) and selective neoadjuvant therapy based on pre-operative tumour characteristics.Among all rectal tumours, low rectal cancers, defined as a tumour with the inferior border below the origo of the levator ani muscle, pose the greatest challenge. With their proximity to the anal sphincter and pelvic floor, they are associated with  increased risk of circumferential resection margin (CRM) involvement, and worsened oncological outcomes compared to mid and high rectal tumours. These tumours demand unparalleled precision in staging and planning.

A national collaborative

The MERCURY-III trial, under the MIRECA study group, responds to this pressing clinical need. This Dutch multicentre initiative builds upon the foundational work of the original MERCURY study group, a European collaborative that first demonstrated the predictive value of MRI features beyond traditional TNM staging. In particular, features such as:

• MRI-detected extramural vascular invasion grade III or grade IV
• Tumour deposits (N1c)
• Tumour location in the anterior quadrant
• An unsafe low rectal plane (mrLRP) — defined by invasion of the mesorectal fascia or intersphincteric plane

have all been shown to predict positive CRM status and adverse oncological outcomes. Yet, most of these features are not yet routinely assessed or documented in all standard radiology reports, despite their high clinical relevance.

Study design highlights

The MERCURY III-trial will evaluate whether structured training of radiologists leads to improved MRI-based prediction of oncological and survival outcomes in low rectal cancer patients.

• 40 radiologists from 21 centres across the Netherlands will participate
• All will receive standardized training in low rectal MRI interpretation
• Over 450 low rectal MRI scans will be re-evaluated post-training
• Results will be analysed and correlated to 5-years follow-up date