Rectal Cancer MRI
High-resolution pelvic MRI is the backbone of rectal cancer planning — it decides who needs pre-operative therapy and whether surgery can achieve a clear margin.
What it stages
A dedicated small-field, high-resolution T2 protocol (perpendicular to the tumour axis) assesses:
- T stage: depth through the bowel wall into perirectal fat and adjacent organs.
- N stage: mesorectal and pelvic side-wall nodes (morphology, not just size).
The features that drive treatment
- CRM (circumferential resection margin): distance from tumour to the mesorectal fascia — a threatened/involved margin predicts local recurrence and prompts neoadjuvant chemoradiotherapy.
- EMVI (extramural venous invasion): tumour in perirectal veins — an adverse prognostic marker MRI detects.
- Sphincter & peritoneal involvement: influences the type of surgery.
The CRM concept, driven by MRI, is central to modern total mesorectal excision planning and reducing local recurrence.
Restaging after therapy
After chemoradiotherapy, MRI (with DWI) restages the tumour and grades response (mrTRG). Excellent responders may be considered for organ-preserving “watch-and-wait” in specialist settings, making accurate MRI response assessment increasingly important.
Reference: Beets-Tan RGH et al. Magnetic resonance imaging for clinical management of rectal cancer: ESGAR consensus. Eur Radiol 2018;28:1465–75.
Educational summary for clinicians; staging is multidisciplinary. Not medical advice.