Four numbers decide the treatment: distance from the anal verge, T stage, mesorectal fascia clearance, and extramural venous invasion.
Orient first
- This report is a surgical and oncological planning document. The MESORECTAL FASCIA is the surgical plane, and the tumour's shortest distance to it is the single most consequential measurement — threatened at 1 mm or less.
- EXTRAMURAL VENOUS INVASION is an independent adverse prognostic factor that is frequently under-reported. It is seen as tumour signal expanding a vein beyond the muscularis propria.
- DEPTH of extramural spread in millimetres matters within T3 — T3a to T3d — and changes neoadjuvant decisions in a way "T3" alone does not.
- HEIGHT from the anal verge and the relationship to the peritoneal reflection determine the operation, including whether the sphincter can be preserved.
- For low tumours the ANAL SPHINCTER complex must be assessed separately: internal sphincter, intersphincteric plane, and external sphincter.
Acquire the study
- High-resolution T2 WITHOUT fat suppression, in planes ORTHOGONAL TO THE TUMOUR — not to the patient. An oblique axial aligned to the rectal wall at the tumour is the defining sequence, and a body-axial plane over-stages by partial volume.
- Sagittal T2 first to plan the oblique axial, and a coronal for low tumours to assess the sphincter.
- Add DWI, which helps identify the tumour and assess response after neoadjuvant therapy.
- No rectal distension for staging — it flattens the tumour and distorts the mesorectal fat.
- Confirm which sequence you are on before judging any signal — see the MRI sequence primer.
The manoeuvre
- Measure the distance from the ANAL VERGE to the lower edge of the tumour, and state whether it is low, mid or high.
- State the relationship to the PERITONEAL REFLECTION.
- Assign T stage, and for T3 give the DEPTH of extramural spread in millimetres.
- Measure the shortest distance from tumour (or a deposit or node) to the MESORECTAL FASCIA and state which structure is closest.
- Assess EXTRAMURAL VENOUS INVASION explicitly, and state present or absent rather than omitting it.
- Assess mesorectal and extramesorectal nodes by morphology — irregular border and mixed signal matter more than size.
- For low tumours, assess the intersphincteric plane and both sphincters.
- Look for tumour deposits, which are counted separately from nodes.
- After neoadjuvant therapy, use a tumour regression grading system and name it.
What confirms it
- A threatened margin is tumour, a deposit or a node within 1 mm of the mesorectal fascia on a correctly-aligned oblique axial image.
What licenses you to exclude it
- ⚠️ A body-axial study cannot exclude margin involvement — partial volume averaging across the rectal wall both over- and under-calls. Report the plane as inadequate and recommend a correctly-aligned study.
- MRI cannot reliably distinguish a small T1 from a T2 tumour; endoanal ultrasound is the study for early disease, and saying so beats guessing.
- After radiotherapy, fibrosis and residual tumour overlap on T2 — assess with DWI and say that only tissue settles it.
The classic misread
- Staging on body-axial rather than tumour-orthogonal images.
- Omitting extramural venous invasion.
- Giving "T3" with no depth in millimetres.
- Distending the rectum for a staging study.