Rectal MRI

Four findings decide the plan. The T stage, which is how far the tumour has grown through the rectal wall. The mesorectal fascia status, which is whether the tumour, an involved node or a tumour deposit comes within one millimetre of the fascia the surgeon removes as an envelope; this is the strongest single predictor of whether the circumferential margin will be clear. Extramural vascular invasion, which is tumour seen growing inside veins outside the bowel wall and which raises the risk of distant spread. And the height of the lower edge of the tumour from the anal verge together with its relation to the sphincter complex, which decides whether the sphincter can be saved.

It means the tumour, an involved lymph node or a tumour deposit lies one millimetre or less from the mesorectal fascia. It predicts that surgery on its own would leave tumour at the circumferential margin, so the usual response is chemoradiotherapy first with repeat imaging before operating, rather than proceeding straight to surgery.

Yes. A restaging MRI, usually six to eight weeks after chemoradiotherapy finishes, shows whether the tumour has shrunk away from the mesorectal fascia and whether a sphincter-preserving operation has become possible when it was not before. In a small group of patients with a complete clinical response on MRI, endoscopy and examination, it also opens the discussion of close surveillance instead of immediate surgery.