The MERCURY trial established that preoperative MRI can accurately define the circumferential resection margin (CRM) in rectal cancer. A threatened CRM on MRI is defined as tumour within:

• A 1 mm of the mesorectal fascia ✓
• B 5 mm of the mesorectal fascia
• C 2 mm of the mesorectal fascia
• D 3 mm of the mesorectal fascia

Explanation

The MERCURY study defined MRI-predicted CRM involvement as tumour within 1 mm of the mesorectal fascia. This threshold guides decisions about neoadjuvant chemoradiotherapy: patients with CRM involvement or threatened CRM (≤1 mm) are typically offered long-course chemoradiation before surgery to facilitate R0 resection. A 2 mm cutoff is sometimes used clinically but the landmark MERCURY trial used 1 mm.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.