A 55-year-old patient with rectal cancer 5 cm from the anal verge has clinical staging T3N1M0. According to MERCURY study criteria, involvement of which structure on MRI mandates preoperative chemoradiotherapy to achieve R0 resection?

• A Internal sphincter compromise
• B Threatened or involved mesorectal fascia (circumferential resection margin ≤1 mm) ✓
• C Levator ani muscle involvement
• D Extension beyond the peritoneal reflection

Explanation

The MERCURY study validated high-resolution MRI for predicting the circumferential resection margin (CRM) in rectal cancer. A threatened CRM (tumor within 1 mm of the mesorectal fascia) is the key criterion mandating preoperative long-course chemoradiotherapy to achieve R0 resection and avoid local recurrence. Involvement of the internal sphincter may require intersphincteric resection but does not by itself mandate radiotherapy. Levator ani involvement indicates T4b disease but the CRM criterion drives the chemoradiation decision in standard TME-eligible tumors.

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

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