A 58-year-old man with chronic ulcerative colitis is found to have a 3-cm mass approximately 5 cm from the anal verge on surveillance colonoscopy. Biopsies reveal moderately differentiated adenocarcinoma. On rectal examination, the mass is fixed and tethered. Staging reveals a T3N0 lesion, with no evidence of distant metastasis. The best next step in management is:

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