A 61-year-old man is found to have a 2-cm mass in the mid-rectum (8 cm from the anal verge) on diagnostic colonoscopy. Biopsy reveals moderately differentiated adenocarcinoma. Computed tomography (CT) reveals no evidence of metastatic disease, and magnetic resonance imaging (MRI) demonstrates a cT3, node-negative cancer. After presentation at a multidisciplinary tumor board, the patient agrees to total neoadjuvant therapy. The patient has a complete clinical response to treatment and opts for close radiologic and endoscopic surveillance in an effort to avoid surgery. On a surveillance proctoscopy 6 months after treatment, the patient is found to have a 7-mm recurrence at the original cancer site. Which of the following is the best next step in management?