A 45-year-old man undergoes a screening colonoscopy, which shows a 1.5-cm polyp in the rectum located 5 cm from the anal verge. Biopsy reveals well-differentiated invasive adenocarcinoma without lymphovascular invasion, perineural innovation, or tumor budding. Staging computed tomography (CT) does not show any evidence of metastatic disease, and his carcinoembryonic antigen (CEA) level is normal. Magnetic resonance imaging (MRI) of the pelvis reveals the polyp to be a T-1 lesion without any evidence of lymph node metastasis. What is the most appropriate next step in treatment?