A 65-year-old man is evaluated after a recent colonoscopy, which disclosed a 2.5-cm pedunculated polyp in the sigmoid colon. The polyp was removed in its entirety in a single piece. Biopsy results showed a well-differentiated adenocarcinoma confined to the submucosa without evidence of lymphovascular involvement and a 1-mm margin. There is no family history of colorectal cancer.

Physical examination findings are unremarkable.

Which of the following is the most appropriate management?