A 62-year-old man was found to have 1-cm posterior rectal lesion that is 3 cm from the anal verge. Histopathologic findings are consistent with a well-differentiated adenocarcinoma without lymphovascular or perineural invasion. Magnetic resonance imaging shows that the tumor is limited to the submucosa and there are no suspicious mesorectal lymph nodes. Metastatic workup uncovers no findings. What is the most appropriate management?