A 72-year-old patient with severe systolic congestive heart failure and oxygen-dependent chronic obstructive pulmonary disease completes chemoradiation for a locally advanced rectal adenocarcinoma 5-cm proximal to the anal verge. Reexamination of the rectum 8 weeks after chemoradiation reveals normal, intact mucosa, and magnetic resonance imaging of the pelvis reveals no visible cancer or suspicious mesorectal nodes. What is the most appropriate therapy for this patient?