How would you treat a patient who got three months of adjuvant CAPOX for T4N0 stage II colon cancer, but had a solitary pulmonary metastasis a year later which was resected?   

For metastasectomy where they have had prior chemotherapy in an adjuvant setting, I would not routinely recommend more chemotherapy. See for example (although there have recently been several studies supporting this approach). Bosma et al., PMID 34420823. Consider using ctDNA for surveillance as par...