A 62-year-old woman from Delhi undergoes subtotal gastrectomy with D2 lymphadenectomy for a T2N1M0 gastric adenocarcinoma of the antrum. Intraoperative findings include a 3.5 cm tumour with 4 positive perigastric lymph nodes and 8 negative distant lymph nodes (total 18 nodes examined). Final histopathology confirms adequate margins (R0 resection) and no involvement of the proximal stomach. The patient recovers well from surgery. What is the most appropriate next step in management?