A 14-year-old girl undergoes unilateral salpingo-oophorectomy for a stage IB dysgerminoma of the left ovary. Histopathology confirms pure dysgerminoma with no other germ cell elements. The contralateral ovary was inspected intraoperatively and appeared normal. Peritoneal washings were negative. Serum AFP and β-hCG are now normal post-operatively. What is the most appropriate next step in management?