A 58-year-old woman from Delhi undergoes staging laparotomy for a 12 cm ovarian mass. Intraoperative findings include a unilateral ovarian tumor with intact capsule, no ascites, negative peritoneal washings, and no involvement of omentum, peritoneum, or distant organs. Histopathology reveals grade 2 serous cystadenocarcinoma. Tumor markers: CA-125 = 95 U/mL. She is otherwise fit with normal renal and hepatic function. What is the most appropriate next step in her management?