A 52-year-old woman from Mumbai presents with vaginal bleeding and pelvic pain for 2 months. She has a history of infertility and endometriosis. On imaging, a 6 cm heterogeneous ovarian mass is noted with ascites. Serum CA-125 is 320 U/mL and inhibin A is normal. Tumour markers: AFP and hCG are negative. Biopsy shows clear cell carcinoma. She undergoes staging laparotomy with total abdominal hysterectomy, bilateral salpingo-oophorectomy, omentectomy, and pelvic lymph node dissection. All nodes are negative. Peritoneal washings are positive for malignant cells. What is the most appropriate next step in management?