About 70% of epithelial ovarian cancer patients are diagnosed at advanced stage. When primary optimal surgery is not possible, neoadjuvant chemotherapy will followed by interval debulking surgery is one treatment option. However, there is no consensus on the optimal timing of the surgery. CA125 is a well-known tumor marker in ovarian cancer. Its kinetic change has been proven to correlate with the patients' response to chemotherapy and chance of optimal resection. This study aims to utilize the kinetic change of CA125 to customize the timing of surgery for individual patients and compare this with the standard clinical practice.

Recruited patients will be randomised into two groups. The control group will receive treatment according to the standard clinical practice. The investigation group will have an additional CA125 at the 5th week after the first cycle of chemotherapy. CA-125 ELIMination Rate Constant K (KELIM) will be determined using online tool. Patients with KELIM =>1 will receive radiological assessment and undergo internal debulking surgery if the disease is operable. Patients with KELIM <1 will have alternative management, such as addition of bevacizumab or changing to dose-dense chemotherapy, and defer the interval debulking surgery.

Inclusion Criteria:

1. Patients aged 18 years old or older
2. Patients with Eastern Cooperative Oncology Group score 0-1 within 28 days prior to recruitment
3. Patients who can sign the informed consent
4. Patients with stage III-IV histologically or cytologically confirmed epithelial ovarian cancer (EOC), fallopian tube or primary peritoneal cancer not amenable for PDS
5. Patients who have baseline computed tomography (CT) of thorax, abdomen and pelvis.
6. Patients who are planned for neoadjuvant chemotherapy (NACT) using 3-weekly carboplatin and paclitaxel. Those who have received one cycle of NACT may be eligible if the CA125 schedule of the study group can be matched.
7. Patients who have an evaluable CA125 level at baseline (i.e., baseline level is at least 2x upper limit of normal)
8. Patients who agree for chemotherapy and interval debulking surgery (IDS) if the disease becomes operable after NACT
9. Patients with adequate hematologic, liver and renal functions for chemotherapy
10. Patients who agree to receive adjuvant chemotherapy after IDS. The total number of NACT and adjuvant chemotherapy should be four or above, up to maximum of 9 cycles.
11. Patients who have childbearing potential should practice highly effective contraception throughout the study until at least 30 days after completion of the treatment.
12. Patients must have either germline and / or somatic BRCA test, or homologous recombination deficiency (HRD) test.

Exclusion Criteria:

1. Patients who have borderline malignancy, or non-EOC like germ cell or sex cord tumor, or metastatic diseases from other origins
2. Patients with mucinous and neuroendocrine histology
3. Patients with history of other malignancies within five years
4. Patients who are eligible for primary debulking surgery (PDS)
5. Patients who cannot undergo PDS because of parametrial and/or vaginal involvement alone
6. Patients who are not fit for PDS because of medical morbidities or refusal of operation
7. Patients who have already started NACT outside the study centers, except those who have received only one cycle within 7 days and the baseline CA125 value within 3 days of NACT (normal cut-off 35 U/ml) is available
8. Patients who participate in other interventional studies
9. Patients who are pregnant or breastfeeding
10. Patients who have contraindications to platinum-based chemotherapy
11. Patents with active tuberculosis, history of positive test for human immunodeficiency virus (HIV) or known acquired immunodeficiency syndrome (AIDS) are excluded.

Completeness of resection is assessed by independent surgeon