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.

Recruited patients will have additional CA125 at 4th (before administration of chemotherapy), and 5th week after the first cycle of chemotherapy. CA-125 ELIMination Rate Constant K (KELIM) will be determined. 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 must be at least 18 years old.
2. Patients who have Eastern Cooperative Oncology Group (ECOG) score 0-1.
3. Patients who are competent to give informed consent.
4. Patients who have stage III-IV histologically or cytologically confirmed epithelial ovarian cancer, fallopian tube or primary peritoneal cancer not amenable for primary debulking surgery (PDS).
5. Patients who are planned for neoadjuvant chemotherapy (NACT) using platinum-based chemotherapy +/- bevacizumab or biosimilar. Those who are receiving NACT before interval debulking surgery (IDS) are also eligible.
6. Patients who have an evaluable CA125 level at baseline (i.e., baseline level is at least 2x upper limit of normal).
7. Patients who have baseline computed tomography of at least abdomen and pelvis, or positron emission tomography (PET)-CT. Magnetic resonance imaging (MRI) is also acceptable but the same modality has to be used when assessing the feasibility of IDS.
8. Patients who agree to undergo IDS, where the time of IDS may differ from the usual clinical practice.
9. Patients who agree to receive adjuvant chemotherapy, if clinically indicated. The total number of chemotherapy should be at least four or above.

Exclusion Criteria:

1. Patients who have borderline malignancy, or non-epithelial ovarian cancer like germ cell or sex cord tumor, or metastatic diseases from other origins like Krukenberg's tumor
2. Patients who are eligible for PDS
3. Patients who are not fit for PDS because of medical morbidities or refusal of operation
4. Patients who have already started NACT outside the study centres, except those who have just had one cycle within 21 days and the baseline CA125 is available.
5. Patients who are pregnant

Patients' disease course based on this novel management algorithm

One year after the end of the study

The study protocol will be available under a reasonable request to the corresponding author