The purpose of this observational study is to learn if a specialized blood test can help predict whether advanced ovarian cancer will return after a patient's initial treatments are finished.

Researchers are inviting women who have been diagnosed with stage III or IV epithelial ovarian cancer and have recently completed their first-line treatments, which include surgery and platinum-based chemotherapy.

The study focuses on "circulating tumor DNA" (ctDNA), which are tiny fragments of genetic material that cancer cells release into the bloodstream as they break down. Finding these DNA fragments in the blood when a patient appears to be cancer-free on standard tests is known as assessing for minimal residual disease (MRD).

Because this is an observational study, participants will receive standard medical care as directed by their doctor. For the research portion, participants will provide blood samples at specific times: at the time of diagnosis, shortly after surgery, right after finishing their first-line chemotherapy, and then every 3 months during regular follow-up visits. Researchers will also analyze a sample of the tumor tissue that was already removed during the patient's standard surgery.

By tracking these participants for up to 3 years, researchers hope to discover if the ctDNA test can accurately identify patients who have a high risk of their cancer returning, and if it can detect this earlier than traditional imaging scans or standard blood tests like CA-125.

Despite high initial response rates to standard first-line therapies (surgery and platinum-based chemotherapy), the majority of patients with advanced high-grade epithelial ovarian cancer experience recurrence within three years. Traditional surveillance methods, including the CA-125 tumor marker and radiological imaging, often lack the sensitivity required to detect minimal residual disease (MRD) at a molecular level, limiting the window for early therapeutic intervention. Circulating tumor DNA (ctDNA) analysis has emerged as a promising, non-invasive technology capable of reflecting tumor-specific genetic mutations and real-time molecular profiling.

This prospective, non-interventional cohort study aims to comprehensively evaluate the clinical utility of blood-based ctDNA as an independent biomarker for predicting recurrence in patients with FIGO stage III-IV epithelial ovarian cancer. The primary objective is to determine whether the presence of ctDNA (MRD positivity) measured immediately after the completion of standard first-line therapy correlates significantly with Relapse-Free Survival (RFS).

To establish a baseline mutational profile, tumor tissue obtained during initial biopsy or debulking surgery (primary or interval) will undergo Next-Generation Sequencing (NGS). Longitudinally, peripheral blood samples (20 mL) will be collected at predefined critical time points: at initial diagnosis (prior to treatment), post-surgery, within 2-4 weeks after the completion of first-line adjuvant or maintenance therapy (baseline for post-treatment MRD), and every 3 months during the follow-up period for up to 36 months or until disease recurrence.

Cell-free DNA (cfDNA) will be extracted from the collected plasma and analyzed using a highly sensitive, validated NGS-based multigene panel (AlphaLiquid® 100). The analysis will quantify major genetic variants (Variant Allele Frequency [VAF] ≥ 0.1%).

By tracking changes in ctDNA dynamics and comparing them with standard follow-up modalities, the study will investigate the lead time of ctDNA detection over radiological recurrence and compare its sensitivity and specificity against CA-125. Ultimately, this research seeks to provide robust scientific evidence to support the integration of ctDNA-based MRD monitoring into standard surveillance protocols, potentially enabling personalized tracking strategies and earlier clinical decision-making for high-risk ovarian cancer patients.

Inclusion Criteria:

Women aged 19 years or older. Pathologically confirmed high-grade serous, endometrioid, clear cell, or mixed type ovarian cancer.

FIGO stage III or IV. Patients who have completed primary debulking surgery (PDS) or interval debulking surgery (IDS) and platinum-based chemotherapy (subsequent maintenance therapy, such as bevacizumab or PARP inhibitors, is allowed).

Patients showing radiological or clinical Complete Response (CR) after the completion of platinum-based chemotherapy.

Written informed consent for the study.

Exclusion Criteria:

Radiological progressive disease during or immediately after treatment. Expected survival of 3 months or less. Immunodeficiency or pathological bleeding tendencies. Unable to undergo blood tests or unwilling to undergo repeated blood sampling. Concurrent other solid tumors or history of malignant tumors within the last 5 years.

Refusal to consent to participate in this study.

The study population consists of adult women (aged 19 years or older) diagnosed with FIGO stage III or IV advanced epithelial ovarian cancer, specifically including high-grade serous, endometrioid, clear cell, or mixed carcinomas. To be eligible, patients must have successfully completed their standard first-line therapy, consisting of primary debulking surgery (PDS) or interval debulking surgery (IDS) paired with platinum-based chemotherapy, and must have achieved a clinical or radiological complete response (CR) at the end of treatment. A total of 200 patients meeting these criteria will be enrolled in this prospective cohort.

Individual participant data (IPD) will not be shared with other researchers to strictly protect patient privacy and confidentiality. According to the study protocol and institutional regulations, access to all research data is strictly limited to the principal investigator and the minimum number of authorized research personnel approved by the Institutional Review Board (IRB). All collected data is coded to prevent external exposure of any personally identifiable information. Furthermore, in compliance with South Korea's Bioethics and Safety Act and Personal Information Protection Act, all study-related records will be securely retained for 3 years after the completion of the study and subsequently permanently destroyed, which precludes external data sharing.