This study is a prospective, multi-omics, observational study aimed at detecting peritoneal metastasis of gastric cancer by combined assays for methylation of cell-free DNA (cfDNA) and other blood-based biomarkers. The study will enroll 384 participants with gastric cancer.

Peritoneal metastasis (PM) in gastric cancer is associated with a poor prognosis.

Laparoscopy with cytology is performed to evaluate for peritoneal spread when considering chemoradiation or surgery. However, the laparoscopy is invasive and the sensitivity of computed tomography (CT) scan is poor for detecting PM. Therefore, it is necessary to evaluate the feasibility of cfDNA methylation and other blood-based biomarkers for the PM diagnosis.

The study will enroll 384 participants with gastric cancer. Baseline blood and diagnosis of PM by laparoscopy with cytology will be collected. Participants with PM will be defined as the case arm and participants without PM will be defined as the control arm. A PM diagnostic model will be developed.

Inclusion Criteria:

• Inclusion Criteria for Case Arm Participants:

  1. Age 18-74 years at the day of consenting to the study.

  2. Able to provide a written informed consent.

  3. No prior cancer treatment (local or systematic) with either of the following:

     A. Pathologically confirmed gastric cancer diagnosis within 42 days prior to blood draw.

     B. High suspicious for cancer diagnosis by imaging tests or other routine clinical examinations, with confirmed pathological cancer diagnosis within 42 days after the blood draw.

  4. Diagnosis of peritoneal metastasis by laparoscopy with cytology.

• Inclusion Criteria for Control Arm Participants:

  1. Age 18-74 years at the day of consenting to the study.

  2. Able to provide a written informed consent.

  3. No prior cancer treatment (local or systematic) with either of the following:

     A. Pathologically confirmed gastric cancer diagnosis within 42 days prior to blood draw.

     B. High suspicious for cancer diagnosis by imaging tests or other routine clinical examinations, with confirmed pathological cancer diagnosis within 42 days after the blood draw.

  4. No peritoneal metastasis detected by laparoscopy with cytology.

Exclusion Criteria:

• Exclusion Criteria for All Participants:

  1. Insufficient qualified blood samples.
  2. During pregnancy or lactation.
  3. Recipient of organ transplant or prior non-autologous (allogeneic) bone marrow or stem cell transplant.
  4. Recipient of blood transfusion within 7 days prior to blood draw.
  5. Recipient of anti-tumor drugs to treat non-cancer diseases within 30 days prior to blood draw.
  6. With other known malignant tumors or multiple primary tumors.

• Exclusion Criteria for Control Arm Participants:

  1. Insufficient qualified blood samples.
  2. During pregnancy or lactation.
  3. Recipient of organ transplant or prior non-autologous (allogeneic) bone marrow or stem cell transplant.
  4. Recipient of blood transfusion within 7 days prior to blood draw.
  5. Recipient of anti-tumor drugs to treat non-cancer diseases within 30 days prior to blood draw.
  6. With other known malignant tumors or multiple primary tumors.

Eligible participants of gastric cancer without any types of tumor treatment will be recruited.