Metastatic gastric cancer is stomach cancer that has spread from the stomach to distant parts of the body, such as the liver, the lining of the abdomen (peritoneum), the lungs, or the bones. It is classified as stage IV, the most advanced stage. Roughly 36% of stomach cancers are already at this distant stage when first diagnosed, largely because early stomach cancer rarely causes noticeable symptoms.
Where Stomach Cancer Spreads
Stomach cancer doesn’t scatter randomly through the body. It follows predictable routes, and the destination matters because it shapes both symptoms and treatment options. In a large study of patients with metastatic disease, the liver was the most common site, involved in 48% of cases. The peritoneum, the thin membrane that lines the inside of the abdomen and wraps around the organs, was next at 32%. The lungs accounted for about 15% and bone for 12%. Some patients had cancer in more than one of these locations at the time of diagnosis.
Peritoneal spread deserves special attention because it behaves differently from cancer that travels through the bloodstream to solid organs like the liver or lungs. When cancer cells seed across the peritoneum, they can cause fluid to accumulate in the abdomen, a condition called ascites. This fluid buildup can make the belly visibly swollen and uncomfortable, and it often requires its own set of treatments separate from the cancer therapy itself.
Symptoms of Advanced Disease
Many symptoms of metastatic stomach cancer depend on where the cancer has landed. When the liver is involved, you may notice discomfort or pain on the right side of the abdomen, nausea, poor appetite, significant weight loss, yellowing of the skin and eyes (jaundice), or persistent itching. Peritoneal spread typically shows up as abdominal swelling from fluid buildup, sometimes accompanied by a feeling of fullness or pressure even without eating much.
More general symptoms include fatigue, unintentional weight loss, and difficulty eating. If the growing tumor partially blocks the stomach or intestine, nausea and vomiting can become persistent. These symptoms often overlap with less serious conditions, which is one reason stomach cancer frequently goes undetected until it reaches an advanced stage.
How It’s Diagnosed
When doctors suspect stomach cancer has spread, they use a combination of imaging and tissue sampling to confirm it. A CT scan with contrast is the preferred first-line tool because it can evaluate the depth of the tumor in the stomach wall, nearby lymph nodes, and distant organs in a single exam. PET scans add another layer, especially for detecting metastases in the liver, lungs, and skeleton that may not be obvious on CT alone.
Endoscopic ultrasound, where a small ultrasound probe is passed down the throat into the stomach, gives a detailed view of the tumor itself and surrounding lymph nodes. Guided by that ultrasound, doctors can also take needle biopsies of suspicious lymph nodes with high accuracy. For peritoneal spread specifically, many cancer centers perform a diagnostic laparoscopy, a small camera inserted through a tiny incision in the abdomen, because even advanced imaging can miss thin sheets of cancer on the peritoneal surface. If fluid is found in the abdomen during this procedure, it’s tested for cancer cells. A positive result is enough to classify the cancer as metastatic.
Biomarker Testing and Why It Matters
Once metastatic stomach cancer is confirmed, a biopsy sample is tested for specific molecular markers that determine which treatments are likely to work. This isn’t optional or experimental; it directly shapes the treatment plan.
The most important marker is HER2, a protein found on the surface of some cancer cells that drives their growth. Patients whose tumors overexpress HER2 are eligible for a targeted drug called trastuzumab, which blocks that protein, added on top of standard chemotherapy. Tumors are also tested for PD-L1, a protein that helps cancer hide from the immune system. When PD-L1 levels are above a certain threshold, immunotherapy drugs that unmask the cancer for the immune system become part of the treatment combination.
A third marker, called MSI-H (or its related finding, mismatch repair deficiency), identifies tumors with a specific type of genetic instability. These cancers tend to respond particularly well to immunotherapy. Together, these three tests, HER2, PD-L1, and MSI-H, help oncologists choose between several different first-line treatment regimens rather than giving every patient the same chemotherapy.
Treatment for Metastatic Stomach Cancer
The foundation of treatment is systemic therapy, meaning drugs that travel through the entire body to reach cancer wherever it has spread. The standard backbone is a combination of two types of chemotherapy: a platinum-based drug and a fluoropyrimidine. These are given intravenously in cycles, typically every two or three weeks.
For the majority of patients whose tumors are HER2-negative, an immunotherapy drug (either nivolumab or pembrolizumab) is now added to that chemotherapy backbone as a first-line approach, provided PD-L1 levels meet the required threshold. Clinical trials have shown that this combination extends survival compared to chemotherapy alone, with the greatest benefit seen in patients with higher PD-L1 scores or MSI-H tumors. For HER2-positive cancers with adequate PD-L1 expression, the combination expands to include trastuzumab alongside immunotherapy and chemotherapy.
If the first-line combination stops working, second-line options exist, though response rates are lower. Treatment decisions at that point depend on what was used first, how the patient tolerated it, and overall health status.
Managing Complications
As the disease progresses, complications like bowel obstruction or worsening ascites can significantly affect quality of life, and managing these becomes a central part of care. Bowel obstruction from peritoneal disease, for example, can cause severe nausea and vomiting. When medications alone can’t control this, doctors may place a venting tube through the skin into the stomach, either with an endoscope or using X-ray guidance, to drain the backed-up fluid and relieve pressure. This approach avoids major surgery, which carries very high risks in this setting.
Ascites is managed by draining the fluid through a needle or catheter inserted into the abdomen. Some patients need this done repeatedly, while others have a semi-permanent catheter placed so fluid can be drained at home. These interventions don’t treat the cancer itself, but they can meaningfully improve comfort and daily functioning.
Survival and Prognosis
The five-year relative survival rate for distant-stage stomach cancer is about 7.5 to 8%, based on data from patients diagnosed between 2015 and 2021. That number reflects an average across all patients, including those diagnosed before immunotherapy became standard first-line treatment. Outcomes vary widely depending on where and how extensively the cancer has spread, the tumor’s molecular profile, and how well it responds to initial treatment. Patients with MSI-H tumors, for example, tend to have significantly better responses to immunotherapy than that overall average would suggest.
These statistics also represent a snapshot in time. The treatment landscape for metastatic stomach cancer shifted substantially starting around 2021 with the approval of immunotherapy combinations, and survival data from patients treated with these newer regimens is still maturing.