The presence of fluid in the abdomen is medically known as ascites, which is the accumulation of excessive fluid within the peritoneal cavity. When caused by cancer, it is specifically termed malignant ascites. Although people often search using the phrase “fluid in the stomach,” the condition involves the entire abdominal space surrounding the organs, not the stomach organ itself. Malignant ascites generally signifies advanced disease and is a common complication in several types of cancer that have spread to the lining of the abdomen.
Understanding Ascites and the Abdominal Cavity
The abdomen is lined by the peritoneum, a thin, two-layered membrane that creates the peritoneal cavity. This cavity normally contains a small amount of fluid, which acts as a lubricant, allowing organs like the stomach and intestines to move without friction.
Ascites occurs when fluid production exceeds the rate at which the body can reabsorb it, leading to abnormal accumulation. The most frequent cause of non-malignant ascites is liver cirrhosis, which creates high pressure (portal hypertension) in the blood vessels leading to the liver. This pressure forces fluid out of the vessels and into the abdominal cavity.
Malignant ascites is caused by the direct effects of cancer, either through the spread of tumor cells or obstruction of the body’s drainage systems. The presence of cancer cells in the fluid or on the peritoneal lining distinguishes malignant ascites from other causes. This fluid accumulation can lead to uncomfortable symptoms like abdominal swelling, pain, and difficulty breathing.
Cancers Most Commonly Linked to Malignant Ascites
Malignant ascites is most frequently associated with adenocarcinomas, cancers that form in glandular tissue, particularly those spreading to the abdominal area. Ovarian cancer is the most common cause of malignant ascites, as it has a high tendency to spread directly to the peritoneal lining, a process called peritoneal seeding. This spread often occurs early due to the close anatomical relationship between the ovaries and the peritoneal cavity.
Gastrointestinal cancers are also significant contributors, including colorectal, stomach (gastric), and pancreatic cancers. These cancers can directly invade the peritoneum or shed cells into the abdominal fluid, leading to widespread tumor implants across the peritoneal surface.
Cancers originating outside the abdomen, such as breast and lung cancer, can also result in malignant ascites, though less commonly than gynecological and gastrointestinal cancers. In these cases, cancer cells travel through the bloodstream or lymphatic system to colonize the peritoneal lining. The development of malignant ascites generally signals a locally advanced stage of the disease.
The Mechanisms of Fluid Accumulation
Cancer causes fluid to accumulate in the abdomen through two primary mechanisms that disrupt the normal balance of fluid production and drainage. The most common mechanism is peritoneal seeding, where cancer cells implant on the peritoneum. These implanted tumor cells release substances, including vascular endothelial growth factor (VEGF).
VEGF increases the permeability of the blood vessels lining the peritoneum, making them “leaky.” This allows fluid and large molecules, like proteins, to escape from the capillaries and rapidly fill the abdominal cavity. The irritation and inflammation caused by the tumor cells also stimulate the peritoneum to produce excessive fluid.
The second mechanism is lymphatic obstruction, which impairs the body’s ability to clear the fluid. The peritoneal cavity is drained by a network of lymphatic channels, especially those beneath the diaphragm. When cancer cells spread to these lymph nodes and channels, they physically block the natural outflow pathways. This obstruction prevents the reabsorption of peritoneal fluid, leading to its accumulation.
Diagnosing the Cause of Abdominal Fluid
When ascites is suspected based on symptoms like abdominal distension, diagnostic imaging confirms the presence of fluid. An ultrasound or computed tomography (CT) scan is typically used to visualize the amount and distribution of the fluid, and to look for signs of a tumor or metastatic spread. These scans can identify masses on the liver or along the peritoneal lining.
The definitive procedure for diagnosing malignant ascites is paracentesis, which involves inserting a thin needle into the abdomen to withdraw a fluid sample. This procedure both relieves patient discomfort and collects fluid for laboratory analysis. The collected fluid is then sent for cytology, where pathologists examine the sample under a microscope for malignant cells.
Positive fluid cytology, meaning cancer cells are identified, confirms the diagnosis of malignant ascites with high accuracy. Further analysis of the fluid, including measuring protein and albumin levels, helps determine the mechanism of fluid formation and may provide clues about the primary site of the original cancer. The combination of imaging and fluid analysis is essential to tailor treatment, which involves addressing the underlying cancer or managing the fluid accumulation.