What Causes Malignant Ascites?

Malignant ascites is caused by cancer affecting the abdominal lining, nearby organs, lymphatic drainage or fluid balance. Cancer cells may irritate the peritoneum, the thin lining inside the abdomen, and make it release extra fluid. A tumour can also obstruct lymph vessels or veins, preventing normal drainage. Cancers that begin in or spread to the abdomen and pelvis can produce this problem, including ovarian cancer and primary peritoneal cancer. The fluid itself is not always proof that cancer cells are present in it; laboratory testing of a sample can help clarify its cause.

The peritoneum normally contains a small amount of lubricating fluid. Cancer deposits on this lining can trigger inflammation and increase fluid production. They may also interfere with the lymphatic channels that normally carry excess fluid away. Pressure from a tumour or enlarged abdominal organs can add to this blockage. In some people, cancer affects the liver or blood vessels, changing the pressure that controls movement of fluid between the bloodstream and abdominal space. These mechanisms can occur alone or together. Ascites can also have non-cancer causes, so clinicians may use an examination, scans, blood tests and analysis of drained fluid to distinguish them.

A healthcare team may ask when the swelling began, whether your weight or appetite has changed, and whether you have pain, nausea or breathlessness. Ultrasound or another scan can show the amount and distribution of fluid. If a paracentesis is recommended, a clinician removes a sample, and sometimes a larger amount, through a needle or catheter after numbing the skin. Treatment depends on the cancer, your symptoms and the reason fluid is collecting. Options may include cancer-directed treatment, medicines that help the body remove fluid, or drainage to ease pressure. Do not start or restrict fluids, salt or medicines without personalised advice.

Contact your oncology or clinical team promptly if abdominal swelling is new, increasing, painful or making eating, moving or sleeping difficult. Seek urgent medical help for severe or rapidly worsening breathlessness, fainting, confusion, intense abdominal pain, repeated vomiting, fever, or inability to pass urine, stool or gas. After drainage, increasing redness, warmth, swelling, discharge or pain at the insertion site may indicate infection and needs medical assessment.

Ask which cause is most likely in your situation, whether the fluid will be tested, and which symptoms should trigger an urgent call. You can also ask how scans will guide treatment, what relief a drainage procedure may provide, how quickly fluid might return, and how to care for the skin afterward.