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Normally, there is a small amount of fluid in the abdomen. This acts like a lubricant, helping to promote normal peristalsis of the intestines. However, there are other causes of free fluid in the pelvis. An example of this would be pelvic TB. Patients with peritoneal carcinomatosis do develop ascites (fluid in the abdominal cavity). When the amount of ascites is massive that there may be a need to do paracentesis to decompress the abdominal cavity. The fluid is then sent to the laboratory for analysis.
You mentioned that your MRI was unremarkable. You may want to observe if there is progressive increase in the amount of fluid in the pelvis and consult your oncologist.
Good luck.