Ovarian Cyst with Rupture or Torsion: This is a common cause of pelvic cysts and can lead to ascites if the cyst ruptures or undergoes torsion, causing fluid to leak into the abdominal cavity.

Ovarian Cancer: Ovarian cancer can present with a pelvic mass and ascites due to metastasis or tumor rupture. The presence of ascites in a postmenopausal woman with a pelvic mass is highly suggestive of ovarian cancer.
Endometriosis: Although less common, endometriosis can cause large cysts (endometriomas) that may rupture, leading to ascites. However, this is less likely to cause significant ascites compared to ovarian cancer or a ruptured ovarian cyst.
Tuberculous Peritonitis: This condition can cause ascites and may be associated with pelvic cysts, especially in endemic areas. It's an important consideration in patients with risk factors for tuberculosis.

Ruptured Ectopic Pregnancy: Although less likely if an ovarian cyst is already identified, a ruptured ectopic pregnancy can cause severe abdominal pain, ascites (due to hemorrhage), and can be life-threatening if not promptly diagnosed and treated.
Pelvic Inflammatory Disease (PID): PID can cause tubo-ovarian abscesses that may rupture, leading to ascites and peritonitis. This diagnosis is critical due to the risk of severe infection and long-term fertility consequences.

Mesenteric Cyst: A rare cause of a pelvic cyst that can cause ascites if it ruptures. These cysts are usually benign but can cause significant symptoms if they become large or rupture.
Lymphangioma: A rare, benign tumor that can occur in the pelvis and cause a cystic mass. If it ruptures, it could potentially lead to ascites, although this is exceedingly rare.
Gastrointestinal Stromal Tumor (GIST) with Metastasis: Although primarily a gastric tumor, GISTs can rarely occur in the small intestine and metastasize to the pelvis, causing a cystic mass and ascites. This diagnosis is rare but should be considered in the differential due to its potential for malignancy.