Like all cancers, this one is most dangerous when discovered late. Hope of a cure in the most advanced stage (stage IV) is only 5 percent (only about 5 percent of those diagnosed with Stage IV ovarian cancer will survive for five years).
Your mother has pleural effusion due to cancer cells spread to the lungs (finding ovarian cancer cells in the fluid around the lungs, called pleural fluid is also evidence of stage IV disease).
Stage IV ovarian cancer means that the cancer has spread to the inside of the liver, the lungs, or other organs located outside of the abdominal cavity.
Because of that, stage IV ovarian cancer is difficult to completely remove with surgery and the currently available chemotherapy is unable to eradicate all of the remaining cancer cells (chemotherapy is usually give after surgery, but also, can be received before to have a chance to further decrease the amount of cancer)