If it's 18cm (not 18mm) then I would think it should be surgically removed. At that large of a size, it could twist your ovary (ovarian torsion) risking death of the ovary. However, it's important to have just the cyst removed versus the whole ovary. Removal of even one ovary has been shown to increase risk for health problems, specifically cognitive impairment, dementia and parkinsonism. Many gynecologists will lop off the whole ovary because it's an easier and more profitable surgery. So you may need to seek out a surgeon with good cystectomy (cyst removal) skills.
We need our ovaries as well as our uterus our whole lives for good health.
I hope surgery goes well and you have an easy recovery. Keep us posted.