Most ovarian cancers start in the epithelial cells, which form the outer layer of tissue around the ovary – called epithelial ovarian cancer. Epithelial ovarian cancer makes up 85% to 90% of ovarian/fallopian tube cancers. Some ovarian cancers start in the fallopian tubes – see the section on this website on fallopian tube cancer. Sometimes it is hard to tell the difference between epithelial ovarian cancer and fallopian tube cancer, so often they are both just referred to as ovarian cancer.^[1] 

Epithelial ovarian cancer can be divided into a number of subtypes, depending on how the cancer cells appear under the microscope, but these subtypes are all treated in the same way. 

Other types of ovarian cancer are: 

• Borderline ovarian cancer, also called a low malignant potential tumour, is a less common type of epithelial ovarian cancer. This type of cancer tends to occur in young women and is mostly confined to the ovary. 
• Ovarian germ cell tumours are a relatively rare type of ovarian cancer. These tumours begin in the reproductive (egg) cells in the ovary. They usually affect only 1 ovary and occur most often in young women and teenagers. 
• Stromal (sex cord) tumours are also rare, and start from structural tissues in the ovary that produce the female hormones oestrogen and progesterone.