The ovaries contain 3 kinds of tissue:

Epithelial cells: These cells cover the ovary. Most ovarian cancers start in this covering.

Germ cells: These cells make eggs (ova) inside of the ovary.

Stromal cells: These cells make most of the female hormones (estrogen and progesterone).

Types of ovarian tumors

Many types of tumors can start in the ovaries. Some are benign (non-cancerous) and never spread beyond the ovary. Women with these types of tumors can be treated by removing either the ovary or the part of the ovary that has the tumor. Other types of tumors are cancerous (malignant) and can spread to other parts of the body. Their treatment is more complicated and is explained later in this article.

As a rule, tumors in the ovary are named for the kinds of cells the tumor started from and whether the tumor is benign or cancerous. There are 3 main types of tumors:

Germ cell tumors: These start from the cells that produce the eggs.

Stromal tumors: These start from cells that hold the ovary together and make the female hormones.

Epithelial tumors: These tumors start from the cells that cover the outer surface of the ovary. Most ovarian tumors are epithelial cell tumors.

Each of these types of tumors is explained in more detail below.

Epithelial ovarian tumors

Epithelial ovarian tumors are further divided into 3 sub-groups:

Benign epithelial tumors: these tumors do not spread and usually do not lead to serious illness.

Tumors of low malignant potential (LMP tumors): These tumors do not clearly appear to be cancerous when looked at under the microscope. They are also known as borderline tumors. They tend to affect women at a younger age than other ovarian cancers. They grow and spread slowly and are less life-threatening than most ovarian cancers.

Epithelial ovarian cancers: Nearly 9 out of 10 ovarian cancers are this type. When someone says they have ovarian cancer, they usually mean this type. Cancer cells of this type have certain features that can be seen under a microscope and which allow doctors to further classify them. These tumors are also given a grade depending on how much the cells look like normal cells. Grade 1 means the cells look more normal; grade 3 look less normal, and grade 2 is in between. Usually the higher the grade the worse the outlook.

Primary peritoneal carcinoma

This condition is a rare cancer much like epithelial ovarian cancer but it starts outside of the ovaries. It grows from the cells that line the pelvis and abdomen. These cells look just like the cells along the surface of the ovaries. Women who have had their ovaries removed can still get this type of cancer. Symptoms of this cancer are much like those of ovarian cancer. Treatment is also similar.

Fallopian tube cancer

This is a very rare cancer. It begins in the tube that carries an egg from the ovary to the uterus (the fallopian tube). Fallopian tube cancer causes symptoms much like those seen in women with ovarian cancer. The treatment and outlook for survival (prognosis) is similar to that for ovarian cancer.

Germ cell tumors

Most germ cell tumors are not cancer, although some can be. There are a number of sub-types of germ cell tumors. The most common are teratoma, dysgerminoma, endodermal sinus tumor, and choriocarcinoma.

Stromal tumors

Stromal tumors can be either benign (not cancer) or cancerous. More than half are found in women over age 50. Some of these tumors make hormones. There are many different types of stromal tumors. Types of malignant (cancerous) stromal tumors include granulosa cell tumors, granulosa-theca tumors, and Sertoli-Leydig cell tumors, which are usually considered low-grade cancers. Thecomas and fibromas are benign stromal tumors.

Ovarian cysts

An ovarian cyst is fluid that collects inside an ovary. Many of these cysts are harmless. The fluid will most often be absorbed and the cyst will go away in time without any treatment. But if the cyst is large, does not go away on its own in a few months, or happens in childhood or after menopause, the doctor may suggest further tests or treatment. This is because a very small number of these cysts can be cancerous.