What is endometrial cancer?
Endometrial cancer is a cancer that starts in the endometrium, the inner lining of the uterus (womb). The picture below shows where the uterus is located.
About the uterus and endometrium
The uterus is a hollow organ, about the size and shape of a medium-sized pear. The uterus is where a fetus grows and develops when a woman is pregnant. The uterus has 2 main parts (see picture above). The cervix is the lower end of the uterus that extends into the vagina. The upper part of the uterus is called the body or the corpus. (Corpus is the Latin word for body.) Although the cervix is technically part of the uterus, when people talk about the uterus, they usually mean the body, not the cervix.
The body of the uterus has 2 main layers. The inner layer or lining is called the endometrium. The outer layer of muscle is known as the myometrium. This thick layer of muscle is needed to push the baby out during birth. The tissue coating the outside of the uterus is the serosa.
Hormone changes during a woman's menstrual cycle cause the endometrium to change. During the early part of the cycle, before the ovaries release an egg (ovulation), the ovaries produce hormones called estrogens. Estrogen causes the endometrium to thicken so that it could nourish an embryo if pregnancy occurs. If there is no pregnancy, estrogen is produced in lower amounts and more of the hormone called progesterone is made after ovulation. This causes the innermost layer of the lining to prepare to shed. By the end of the cycle, the endometrial lining is shed from the uterus and becomes the menstrual flow (period). This cycle repeats throughout a woman's life until menopause (change of life).
Cancers of the uterus and endometrium
Nearly all cancers of the uterus start in the endometrium and are called endometrial carcinomas. Cancers can also start in the muscle layer or supporting connective tissue of the uterus. These cancers belong to the group of cancers called sarcomas.
Endometrial cancers start in the cells that line the uterus and belong to the group of cancers called carcinomas. Most endometrial carcinomas are cancers of the cells that form glands in the endometrium. These are called adenocarcinomas. The most common type of endometrial cancer is called endometrioid adenocarcinoma. Other less common types of endometrial carcinomas include squamous cell and undifferentiated.
Over 80% of endometrial cancers are typical adenocarcinomas -- also known as endometrioid. Endometrioid cancers are made up of cells in glands that look much like the normal uterine lining (endometrium). Some of these cancers contain squamous cells (squamous cells are flat, thin cells that can be found on the outer surface of the cervix), as well as glandular cells. A cancer with both types of cells is called an adenocarcinoma with squamous differentiation. If, under the microscope, the glandular cells look cancerous but the squamous cells don't, the tumor may be called an adenoacanthoma. If both the squamous cells and the glandular cells look malignant (cancerous), these tumors can be called adenosquamous carcinomas. There are other types of endometrioid cancers, such as secretory carcinoma, ciliated carcinoma, and mucinous adenocarcinoma.
The grade of an endometrioid cancer is based on how much the cancer forms glands that look similar to the glands found in normal, healthy endometrium. In lower-grade cancers, more of the cancerous tissue forms glands. In higher-grade cancers, more of the cancer cells are arranged in a haphazard or disorganized way and do not form glands.
- Grade 1 tumors have 95% or more of the cancerous tissue forming glands.
- Grade 2 tumors have between 50% and 94% of the cancerous tissue forming glands.
- Grade 3 tumors have less than half of the cancerous tissue forming glands. Grade 3 cancers are called "high-grade." They tend to be aggressive and have a poorer outlook than lower grade cancers (grades 1 and 2).
Some less common forms of endometrial adenocarcinoma are clear-cell carcinoma, serous carcinoma (also called papillary serous carcinoma), and poorly differentiated carcinoma. These cancers are more aggressive than most endometrial cancers. They tend to grow quickly and often have spread outside the uterus at the time of diagnosis.
Doctors sometimes divide endometrial carcinoma into 2 types based on their outlook and underlying causes. “Type 1” cancers are thought to be caused by excess estrogen. They are usually not very aggressive and are slow to spread to other tissues. Grades 1 and 2 endometrioid cancers are “type 1” endometrial cancers. A small number of endometrial cancers are “type 2.” Experts aren't sure what causes type 2 cancers, but they don't seem to be caused by too much estrogen. Serous carcinoma, clear-cell carcinoma, poorly differentiated carcinoma, and grade 3 endometrioid carcinoma are all type 2 cancers. These cancers don't look at all like normal endometrium and so are called "poorly differentiated" or “high-grade.” Because type 2 cancers are more likely to grow and spread outside of the uterus, they have a poorer outlook (than type 1 cancers). Doctors tend to treat these cancers more aggressively.
Uterine carcinosarcoma (CS) is another cancer that starts in the endometrium and is included in this document. When looked at under the microscope, this cancer has features of both endometrial carcinoma and sarcoma. In the past, CS was considered a type of uterine sarcoma, but many doctors now believe that CS may actually be a form of poorly differentiated carcinoma.
Uterine CS has many things in common with type 2 endometrial carcinoma. For example, they have similar risk factors. These cancers are also similar in how they spread and are treated. CSs are also known as malignant mixed mesodermal tumors or malignant mixed mullerian tumors (MMMTs). They make up about 4% of uterine cancers.
Cancer can also start in the supporting connective tissue (stroma) and muscle cells of the uterus. These cancers are called uterine sarcomas. They are much less common than endometrial carcinoma. These include:
- Stromal sarcomas, which start in the supporting connective tissue of the endometrium
- Leiomyosarcomas, which start in the myometrium or muscular wall of the uterus
These cancers are not discussed in this document because their treatment and prognosis (outlook) are different from the most common cancers of the endometrium. These cancers are discussed in our document Uterine Sarcoma.
Cancers that start in the cervix and then spread to the body of the uterus are different from cancers that start in the body of the uterus; the former are described in our document Cervical Cancer.
Last Medical Review: 11/04/2013
Last Revised: 01/08/2015