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Endometrial cancer starts when cells in the inner lining of the uterus (endometrium) begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?

The picture below shows where the uterus is located.

About the uterus and endometrium

The uterus is a hollow organ, which is normally 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 image below):

  • 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 cancer of 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.

During a woman's menstrual cycle, hormones 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 prepares the innermost layer of the lining 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 until the woman’s goes through menopause (change of life).

Types of cancers of the uterus and endometrium

The 2 main types of cancer of the uterus are:

  • Uterine sarcomas, which start in the muscle layer (myometrium) or supporting connective tissue of the uterus. These include uterine leiomyosarcomas and endometrial stromal sarcomas. These cancers are not covered here, but are discussed in detail in Uterine Sarcomas.
  • Endometrial carcinomas, which start in the cells of the inner lining of the uterus (the endometrium). Nearly all cancers of the uterus are this type. These cancers are the focus of the remainder of this information.

Endometrial carcinomas can be divided into different types based on how the cells look under the microscope (histologic types). These include:

  • Adenocarcinoma, (most endometrial cancers are adenocarcinomas)
  • Carcinosarcoma (discussed more below)
  • Squamous cell carcinoma
  • Undifferentiated carcinoma
  • Small cell carcinoma
  • Transitional carcinoma

The most common type of adenocarcinoma is endometrioid cancer. Endometrioid cancers are made up of cells in glands that look much like the normal uterine lining (endometrium). Some of these cancers have squamous cells (squamous cells are flat, thin cells that can be found on the outer surface of the cervix), as well as glandular cells.

There are many variants (or sub-types) of endometrioid cancers including:

  • Adenocarcinoma, (with squamous differentiation)
  • Adenoacanthoma
  • Adenosquamous (or mixed cell)
  • Secretory carcinoma
  • Ciliated carcinoma
  • Villoglandular adenocarincoma

Clear-cell carcinoma, mucinous adenocarcinoma, and papillary serous adenocarcinoma are less common types of endometrial adenocarcinomas. These types tend to be more aggressive than most endometrial cancers. They tend to grow quickly and often have spread outside the uterus at the time of diagnosis.

Grading endometrial carcinomas

The grade of an endometrial cancer is based on how much the cancer forms glands that look similar to those found in normal, healthy endometrium.

In lower-grade cancers (grades 1 and 2), more of the cancerous tissue forms glands.

In higher-grade cancers (grade 3), 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 tend to be aggressive and have a poorer outlook than lower-grade cancers.

Grades 1 and 2 endometrioid cancers are type 1 endometrial cancers. Type 1 cancers are usually not very aggressive and do not spread to other tissues quickly. Type 1 endometrial cancers are thought to be caused by excess estrogen. They sometimes develop from atypical hyperplasia, an abnormal overgrowth of cells in the endometrium (see the risk factors section).

A small number of endometrial cancers are type 2 endometrial cancer . Type 2 cancers are more likely to grow and spread outside the uterus, they have a poorer outlook (than type 1 cancers). Doctors tend to treat these cancers more aggressively. They don’t seem to be caused by too much estrogen. Type 2 cancers include all endometrial carcinomas that aren’t type 1, such as papillary serous carcinoma, clear-cell carcinoma, undifferentiated carcinoma, and grade 3 endometrioid carcinoma. These cancers don’t look at all like normal endometrium and so are called poorly differentiated or high-grade.

Uterine carcinosarcoma (CS) starts in the endometrium and has features of both endometrial carcinoma and sarcoma. In the past, CS was considered a type of uterine sarcoma, but doctors now believe that CS is a carcinoma that is abnormal and so no longer looks much like the cells it came from (poorly differentiated).

Uterine CS is considered a type 2 endometrial carcinoma. CS tumors are also known as malignant mixed mesodermal tumors or malignant mixed mullerian tumors (MMMTs). They make up about 4% of uterine cancers.

Cervical cancers

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; they are described in Cervical Cancer.

Last Medical Review: 02/10/2016
Last Revised: 02/29/2016