Endometrial cancer develops from the endometrium, the inner lining of the uterus. It is the most common form of uterine cancer: 19 out of 20 uterine cancers affect the endometrium.

The Endometrium

During a woman's menstrual cycle, the level of hormones released by the ovaries triggers changes in the endometrium. When a woman has her period, the lining of the uterus is shed. After each period, the endometrium grows thicker to replace that lost lining and prepare for a possible pregnancy.

Adenocarcinomas

Nearly all endometrial cancers are adenocarcinomas, cancers of glandular cells. There are several types of adenocarcinomas.

• Endometrioid adenocarcinomas are the most common type, accounting for about 75 percent of the adenocarcinomas that develop in the lining of the uterus. Endometrioid cancers involve cells from the glandular areas of the uterus as well as areas formed by squamous cells (the type of cells found on the surface of the cervix and the skin). These cancers are often diagnosed at an early stage and treated successfully.
• Adenosquamous carcinomas are tumours in which both the squamous and glandular cells are malignant (cancerous). These tend to be more aggressive than endometrioid adenocarcinomas.
• Papillary serous adenocarcinomas and clear cell adenocarcinomas are relatively rare. These often grow and spread more rapidly and are more likely to recur.

In addition to adenocarcinomas, there are less common uterine cancers, called uterine sarcomas, which can involve the endometrium. These include:

• stromal sarcomas, which develop in the stroma (supporting connective tissue) of the endometrium
• malignant mixed mesodermal tumours (carcinosarcomas), which involve both the glands and the stroma or the supporting connective tissue of the endometrium
• leiomyosarcomas, which start in the muscular wall of the uterus