by Jason Wasserman MD PhD FRCPC
February 7, 2025
Endometrial endometrioid carcinoma is a type of cancer that starts in the endometrium, the inner lining of the uterus. It is the most common type of endometrial cancer and typically affects women over the age of 50 years. Endometrial endometrioid adenocarcinoma is believed to develop from a pre-cancerous condition called atypical endometrial hyperplasia.
The most common symptoms of endometrial endometrioid carcinoma include:
If you experience any of these symptoms, it’s important to consult a doctor for further evaluation.
The exact cause of endometrial endometrioid carcinoma is not fully understood, but several risk factors may contribute to its development, including:
A biopsy is used to diagnose endometrial endometrioid carcinoma. A small tissue sample is taken from the endometrium and examined under a microscope by a pathologist. The pathologist looks for abnormal cells and their growth patterns, which helps confirm the diagnosis.
When examined under a microscope, endometrial endometrioid carcinoma displays a variety of growth patterns, reflecting the abnormal way the tumour cells are organized. The tumour typically shows a combination of glandular, papillary, and solid growth:
In addition to these patterns, squamous differentiation is common in endometrial endometrioid carcinoma. This means that some areas of the tumour start to look like squamous cells, which are flat cells that typically line the surface of certain tissues, such as the skin. Squamous differentiation can give parts of the tumour a more solid appearance and is a feature frequently observed in this type of cancer.
The FIGO grade is a system pathologists uses to assess how much the tumour cells in endometrial endometrioid carcinoma differ from normal endometrial cells. This assessment is done by examining the tumour under a microscope and determining the percentage of non-squamous solid growth (areas where the tumour cells form solid clusters rather than the organized glands seen in healthy tissue).
The FIGO grade is important because it helps guide treatment decisions and provides information about the tumour’s likely behaviour. Higher-grade tumours grow more quickly and have a greater chance of spreading (metastasizing), while lower-grade tumours are generally less aggressive.
The FIGO system divides tumours into two main categories:
Mismatch repair proteins (MMR) are a system inside normal, healthy cells that fix mistakes in our genetic material (DNA). The system comprises different proteins, the four most common being MSH2, MSH6, MLH1, and PMS2. The four MMR proteins work in pairs to fix damaged DNA. Specifically, MSH2 works with MSH6, and MLH1 works with PMS2. If one protein is lost, the pair cannot function normally, and the risk of developing cancer increases.
The most common way to test for mismatch repair proteins is immunohistochemistry. This test allows pathologists to see if the tumour cells produce all four mismatch repair proteins. The results of this test are typically reported as follows:
Mismatch repair testing is important because it can help predict how well specific treatments may work. For instance, cancers with a loss of mismatch repair protein expression are more likely to respond to immunotherapy treatments like PD-1 or PD-L1 inhibitors. This is because the many mutations often found in deficient tumours can produce new antigens that make the tumour more visible and vulnerable to the immune system.
Mismatch repair testing is also performed to identify patients who may have Lynch syndrome, also known as hereditary nonpolyposis colorectal cancer (HNPCC). Lynch syndrome is a genetic disorder that increases the risk of developing various types of cancer, including esophageal cancer, colon cancer, endometrial cancer, ovarian cancer, and stomach cancer.
Next-generation sequencing (NGS) may be performed to look for genetic changes in endometrial endometrioid carcinoma. This test examines multiple genes at once to identify mutations that may affect prognosis or guide treatment decisions. However, next-generation sequencing is not performed in all cases, and the genes assessed may vary depending on the institution.
Mutations in CTNNB1 are commonly found in low grade endometrial endometrioid carcinoma. These mutations may indicate a tumour with a higher risk of recurrence, even in early-stage disease. The result is typically reported as mutated or wild-type (normal).
KRAS mutations occur in a subset of endometrial endometrioid carcinomas. They may be associated with more aggressive tumour behaviour and resistance to certain therapies.
PIK3CA is involved in cell growth and survival. Mutations in PIK3CA are frequently found in endometrial endometrioid carcinoma and may influence how the tumour responds to specific targeted therapies.
Mutations in POLE are seen in a subset of endometrial endometrioid carcinomas and are associated with an excellent prognosis and a low risk of recurrence. These tumours typically have many mutations, which may make them more responsive to the body’s immune system.
PTEN is a tumour suppressor gene that helps regulate cell growth. Mutations in PTEN are very common in endometrial endometrioid carcinoma and are considered an early genetic event in tumour development.
Mutations in p53 are rare in low-grade endometrial endometrioid carcinoma but are frequently found in high grade tumours. An abnormal p53 result suggests a more aggressive tumour, and these tumours may be treated similarly to serous carcinoma.
The myometrium is the thick muscular layer of the uterus. Myometrial invasion occurs when the cancer spreads from the inner lining of the uterus (the endometrium) into the myometrium. The depth of myometrial invasion is important because the more deeply the tumour invades, the higher the risk of spreading to other body parts.
Most pathology reports for endometrial endometrioid carcinoma will describe the amount of myometrial invasion in millimetres and as a percentage of the total myometrial thickness. This information is used to stage the tumour and to plan treatment.
Cervical stromal invasion means that the cancer has spread from the body of the uterus into the cervix, which is the lower part of the uterus that connects to the vagina. This type of invasion indicates a more advanced stage of cancer and may influence treatment decisions, such as the need for more extensive surgery or radiation therapy.
The uterus is closely connected to several other organs and tissues, such as the ovaries, fallopian tubes, vagina, bladder, and rectum. The term “adnexa” refers to the fallopian tubes, ovaries, and ligaments directly linked to the uterus. As a tumour grows, it can spread into any of these organs or tissues. In such cases, some parts of these organs or tissues may have to be removed along with the uterus. A pathologist will thoroughly examine these organs or tissues for tumour cells, and the findings will be detailed in your pathology report. The presence of tumour cells in other organs or tissues is significant, as it raises the pathologic tumour stage and is linked with a poorer prognosis.
Lymphatic invasion occurs when cancer cells enter the lymphatic system, a network of vessels that helps fight infection. Vascular invasion refers to cancer cells entering the blood vessels. Both lymphatic and vascular invasion are important because they indicate that the cancer is more likely to spread (metastasize) to other body parts, including lymph nodes and distant organs. These findings are often included in a pathology report to help guide treatment decisions.
A margin refers to the edge of the tissue removed during surgery, such as a hysterectomy. After the surgery, pathologists examine the margins of the tissue under a microscope to check for any remaining cancer cells. In the case of endometrial endometrioid carcinoma, several specific margins are carefully evaluated:
If any of these margins contain cancer cells, it is referred to as a positive margin, which may mean that some tumour cells were left behind after surgery. A negative margin means no cancer cells were found at the edges, suggesting that the tumour was completely removed. Clear margins are important for reducing the risk of the cancer returning, and positive margins may lead to recommendations for additional treatments, such as radiation therapy.
Lymph nodes are small, bean-shaped structures that are part of the lymphatic system, which helps fight infection and remove waste from the body. Lymph nodes contain immune cells that filter lymph fluid, which travels through lymphatic vessels, and help trap harmful substances like bacteria or cancer cells. Lymph nodes are located throughout the body, including in the pelvis and abdomen, close to the uterus.
In the context of endometrial endometrioid carcinoma, lymph nodes are examined because this type of cancer has a higher risk of spreading beyond the uterus, particularly to nearby lymph nodes. For this reason, your surgeon may remove lymph nodes from the pelvis or abdomen, which are then sent to the pathologist for examination under a microscope. This is done to check for the presence of metastatic cancer (cancer that has spread from the primary tumour to other areas of the body).
Examining lymph nodes is important for several reasons:
Pathologists use the term ‘isolated tumour cells’ to describe a group of tumour cells that measures 0.2 mm or less and is found in a lymph node. If only isolated tumour cells are found in all the lymph nodes examined, the pathologic nodal stage is pN1mi.
A ‘micrometastasis’ is a group of tumour cells measuring from 0.2 mm to 2 mm that is found in a lymph node. If only micrometastases are found in all the lymph nodes examined, the pathologic nodal stage is pN1mi.
A ‘macrometastasis’ is a group of tumour cells measuring more than 2 mm and found in a lymph node. Macrometastases are associated with a worse prognosis and may require additional treatment.
The pathologic stage for endometrial endometrioid carcinoma is based on the TNM staging system, an internationally recognized system created by the American Joint Committee on Cancer. This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M) to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means a more advanced disease and a worse prognosis.
Endometrial endometrioid carcinoma is given a tumour stage between T1 and T4 based on the depth of myometrial invasion and growth of the tumour outside of the uterus.
Based on the examination of lymph nodes from the pelvis and abdomen, endometrial endometrioid carcinoma is given a nodal stage from N0 to N2.
The FIGO staging system, developed by the International Federation of Gynecology and Obstetrics, is a standardized way of classifying endometrial cancers based on how far they have spread. This system is important because it helps doctors determine the extent of the cancer, plan appropriate treatment, and estimate the prognosis (the likely disease outcome).