Endometrioid Carcinoma of the Ovary: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
March 5, 2026

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Endometrioid carcinoma of the ovary is a type of ovarian cancer that resembles endometrioid carcinoma of the uterus when examined under the microscope. It develops from epithelial cells that line the ovary.

This tumour accounts for about 10% of ovarian carcinomas. Many cases are diagnosed at an early stage and are confined to the ovary at the time of diagnosis.

Endometrioid carcinoma of the ovary is often associated with endometriosis, a condition in which tissue similar to the lining of the uterus grows outside the uterus. In some patients, an endometrioid carcinoma of the ovary occurs at the same time as an endometrioid carcinoma of the uterus.

What are the symptoms of endometrioid carcinoma of the ovary?

Many patients with endometrioid carcinoma of the ovary develop symptoms related to a pelvic mass. These symptoms may include abdominal swelling, pelvic pressure, pelvic pain, or a feeling of abdominal fullness.

Some tumours are discovered incidentally during imaging or surgery performed for another reason.

Because ovarian tumours may grow for some time before causing symptoms, they can become relatively large before detection.

What causes endometrioid carcinoma of the ovary?

The exact cause of endometrioid carcinoma of the ovary is not fully understood. Most ovarian endometrioid carcinomas develop from endometriosis. In these cases, the tumour cells often share genetic changes with the surrounding endometriotic tissue, suggesting that the cancer develops from the endometriosis over time.

A smaller number of tumours arise from benign or borderline tumours called endometrioid adenofibromas.

Endometrioid carcinoma of the ovary may also occur in people with Lynch syndrome. This inherited condition increases the risk of several cancers, including cancers of the uterus, ovary, and colon.

Relationship to endometriosis

Endometriosis is a condition in which tissue similar to the lining of the uterus grows outside the uterus, often on the ovaries.

Most ovarian endometrioid carcinomas develop in areas of endometriosis. In these cases, the tumour may arise from an endometriotic cyst, sometimes called a “chocolate cyst” because it contains old blood.

Under the microscope, the tumour may appear as a polyp-like growth projecting into the cyst. Genetic studies have shown that the tumour and the surrounding endometriosis often share the same mutations, supporting the idea that the cancer develops from endometriotic tissue.

How is this diagnosis made?

The diagnosis of endometrioid carcinoma of the ovary usually begins when a mass in the ovary is identified by imaging or during surgery.

The tumour is removed and examined under the microscope by a pathologist. The pathologist studies the appearance of the tumour cells and their growth pattern to determine the type of ovarian cancer.

If surgery is performed, the pathologist also examines other tissues removed during the operation, such as the fallopian tubes, uterus, lymph nodes, and abdominal tissues. This examination helps determine how far the tumour has spread and is important for staging.

Microscopic features

Under the microscope, ovarian endometrioid carcinoma often resembles endometrioid carcinoma of the uterus.

Most tumours show a back-to-back arrangement of glands, meaning the tumour glands grow very close together with little normal tissue between them. The glands are lined by endometrioid-type epithelial cells with round-to-oval nuclei and mild-to-moderate atypia.

Solid growth may be present in some tumours, particularly in higher-grade cases.

About half of tumours show squamous differentiation, meaning that some areas of the tumour contain cells that resemble squamous cells. These areas often form rounded groups of cells called morules.

Other patterns may occasionally be seen, including mucinous differentiation, clear cell change, or unusual gland shapes.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside tumour cells. These tests help confirm the diagnosis and distinguish ovarian endometrioid carcinoma from other types of ovarian cancer.

Most ovarian endometrioid carcinomas show positive staining for estrogen receptor (ER) or progesterone receptor (PR).

Endometrioid carcinomas are usually negative for WT1, which helps distinguish them from high-grade serous carcinoma of the ovary. Most tumours also show a normal (wild-type) p53 staining pattern, whereas high-grade serous carcinoma typically shows abnormal p53 staining.

To distinguish endometrioid carcinoma from clear cell carcinoma, pathologists may use markers such as napsin A and PR.

Biomarkers

Biomarker testing examines proteins or genetic changes in tumour cells that may help guide treatment decisions. Some tests use immunohistochemistry to detect proteins in tumour cells, while others evaluate changes in tumour DNA.

Estrogen receptor (ER) and progesterone receptor (PR)

ER and PR are proteins that allow tumour cells to respond to the hormones estrogen and progesterone.

These markers are tested using immunohistochemistry and are usually reported as positive or negative, sometimes with a percentage showing how many tumour cells express the receptor.

Most ovarian endometrioid carcinomas are ER- or PR-positive.

Mismatch repair proteins (MMR)

Mismatch repair proteins help correct errors that arise during DNA replication.

Testing evaluates the proteins MLH1, PMS2, MSH2, and MSH6. Results are reported as retained expression (normal) or loss of expression (abnormal).

Loss of one or more mismatch repair proteins suggests mismatch repair deficiency, which may indicate Lynch syndrome and may predict response to immunotherapy.

p53

p53 is a protein that helps regulate cell growth and repair damaged DNA.

Results are reported as either wild-type (normal) or abnormal (mutant-type) expression. Most ovarian endometrioid carcinomas show wild-type p53 staining.

PD-L1

PD-L1 is a protein that can help tumour cells evade the immune system.

Testing is usually reported using a combined positive score (CPS). Tumours with PD-L1 expression may respond to immune checkpoint inhibitor therapy in some situations.

Folate receptor alpha (FOLR1)

Folate receptor alpha is a protein that helps transport folate into cells.

Testing is performed using immunohistochemistry. Tumours are considered positive when at least 75% of tumour cells show moderate or strong membrane staining.

Patients with FOLR1-positive ovarian cancer may be eligible for treatment with mirvetuximab soravtansine.

What are the subtypes of endometrioid carcinoma of the ovary?

Doctors have identified four molecular subtypes of ovarian endometrioid carcinoma based on genetic changes in the tumour.

• Hypermutated tumours: These tumours develop because of defects in the mismatch repair system. About 13% of cases fall into this group.
• Ultramutated tumours: These tumours have mutations in the POLE gene, which leads to a very high number of DNA mutations. About 5% of cases belong to this group.
• TP53-mutated tumours: These tumours have mutations in the TP53 gene, which normally helps control cell growth and repair DNA damage.
• No specific molecular profile (NSMP): The largest group, accounting for about 70% of ovarian endometrioid carcinomas.
• Other genetic changes are also common, including alterations affecting CTNNB1, PIK3CA, PTEN, KRAS, and ARID1A.

Other features to look for in your pathology report

Tumor spread

Pathologists examine the tumour to determine whether it has spread beyond the ovary.

Tumour cells may invade nearby structures such as the fallopian tube, uterus, or abdominal tissues. Tumour cells may also spread to the peritoneum, which lines the abdominal cavity.

The presence of tumour cells outside the ovary increases the stage of the tumour.

Intact versus ruptured ovary

Whether the ovary was intact or ruptured during surgery is important for staging. If the tumour is confined to the ovary and the capsule is intact, the cancer may be stage I. If the capsule is ruptured or tumour cells are present on the ovarian surface, the stage may be higher.

Lymphatic and vascular invasion

Lymphatic and vascular invasion means tumour cells are present inside small lymphatic channels or blood vessels. This finding increases the risk that tumour cells may spread to lymph nodes or distant organs.

Lymph nodes

Lymph nodes are small, bean-shaped structures in the lymphatic system. They help filter harmful substances from the body and play an important role in the immune system.

In ovarian cancer surgery, lymph nodes from the pelvis and abdomen may be removed and examined under the microscope. These are called regional lymph nodes. They include pelvic lymph nodes (such as obturator, internal iliac, external iliac, common iliac, and sacral nodes) and para-aortic lymph nodes.

If tumour cells are found in these lymph nodes, the cancer is considered to have spread beyond the ovary, and the tumour stage increases. Lymph node involvement may also influence treatment decisions, such as the use of chemotherapy or other systemic therapies.

When tumour cells are found in lymph nodes, the pathology report often describes the size of the tumour deposits. The size helps doctors determine the extent of lymph node involvement.

Isolated tumour cells (ITCs)

These are very small clusters of tumour cells measuring 0.2 mm or less. When only isolated tumour cells are present, the lymph nodes are often reported as N0(i+), meaning that only very small deposits of tumour cells were found.

Small lymph node metastases

These tumour deposits measure more than 0.2 mm but 10 mm or less. These are considered true lymph node metastases, indicating that the cancer has spread to the lymph nodes.

Large lymph node metastases

These tumour deposits measure more than 10 mm. Larger tumour deposits generally indicate greater tumour involvement of the lymph node.

Your pathology report may also describe:

• The number of lymph nodes examined.

• The number of lymph nodes containing tumour cells.

• The location of the involved lymph nodes.

• The size of the largest tumour deposit.

These findings are important because they help determine the pathologic stage of the tumour, which guides treatment decisions and helps estimate prognosis.

How do doctors stage ovarian cancer?

Staging describes how far a cancer has spread in the body. For ovarian cancer, two main systems are used: the TNM and FIGO systems. Both are internationally accepted and provide important information about prognosis (the expected outcome) and treatment planning.

The TNM system

The TNM system was developed by the American Joint Committee on Cancer (AJCC). It looks at three main factors:

• T (tumour): Describes the size of the tumour and how far it has spread in or around the ovary or fallopian tube.

• N (lymph nodes): Describes whether cancer cells have spread to nearby lymph nodes.

• M (metastasis): Describes whether cancer has spread to distant parts of the body.

Breakdown of the T stage:

• T1: The tumour is limited to one or both ovaries or fallopian tubes.

  • T1a: Tumour is inside one ovary or fallopian tube, with the outer surface intact and no cancer cells in fluid taken from the abdomen.

  • T1b: Tumour is inside both ovaries or fallopian tubes, but the outer surfaces are intact, and no cancer cells are found in the fluid.

  • T1c: The tumour is limited to one or both ovaries or tubes, but there has been a rupture, tumour on the outer surface, or cancer cells found in abdominal fluid.

• T2: The tumour has grown into tissues in the pelvis, such as the uterus or bladder.

  • T2a: Spread to the uterus or other fallopian tube or ovary.

  • T2b: Spread to other pelvic tissues.

• T3: The tumour has spread beyond the pelvis into the abdomen or to regional lymph nodes.

  • T3a: Cancer cells are found microscopically outside the pelvis or in nearby lymph nodes.

  • T3b: Visible tumour deposits up to 2 cm outside the pelvis or in nearby lymph nodes.

  • T3c: Visible tumour deposits larger than 2 cm outside the pelvis or involving the capsule of the liver or spleen (without entering the organ itself).

Breakdown of the N stage:

• N0: No cancer cells are seen in regional lymph nodes.

• N0(i+): Only isolated tumour cells smaller than 0.2 mm are seen in the lymph nodes.

• N1: Cancer cells are found in regional lymph nodes.

  • N1a: Deposits up to 10 mm.

  • N1b: Deposits larger than 10 mm.

The FIGO system

The FIGO (International Federation of Gynecology and Obstetrics) system is specifically designed for gynecologic cancers like ovarian cancer. It uses similar criteria to the TNM system but is grouped into broader stages that are easier to interpret clinically.

Breakdown of FIGO stages:

• Stage I: Cancer is limited to the ovaries or fallopian tubes.

  • IA: In one ovary or fallopian tube only.

  • IB: In both ovaries or fallopian tubes.

  • IC: Cancer is still limited to the ovaries or tubes, but there has been a rupture, tumour on the surface, or cancer cells found in fluid.

• Stage II: Cancer involves one or both ovaries or tubes with spread to pelvic organs such as the uterus, bladder, or rectum.

  • IIA: Spread to the uterus or other ovary/tube.

  • IIB: Spread to other pelvic tissues.

• Stage III: Cancer has spread outside the pelvis into the abdominal cavity or to regional lymph nodes.

  • IIIA1: Cancer in lymph nodes only.

  • IIIA2: Microscopic spread outside the pelvis.

  • IIIB: Visible spread outside the pelvis up to 2 cm.

  • IIIC: Visible spread larger than 2 cm or spread to the capsule of the liver or spleen.

• Stage IV: Cancer has spread to distant organs outside the abdomen.

  • IVA: Cancer cells are found in the fluid around the lungs.

  • IVB: Cancer has spread to organs such as the liver, spleen, or lymph nodes outside the abdomen.

Why staging is important

Both TNM and FIGO staging systems provide doctors with essential information about how far the cancer has spread. This helps guide treatment choices, such as whether surgery alone is enough or if chemotherapy or other treatments are needed.

Staging also helps predict prognosis. Early-stage disease (stage I) has a much better survival rate compared to advanced-stage disease (stage III or IV). By using staging information, doctors can personalize care and discuss treatment options and expectations with patients.

Questions to ask your doctor

• What stage is my ovarian cancer?

• Was the tumour confined to the ovary or had it spread beyond the ovary?

• Was the ovarian capsule intact or ruptured?

• Were lymph nodes involved?

• Were biomarker tests performed, and what do the results mean?

• Do my biomarker results affect treatment options?