by Jason Wasserman MD PhD FRCPC
April 16, 2026
Endometrioid carcinoma of the ovary is a type of ovarian cancer whose cells resemble the cells of endometrioid carcinoma of the uterus when examined under the microscope. It develops from epithelial cells and accounts for about 10% of all ovarian carcinomas. Unlike the more common high-grade serous carcinoma, most endometrioid carcinomas of the ovary are diagnosed at an early stage and are still confined to the ovary at the time of diagnosis. This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.
What are the symptoms?
Many patients with endometrioid carcinoma of the ovary develop symptoms related to a pelvic mass. These may include abdominal swelling, pelvic pressure, pelvic or abdominal pain, or a feeling of fullness. Some tumors are discovered incidentally during imaging or surgery performed for another reason. Because ovarian tumors may grow for some time before causing symptoms, they can become relatively large before they are detected.
What causes endometrioid carcinoma of the ovary?
The exact cause is not fully understood. The majority of ovarian endometrioid carcinomas develop from endometriosis — a condition in which tissue similar to the lining of the uterus grows outside the uterus, commonly on or within the ovaries. In these cases, the cancer cells often share genetic mutations with the surrounding endometriotic tissue, suggesting that the cancer develops gradually from the endometriosis over time. The tumor may arise from an endometriotic cyst (sometimes called a “chocolate cyst” because it contains old blood), appearing as a polyp-like growth projecting into the cyst.
A smaller number of tumors arise from benign or borderline tumors called endometrioid adenofibromas.
Endometrioid carcinoma of the ovary may also occur in people with Lynch syndrome, an inherited condition caused by mutations in DNA mismatch repair genes. Lynch syndrome increases the lifetime risk of several cancers, including cancers of the uterus, ovary, and colon.
Relationship to endometrial cancer
In about 15–20% of cases, endometrioid carcinoma of the ovary occurs at the same time as an endometrioid carcinoma of the uterus. When both tumors are present, pathologists must determine whether one cancer spread from one organ to the other, or whether two separate primary cancers developed independently. This distinction has important implications for staging and treatment.
In most cases where both ovarian and uterine endometrioid carcinomas are found together, molecular studies show they are two independent primary tumors rather than one cancer that spread. This is because endometrioid carcinoma of the ovary often arises from endometriosis in the ovary, which is a separate process from the development of endometrial cancer in the uterus. Independent synchronous primaries are generally staged separately and carry a more favorable prognosis than metastatic disease.
How is the diagnosis made?
The diagnosis is usually made after a tissue sample is examined under the microscope by a pathologist. The sample is most often obtained when a mass in the ovary is removed at surgery. In some cases, a biopsy of an abdominal mass is performed before definitive surgery to confirm the diagnosis. If surgery is performed, the pathologist examines all removed tissues — including the fallopian tubes, uterus, lymph nodes, omentum, and any additional abdominal samples — to determine how far the tumor has spread.
Under the microscope, ovarian endometrioid carcinoma closely resembles endometrioid carcinoma of the uterus. Most tumors show crowded, back-to-back glands — meaning the tumor glands are packed together with very little normal tissue between them. The glands are lined by cells with round-to-oval nuclei and mild-to-moderate atypia (abnormality). In higher-grade tumors, solid sheets of cells replace the gland-forming pattern. About half of tumors show squamous differentiation — areas where some tumor cells resemble squamous cells, often forming rounded clusters called morules. Other occasional features include mucinous differentiation or clear cell change.
To confirm the diagnosis and distinguish endometrioid carcinoma from other ovarian cancer types, the pathologist uses immunohistochemistry (IHC) — a technique that uses antibodies to detect specific proteins in tumor cells. Most ovarian endometrioid carcinomas are positive for estrogen receptor (ER) and/or progesterone receptor (PR), and are negative for WT1 (a protein strongly expressed in high-grade serous carcinoma). Most tumors show a normal (wild-type) p53 staining pattern, in contrast to high-grade serous carcinoma, which typically shows abnormal p53. When distinguishing endometrioid carcinoma from clear cell carcinoma, the pathologist may also use markers such as napsin A (typically positive in clear cell carcinoma and negative in endometrioid carcinoma) and PR.
Once the cancer is confirmed, imaging — typically CT of the abdomen and pelvis — is performed to determine the extent of disease and guide staging and treatment planning.
Histologic grade
Ovarian endometrioid carcinoma is assigned a histologic grade based on the proportion of the tumor that grows as solid sheets of cells rather than as recognizable glands. The grading system used is the FIGO (International Federation of Gynecology and Obstetrics) system, the same system used for endometrioid carcinoma of the uterus:
- FIGO grade 1 (well differentiated) — Less than 5% of the tumor grows as solid sheets. The tumor forms well-developed glands and tends to behave in a relatively predictable, less aggressive way.
- FIGO grade 2 (moderately differentiated) — Between 6% and 50% of the tumor grows as solid sheets. These tumors have intermediate behavior between grade 1 and grade 3.
- FIGO grade 3 (poorly differentiated) — More than 50% of the tumor grows as solid sheets. Solid growth is associated with more aggressive behavior and a higher risk of recurrence. Note: if the tumor shows marked nuclear atypia (very abnormal nuclei) that seems out of proportion to the amount of solid growth, the grade may be increased by one.
Squamous areas are not counted when calculating the grade — only non-squamous solid growth is used. The histologic grade is an important factor in treatment planning and helps predict the likelihood of spread and recurrence.
Tumor spread
The pathologist examines all tissue samples to determine whether the tumor has spread beyond the ovary. Tumor cells may invade nearby structures such as the fallopian tube, uterus, or surrounding pelvic tissues. Tumor cells may also spread to the peritoneum — the thin lining of the abdominal cavity — or to the omentum. Because most ovarian endometrioid carcinomas are diagnosed at an early stage, spread beyond the ovary is less common than in high-grade serous carcinoma, but it still occurs and must be carefully assessed.
Ovarian capsule status
The outer covering of the ovary is called the capsule. The pathologist will note whether the capsule is intact or ruptured, and whether tumor is present on the outer surface of the ovary. These findings affect the stage:
- Intact capsule, no surface tumor — Suggests the cancer may still be contained within the ovary, which is associated with an earlier stage and better prognosis.
- Ruptured capsule or tumor on the surface — Even if no other spread is found, capsule rupture or surface involvement increases the stage.
- Intraoperative rupture — If the capsule ruptures during surgery rather than before, this is noted separately and also affects staging.
Lymphovascular invasion
Lymphovascular invasion means that tumor cells have been found inside small blood vessels or lymphatic channels within the tissue. This finding suggests that tumor cells have had an opportunity to travel to lymph nodes or distant sites. Its presence can influence staging and treatment planning.
Lymph nodes
Lymph nodes are small, bean-shaped structures that help filter the body’s lymphatic fluid and support the immune system. In ovarian cancer surgery, lymph nodes from the pelvis and along the major abdominal blood vessels (para-aortic nodes) are often removed and examined. If tumor cells are found in the lymph nodes, the cancer is considered to have spread beyond the ovary and the stage increases.
The pathology report will describe the total number of lymph nodes examined, the number containing tumor cells, the size of the largest deposit, and the location of any involved nodes. Lymph node deposits are classified by size:
- Isolated tumor cells (pN0(i+)) — Clusters measuring 0.2 mm or less; not counted as definitive nodal metastases in all staging systems.
- Small metastases (pN1a) — Deposits measuring more than 0.2 mm and up to 10 mm.
- Large metastases (pN1b) — Deposits measuring more than 10 mm.
Biomarker and molecular testing
Biomarker testing in ovarian endometrioid carcinoma examines proteins and genetic changes in tumor cells that help guide treatment decisions, identify inherited cancer risk, and — increasingly — classify tumors into molecular subtypes that predict behavior and response to specific therapies.
Mismatch repair proteins (MMR) and POLE mutation testing
Mismatch repair (MMR) proteins — MLH1, PMS2, MSH2, and MSH6 — work together to fix small copying errors that arise when cells divide and duplicate their DNA. When one or more of these proteins is absent or non-functional, errors accumulate throughout the genome, and the tumor is described as mismatch repair–deficient (dMMR) or microsatellite instability–high (MSI-H).
Mismatch repair deficiency occurs in approximately 13% of ovarian endometrioid carcinomas — a substantially higher rate than in high-grade serous carcinoma. This has two important implications. First, dMMR/MSI-H tumors may be eligible for immunotherapy with pembrolizumab (Keytruda), which is approved across multiple tumor types for dMMR/MSI-H solid tumors that have progressed despite prior treatment. Second, dMMR may indicate Lynch syndrome — an inherited condition caused by a germline mutation in one of the MMR genes. Lynch syndrome significantly raises the lifetime risk of ovarian, uterine, colorectal, and other cancers, and its implications extend to blood relatives who may not yet know they are at risk.
Testing is performed by immunohistochemistry on tumor tissue. Results are reported as retained expression (normal) or loss of expression (abnormal) for each of the four MMR proteins. When MLH1 and PMS2 are both lost, additional testing — for a BRAF V600E mutation or MLH1 promoter methylation — is typically performed to distinguish sporadic, non-inherited loss from Lynch syndrome. Loss of MSH2 or MSH6, or MLH1/PMS2 loss that is not explained by methylation or BRAF mutation, should prompt referral to a genetic counsellor for Lynch syndrome assessment.
A related but distinct group of tumors carries mutations in the POLE gene, which encodes a protein responsible for proofreading DNA during copying. POLE-mutated tumors accumulate an extremely high number of mutations (sometimes called ultramutated), but — paradoxically — tend to behave less aggressively than their mutation burden might suggest. POLE-mutated ovarian endometrioid carcinomas account for approximately 5% of cases and are associated with an excellent prognosis even when high grade. POLE mutation status is typically assessed by molecular sequencing. When a POLE mutation is identified, immunotherapy may also be beneficial, as the high mutation burden makes these tumors more visible to the immune system.
Estrogen receptor (ER) and progesterone receptor (PR)
ER and PR are proteins that allow cells to respond to the hormones estrogen and progesterone. Most ovarian endometrioid carcinomas are positive for ER and/or PR — a finding that helps confirm the diagnosis and distinguish this tumor type from other ovarian cancers. In the pathology report, ER and PR are tested by immunohistochemistry and typically reported as positive or negative, sometimes with a percentage reflecting how many tumor cells express the receptor.
In ovarian endometrioid carcinoma, ER and PR positivity is primarily a diagnostic and prognostic marker. The role of hormone-blocking therapies (such as aromatase inhibitors or letrozole) in ovarian endometrioid carcinoma is more limited than in breast cancer, but hormonal therapy may be considered in some patients with recurrent or advanced disease who are not candidates for chemotherapy. Your oncologist will advise you on whether this option is relevant to your situation.
p53
p53 is a protein that controls cell growth and helps repair DNA damage. It is produced by the TP53 gene. In ovarian endometrioid carcinoma, most tumors show a normal (wild-type) p53 staining pattern, in contrast to high-grade serous carcinoma, where p53 is almost always abnormal. An abnormal p53 result in an otherwise typical endometrioid carcinoma may indicate that the tumor is higher grade, more aggressive, or has features overlapping with serous carcinoma. p53 testing is performed by immunohistochemistry and results are reported as wild-type (normal) or mutant-type (abnormal).
PD-L1
PD-L1 is a protein that some tumor cells use to shield themselves from the immune system. Testing is performed by immunohistochemistry and is typically reported as a Combined Positive Score (CPS), which counts PD-L1-positive tumor and immune cells relative to the total tumor cell count. In ovarian cancer, PD-L1 testing is most relevant in the setting of advanced or recurrent disease, where immunotherapy with agents such as pembrolizumab may be considered. PD-L1 results are interpreted alongside other clinical and molecular factors rather than used as a standalone treatment decision marker.
Folate receptor alpha (FOLR1)
Folate receptor alpha (FOLR1) is a protein found on the surface of some cancer cells that helps transport folate — a B vitamin — into the cell. Testing is performed by immunohistochemistry. A tumor is considered FOLR1-positive when at least 75% of viable tumor cells show moderate-to-strong staining on the cell membrane. Patients with FOLR1-positive ovarian cancer may be eligible for treatment with mirvetuximab soravtansine (Elahere), an antibody-drug conjugate approved for platinum-resistant ovarian cancer. FOLR1 expression rates in endometrioid carcinoma are generally lower than in high-grade serous carcinoma, but testing is performed because it identifies patients who may benefit from this treatment option.
For more information about biomarker testing in ovarian cancer, see the Biomarkers and Molecular Testing section.
Pathologic stage (pTNM)
Staging describes how far the cancer has spread. For ovarian cancer, the pathologic stage is based on the AJCC TNM system, which closely corresponds to the FIGO staging system used by gynecologic oncologists. The stage is determined by three components: T (local tumor extent), N (lymph node involvement), and M (distant spread). M stage is determined by imaging and is not typically assigned in the pathology report unless distant spread was sampled at surgery.
Tumor stage (pT)
- pT1 (FIGO Stage I) — Tumor is limited to one or both ovaries or fallopian tubes.
- pT1a — Tumor in one ovary or fallopian tube only; capsule intact; no tumor cells in abdominal fluid.
- pT1b — Tumor involves both ovaries or fallopian tubes; capsules intact; no tumor cells in abdominal fluid.
- pT1c — Tumor limited to ovary/fallopian tube but with capsule rupture, tumor on the outer surface, or cancer cells in abdominal fluid or washings.
- pT2 (FIGO Stage II) — Tumor has spread beyond the ovaries or fallopian tubes into the pelvis.
- pT2a — Spread to the uterus, the other fallopian tube, or the other ovary.
- pT2b — Spread to other pelvic tissues such as the bladder or rectum.
- pT3 (FIGO Stage III) — Tumor has spread beyond the pelvis to the peritoneum or regional lymph nodes.
- pT3a — Microscopic spread to the peritoneum outside the pelvis, with or without regional lymph node involvement.
- pT3b — Visible tumor deposits up to 2 cm on the peritoneum outside the pelvis, with or without lymph node involvement.
- pT3c — Visible tumor deposits larger than 2 cm outside the pelvis, or spread to the outer surface (capsule) of the liver or spleen, with or without lymph node involvement.
Note: Spread inside the substance of the liver or spleen (rather than only on their surface) is classified as M1 (Stage IVB).
Nodal stage (pN)
- pN0 — No cancer cells found in regional lymph nodes.
- pN0(i+) — Only isolated tumor cells (0.2 mm or less) found in lymph nodes; not counted as definitive metastases in all staging systems.
- pN1 — Cancer cells present in regional lymph nodes.
- pN1a — Tumor deposits up to 10 mm.
- pN1b — Tumor deposits larger than 10 mm.
What is the prognosis?
The prognosis for ovarian endometrioid carcinoma is generally more favorable than for high-grade serous carcinoma, primarily because most cases are diagnosed at an early stage. Overall, stage is the most important prognostic factor. Approximate five-year survival rates are:
- Stage I — 80–90%. The majority of patients with ovarian endometrioid carcinoma are diagnosed at this stage.
- Stage II — 65–75%.
- Stage III — 35–55%.
- Stage IV — 15–25%.
Beyond stage, the following features influence prognosis:
- Histologic grade — Grade 1 tumors have a significantly better prognosis than grade 3. Grade 3 ovarian endometrioid carcinoma is associated with a higher rate of recurrence and poorer survival.
- POLE mutation — POLE-mutated tumors have an excellent prognosis even when high grade, with very low rates of recurrence.
- Mismatch repair status — dMMR/MSI-H tumors may also have a more favorable prognosis in the context of immunotherapy eligibility.
- Lymphovascular invasion — The presence of tumor cells in blood vessels or lymphatic channels is associated with a higher risk of spread.
- Capsule status — Capsule rupture before or during surgery is associated with a higher stage and a greater risk of recurrence.
- Synchronous uterine cancer — When an independent synchronous uterine endometrioid carcinoma is also present, outcomes are generally still favorable if both tumors are low grade and early stage, as the two cancers are staged and treated separately.
What happens after the diagnosis?
Treatment is planned by a multidisciplinary team that typically includes a gynecologic oncologist, medical oncologist, radiation oncologist, pathologist, and radiologist. The approach depends on the stage, grade, and molecular features of the tumor, as well as the patient’s overall health.
Surgery is the cornerstone of treatment and typically involves removal of both ovaries, both fallopian tubes, the uterus, and the omentum (total abdominal hysterectomy with bilateral salpingo-oophorectomy and omentectomy). For early-stage, low-grade tumors, surgery alone may be sufficient. For younger patients who wish to preserve fertility and have a low-grade, stage IA tumor, a more limited surgical approach may be discussed with the gynecologic oncologist.
For patients with higher-stage or higher-grade disease, chemotherapy with carboplatin and paclitaxel is typically recommended after surgery. Radiation therapy to the pelvis may also be considered in some cases, particularly for stage II disease. The specific treatment plan will depend on the full pathology results, including grade, stage, capsule status, and biomarker findings.
Patients whose tumors carry mismatch repair deficiency may be eligible for immunotherapy with pembrolizumab in the recurrent or advanced setting, and this option should be discussed with the oncologist. Patients whose tumors show POLE mutations may have a particularly favorable course and may be candidates for treatment de-escalation in the future as clinical evidence continues to build.
All patients with ovarian endometrioid carcinoma should be assessed for Lynch syndrome risk, particularly those whose tumors show MMR deficiency. Referral to a genetic counsellor is recommended when MMR deficiency is identified or when there is a personal or family history suggestive of a hereditary cancer syndrome.
Follow-up after treatment typically involves regular clinical assessments and CA-125 monitoring, with imaging performed if there are signs of potential recurrence.
Questions to ask your doctor
- What is the stage and grade of my ovarian cancer, and what do they mean for my prognosis?
- Has the tumor spread beyond the ovary — to the lymph nodes, peritoneum, or other organs?
- Was the ovarian capsule intact, or did it rupture before or during surgery?
- Was mismatch repair testing performed, and were any of the MMR proteins lost?
- If MMR deficiency was found, do I need testing for Lynch syndrome, and should my family members be tested?
- Was POLE mutation testing performed, and what were the results?
- Was FOLR1 testing performed, and does the result affect my treatment options?
- Do I have a synchronous uterine cancer as well, and if so, how are the two cancers staged and treated?
- Do I need chemotherapy or radiation after surgery, based on the grade, stage, and biomarker results?
- Am I eligible for immunotherapy based on the MMR or PD-L1 results?
- What follow-up schedule do you recommend after I complete treatment?
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