Endometrioid Borderline Tumor of the Ovary: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
May 25, 2026

Endometrioid borderline tumor is a type of ovarian tumor that is not cancer, but is also not a completely benign growth. It belongs to a group of tumors called borderline tumors, which sit between clearly benign tumors and cancer. The behavior of an endometrioid borderline tumor falls somewhere between an endometrioid cystadenofibroma, which is a benign (noncancerous) tumor, and endometrioid carcinoma, which is a type of ovarian cancer. The tumor is called “endometrioid” because, under the microscope, its cells resemble those that normally line the inside of the uterus (the endometrium).

Endometrioid borderline tumor is the third most common type of borderline ovarian tumor, after the serous and mucinous types, although it is much less common than either of those. It is usually found in only one ovary, and most cases are diagnosed in women in their 50s. The outlook is excellent, and most patients are cured by surgery alone.

This article will help you understand what this diagnosis means on your pathology report, what each term means, and why it matters for your care.

What causes an endometrioid borderline tumor?

The exact cause of endometrioid borderline tumor is not known. The most important known association is with endometriosis, a condition in which tissue similar to the lining of the uterus grows outside the uterus, including on or within the ovary. Many patients with an endometrioid borderline tumor have a history of endometriosis, or their ovary shows evidence of endometriosis when it is examined under the microscope.

For this reason, it is believed that in some women, endometriosis acts as the “seed” from which an endometrioid borderline tumor develops. It is important to keep this risk in perspective: endometriosis is common, and the great majority of women who have endometriosis will never develop an endometrioid borderline tumor or any ovarian cancer. The tumor cells often contain changes (mutations) in genes such as CTNNB1 and PTEN, which are involved in controlling how cells grow. There are no clearly established lifestyle causes.

What are the symptoms?

Many endometrioid borderline tumors cause no symptoms and are discovered during an imaging test or examination performed for another reason. When symptoms do occur, they are usually related to the presence of a mass in the ovary and may include:

Abdominal or pelvic pain — Discomfort or pain in the lower abdomen or pelvis.
Abdominal swelling or bloating — A feeling of fullness, distension, or an increase in abdominal size.
Pressure symptoms — A larger tumor can press on nearby organs, sometimes causing changes in urination or bowel habits.

Some patients also have symptoms related to endometriosis, such as painful menstrual periods or long-standing pelvic pain. Because these symptoms are common and can have many causes, they are not specific to endometrioid borderline tumor. Any persistent abdominal or pelvic symptom should be evaluated by a doctor.

How is the diagnosis made?

For most women, the diagnosis of an endometrioid borderline tumor is made after the entire tumor is surgically removed and sent to a pathologist for examination under the microscope. The fallopian tube on the same side, and sometimes the uterus and other tissues, may be removed at the same time, depending on the situation.

During the operation, the surgeon may request an intraoperative consultation (also called a frozen section). In this situation, the pathologist examines a sample of the tumor while the patient is still in the operating room and provides a preliminary diagnosis within minutes. The result of an intraoperative consultation can change the type of surgery performed or the treatment offered afterward. A final diagnosis is made later, once the entire tumor has been examined in detail.

What does an endometrioid borderline tumor look like under the microscope?

When examined under the microscope, an endometrioid borderline tumor shows several characteristic features:

Crowded endometrioid glands — The tumor consists of crowded glands lined by abnormal cells. The cells look similar to the cells that normally line the inside of the uterus, which is why they are called endometrioid.
Fibrous background — The glands are often set within a fibrous background. A tumor with this pattern may be described as adenofibromatous in appearance.
Squamous metaplasia — Many endometrioid borderline tumors contain small areas where the cells take on a different, flattened appearance called squamous metaplasia. This is a common and expected finding.
Mild to moderate atypia — The tumor cells look somewhat abnormal, but not as abnormal as the cells of a cancer. Importantly, there is no destructive growth of tumor cells into the supporting tissue of the ovary, which is what separates a borderline tumor from a carcinoma.

Intraepithelial carcinoma and microinvasion

While examining the tumor, the pathologist looks for two specific findings that are sometimes seen in an endometrioid borderline tumor:

Intraepithelial carcinoma — This term describes areas where the tumor cells appear more abnormal (similar to cancer cells) but remain within the lining of the glands and have not invaded deeper tissue.
Microinvasion — This term describes tiny groups of tumor cells, or small irregular glands, that have moved out of the lining and into the supporting tissue beneath the lining, called the stroma. The movement of tumor cells into the stroma is called invasion, and when these foci are very small, the finding is called microinvasion.

Based on current evidence, the presence of intraepithelial carcinoma or microinvasion in an endometrioid borderline tumor does not appear to meaningfully change the outlook when the tumor is confined to the ovary and completely removed. These findings are recorded in the pathology report and may prompt closer follow-up.

Capsule status and ovarian surface involvement

All ovarian tumors are examined to see whether there are any holes or tears in the outer surface of the tumor or ovary. This outer surface is called the capsule.

Intact capsule — No holes or tears are identified in the outer surface. The tumor is fully enclosed.
Ruptured capsule — The outer surface contains a hole or tear. Rupture may happen on its own before surgery or during the operation to remove the tumor.

The pathologist also examines the surface of the ovary under the microscope to determine whether any tumor cells are present. A ruptured capsule or tumor cells on the surface of the ovary raise the pathologic stage because both increase the chance that tumor cells could reach other surfaces in the abdomen or pelvis.

Lymph nodes

Lymph nodes are small immune organs located throughout the body. The risk that an endometrioid borderline tumor will spread to lymph nodes is very low. For this reason, lymph nodes are often not removed during surgery for this type of tumor. If lymph nodes are removed, the pathologist examines them under the microscope and reports the number examined and, if any, the number that contain tumor cells.

Pathologic stage

Even though an endometrioid borderline tumor is not cancer, it is given a pathologic stage using the same system used for ovarian cancers, the FIGO staging system. The stage describes how much of the tumor was found beyond the ovary itself. The great majority of endometrioid borderline tumors (approximately 90%) are stage I, meaning the tumor is confined to the ovary.

Stage I — The tumor is confined to one or both ovaries. Stage I is divided based on whether one or both ovaries are involved, whether the capsule is intact or ruptured, whether tumor cells are present on the surface of the ovary, and whether tumor cells are found in fluid collected from the abdomen.
Stage II — The tumor involves one or both ovaries and has spread to other organs within the pelvis.
Stage III — The tumor has spread to surfaces in the abdomen beyond the pelvis, or tumor cells are found in lymph nodes.
Stage IV — Tumor is found in distant locations. This is very rare for an endometrioid borderline tumor.

To help determine the stage, small tissue samples, called biopsies, may be taken during surgery from the omentum (a sheet of fatty tissue over the intestines) and the peritoneum (the lining of the abdominal cavity), and fluid may be collected from the abdomen. These are examined for tumor cells.

What is the prognosis?

The prognosis for endometrioid borderline tumor is excellent. The great majority of these tumors are confined to one ovary at the time of diagnosis, and surgical removal alone is curative for nearly all patients. Recurrence and spread to other organs are uncommon. As noted above, even when the tumor contains areas of intraepithelial carcinoma or microinvasion, the outlook for a tumor that is confined to the ovary and completely removed remains very good.

A few factors are associated with a need for closer follow-up:

Incomplete removal — Tumor left behind after surgery increases the chance of regrowth.
Capsule rupture or surface involvement — These findings raise the stage and may prompt closer monitoring.
Advanced stage — A tumor that has spread beyond the ovary (stage II or higher) is followed more closely, although advanced-stage endometrioid borderline tumor is uncommon.

What happens after this diagnosis?

Surgery is the main treatment for endometrioid borderline tumor, and for most patients it is the only treatment needed. The discussion between you and your gynecologic team about the type of surgery depends on your age, whether you wish to preserve the ability to become pregnant, and the findings on your pathology report.

Options that the team may discuss include:

Removal of the affected ovary — Because endometrioid borderline tumors almost always involve only one ovary, removing the affected ovary and its fallopian tube is often the main procedure. For patients who have completed childbearing, the team may also discuss removing the uterus, the other ovary, and the fallopian tubes.
Fertility-sparing surgery — For younger patients who wish to preserve fertility, removing only the affected ovary and fallopian tube while leaving the unaffected ovary and the uterus in place is often possible, because these tumors are usually one-sided.
Surgical staging — Examining and sampling other sites in the abdomen and pelvis, and collecting fluid from the abdomen, helps determine the stage. Because the risk of lymph node spread is very low, lymph node removal is often unnecessary.
Observation and follow-up — Chemotherapy is generally not used for endometrioid borderline tumor, because the tumor is not cancer and does not respond well to it. After surgery, regular follow-up with examinations and sometimes imaging or blood tests is used to monitor for recurrence.

After treatment, follow-up with a gynecologist or gynecologic oncologist is recommended to promptly identify and manage any recurrence.

Questions to ask your doctor

Was the tumor confined to one ovary, or were both ovaries involved?
What was the stage of my tumor?
Was the capsule intact or ruptured, and were tumor cells found on the surface of the ovary?
Did my tumor contain areas of intraepithelial carcinoma or microinvasion?
Was there evidence of endometriosis in my ovary or elsewhere?
Was all of the tumor removed during surgery?
If I would like to preserve my fertility, what are my surgical options?
Will I need any treatment after surgery, or is surgery alone sufficient?
What is my chance of the tumor coming back?
How often will I need follow-up appointments, and what will they involve?
What symptoms should prompt me to contact you between visits?