Mucinous Borderline Tumor of the Ovary: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
May 25, 2026

Mucinous 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 a mucinous borderline tumor falls somewhere between a mucinous cystadenoma, which is a benign (noncancerous) tumor, and mucinous carcinoma, which is a type of cancer.

Mucinous borderline tumors are usually found in only one ovary and are often very large by the time they are discovered. They tend to occur in women younger than those who develop ovarian cancer. The outlook is very good, and most patients are cured by surgery alone. A small number of mucinous borderline tumors contain areas that, if left untreated, could develop into cancer over time.

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 a mucinous borderline tumor?

The exact cause of mucinous borderline tumor is not known. It is not caused by an infection and is not contagious. Research has shown that the tumor cells often contain a change (mutation) in a gene called KRAS, which is involved in controlling how cells grow and divide. This genetic change is thought to be an early step in the development of the tumor. There are no clearly established lifestyle causes, and in most cases there is no identifiable reason why a particular person develops this tumor.

What are the symptoms?

Many mucinous borderline tumors cause no symptoms and are discovered during an imaging test or examination performed for another reason. Because these tumors can grow very large, symptoms, when they occur, are usually related to the presence of a large mass and may include:

Abdominal swelling or bloating — An increase in abdominal size or a feeling of fullness or distension. Because mucinous borderline tumors can become very large, this is a common reason for seeking medical attention.
Abdominal or pelvic pain — Discomfort or pain in the lower abdomen or pelvis.
Pressure symptoms — A large tumor can press on nearby organs, sometimes causing changes in urination or bowel habits.

Because these symptoms are common and can have many causes, they are not specific to mucinous 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 a mucinous 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. Because mucinous borderline tumors are often very large and can contain different areas with different appearances, a final diagnosis is made later, once the entire tumor has been examined in detail and carefully sampled.

What does a mucinous borderline tumor look like under the microscope?

When the tumor is examined under the microscope, it is usually made up of many spaces called cysts. The walls of the cysts can be thin or thick, and some cysts contain more solid areas. Several features are characteristic of a mucinous borderline tumor:

Mucinous epithelium — The inside of the cysts and the solid areas are lined by an abnormal type of epithelium that forms glands and produces a thick, gelatinous, mucus-like fluid called mucin. The mucin fills the inside of the tumor.
Gastrointestinal-type cells — The cells resemble those that normally line parts of the digestive tract, such as the stomach and intestines. For this reason, mucinous borderline tumors are described as being of “intestinal” or “gastrointestinal” type.
Increased complexity — Compared with a benign mucinous cystadenoma, a borderline tumor shows more crowding, layering, and tufting of the cells, with small papillary projections extending into the cysts.
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 a mucinous 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 cysts and have not invaded deeper tissue. A mucinous borderline tumor with these areas may be described as a mucinous borderline tumor with intraepithelial carcinoma.
Microinvasion — This term describes tiny groups of tumor cells that have grown into the supporting tissue beneath the surface, called the stroma. To be classified as microinvasion, each focus must measure less than 5 mm. If an area of invasion measures 5 mm or larger, the tumor is diagnosed as mucinous carcinoma instead.

Even when intraepithelial carcinoma or microinvasion is present, the outlook for a mucinous borderline tumor that is confined to the ovary and completely removed remains very good. 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. If the tumor is received in multiple pieces, it may not be possible for the pathologist to tell whether the capsule was ruptured.

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.

The appendix and other tissues

Mucinous tumors that start in other organs, particularly the appendix and other parts of the digestive tract, can spread to the ovary and look very similar to a mucinous borderline tumor under the microscope. For this reason, the pathologist and surgeon take care to determine where the tumor actually started.

If a mucinous tumor is found in the ovary, the surgeon may also remove the appendix for microscopic examination, particularly if the appendix appears abnormal during surgery or if there is mucin spread within the abdomen (a finding sometimes called pseudomyxoma peritonei). When mucin and mucinous tumor cells are found spread throughout the abdomen, the source is almost always the appendix or another part of the digestive tract rather than the ovary. Small tissue samples, called biopsies, may also be taken from the omentum (a sheet of fatty tissue over the intestines) and the peritoneum (the lining of the abdominal cavity) to check for tumor cells. Features that suggest a tumor may have spread to the ovary from another organ include involvement of both ovaries, a smaller tumor size, and a tumor growing on the surface of the ovary.

Pathologic stage

Even though a mucinous 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 mucinous 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 a mucinous borderline tumor.

A mucinous borderline tumor found at an advanced stage is unusual, and when this appears to be the case, the pathologist and treating team carefully reconsider whether the tumor may actually be a mucinous carcinoma or a tumor that has spread to the ovary from another organ.

What is the prognosis?

The prognosis for a mucinous borderline tumor is very good. 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. Reported survival rates for stage I tumors are approximately 95 to 100%, and recurrence after complete removal is uncommon. Even when the tumor contains areas of intraepithelial carcinoma or microinvasion, the outlook for a tumor confined to the ovary and completely removed remains excellent.

A few factors are associated with a somewhat higher chance of the tumor returning or 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.
Intraepithelial carcinoma or microinvasion — These findings are followed more closely, although the outlook remains very good when the tumor is confined to the ovary.
Fertility-sparing surgery — When only the affected ovary or only the cyst is removed, there is a small chance that a new tumor may develop, although this approach does not appear to reduce overall survival.

What happens after this diagnosis?

Surgery is the main treatment for mucinous 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 mucinous 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 and the other ovary and fallopian tube.
Fertility-sparing surgery — For younger patients who wish to preserve fertility, removing only the affected ovary and fallopian tube (or, in some cases, only the cyst) 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. The appendix may also be removed and examined, particularly if it appears abnormal or if there is mucin spread within the abdomen.
Observation and follow-up — Chemotherapy is generally not used for mucinous 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 watch 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 my appendix examined, and were there any findings?
How did the team confirm that the tumor started in the ovary and did not spread there from another organ?
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?