Serous Borderline Tumour of the Ovary: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
May 25, 2026

Serous 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. Serous borderline tumor is the most common borderline tumor of the ovary. It is called “borderline” because, although it is not cancer, it can behave in ways that ordinary benign tumors do not. For example, groups of tumor cells can attach to other surfaces in the abdomen and pelvis, and in a small number of cases the tumor can come back or develop into a slow-growing cancer called low grade serous carcinoma.

Serous borderline tumor tends to occur in women younger than those who develop ovarian cancer, often between the ages of 30 and 50, and about one-third of cases involve both ovaries. The outlook for serous borderline tumor is very good.

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 serous borderline tumor?

The exact cause of serous 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 changes (mutations) in genes called KRAS and BRAF, which are involved in controlling how cells grow and divide. These genetic changes are thought to be early steps 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 serous 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 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 serous 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 serous 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 a serous borderline tumor look like under the microscope?

Serous borderline tumors develop from the epithelial cells on the surface of the ovary. When examined under the microscope, the tumor shows several characteristic features:

Cysts — The tumor is usually made up of many small spaces called cysts. The walls of the cysts can be thin or thick, and some cysts contain more solid areas, which pathologists sometimes call excrescences.
Serous lining cells — The inside of the cysts is lined by specialized cells called serous cells, which produce a clear fluid that fills the tumor.
Papillary projections — The thick-walled cysts and solid areas often contain small finger-like projections of tissue called papillae. The presence of these papillary projections and solid areas is what distinguishes a serous borderline tumor from a completely benign tumor called a serous cystadenoma.
Mild to moderate atypia — The tumor cells look somewhat abnormal but not as abnormal as the cells of a cancer. Cell division is limited, and there is no destructive growth into the supporting tissue of the ovary.

The micropapillary pattern

Some serous borderline tumors show a distinctive growth pattern called micropapillary, in which the tumor forms long, thin, finger-like projections. A tumor with this pattern may be described as a serous borderline tumor with micropapillary features. The micropapillary pattern is associated with a higher likelihood of involvement of both ovaries, a more advanced stage, and the presence of implants (described below). When matched for stage, however, tumors with a micropapillary pattern often behave similarly to those without it. Your pathology report will note whether this pattern is present.

Microinvasion

The tumor cells in a serous borderline tumor are normally confined to the surface lining of the cysts. In a small number of tumors, tiny groups of tumor cells are found in the supporting tissue beneath the surface, which is called the stroma. This finding is called microinvasion. To be classified as microinvasion, each focus of tumor cells in the stroma must measure less than 5 mm. If an area of invasion measures 5 mm or larger, the tumor is diagnosed as low grade serous carcinoma instead. Microinvasion does not appear to meaningfully alter the generally favorable outlook for a serous borderline tumor, but it is documented in the pathology report and may prompt closer follow-up.

Capsule status: intact or ruptured

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.

Capsule status is important because rupture or tumor cells on the surface of the ovary raise the pathologic stage and may influence the discussion of follow-up.

Implants

In some cases, groups of tumor cells from a serous borderline tumor are found attached to other surfaces in the abdomen or pelvis, such as the omentum (a sheet of fatty tissue that hangs over the intestines) or the peritoneum (the lining of the abdominal cavity). These groups of cells are called implants.

By the current definition, the implants associated with a serous borderline tumor are non-invasive, meaning the tumor cells rest on the surface of an organ without growing into the deeper tissue. Non-invasive implants raise the pathologic stage of the tumor and are associated with a somewhat higher chance of the tumor returning, but they do not change the diagnosis. If the pathologist finds that tumor cells have grown into the deeper tissue of one of these sites, the finding is no longer considered an implant of a borderline tumor; instead, it is diagnosed as low grade serous carcinoma. For this reason, the pathologist examines any implants carefully to determine whether invasion is present.

Lymph nodes

Lymph nodes are small immune organs located throughout the body. Although uncommon, tumor cells from a serous borderline tumor can be found in lymph nodes, and if this occurs it will be described in your pathology report. Importantly, and unlike many other types of tumors, the presence of serous borderline tumor cells in a lymph node is not associated with a worse outcome.

Pathologic stage

Even though a serous 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 indicates how much of the tumor was found beyond the ovary. Most serous borderline tumors are stage I, meaning the tumor is confined to the ovary or ovaries.

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 there are implants on other organs within the pelvis, such as the uterus, fallopian tubes, or the lining of the pelvis.
Stage III — There are implants on 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 serous borderline tumor.

For serous borderline tumors, a higher stage reflects the presence of non-invasive implants beyond the ovary. Even tumors found at stage II or III have a favorable outlook, although they are followed more closely than stage I tumors.

What is the prognosis?

The prognosis for serous borderline tumor is very good. Most tumors are found at an early stage, when the tumor is confined to the ovary, and the great majority of patients are cured by surgery alone. Reported survival rates are high, with five-year survival generally around 95 to 99%, and long-term survival also remains high. Even when the tumor is found at a more advanced stage, the outlook is still favorable.

A small number of serous borderline tumors return after treatment, sometimes many years later. When the tumor does come back, it may return as another borderline tumor or, less commonly, as low grade serous carcinoma. Several features are associated with a somewhat higher chance of the tumor returning:

Advanced stage — Tumors with non-invasive implants beyond the ovary (stage II or III) are more likely to recur than tumors confined to the ovary.
Micropapillary pattern — Tumors with a micropapillary pattern are more often found at an advanced stage and may be more likely to recur.
Incomplete removal — Tumor left behind after surgery increases the chance of regrowth.
Fertility-sparing surgery — When only the affected ovary or only the cyst is removed (rather than both ovaries), the chance of a new tumor developing is higher, although this approach does not appear to reduce overall survival.

Because recurrences can occur late, long-term follow-up is recommended for all patients.

What happens after this diagnosis?

Surgery is the main treatment for serous 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, whether one or both ovaries are involved, and the stage of the tumor.

Options that the team may discuss include:

Complete surgery — For patients who have completed childbearing, the team may discuss removing both ovaries and fallopian tubes and the uterus, along with surgical staging. Staging involves examining and sampling other surfaces in the abdomen and pelvis, such as the omentum and peritoneum, and collecting abdominal fluid to check for tumor cells.
Fertility-sparing surgery — For younger patients who wish to preserve fertility, the team may discuss 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. This approach is associated with a higher chance of a new tumor developing but does not appear to reduce overall survival.
Surgical staging — Examining and sampling other sites in the abdomen and pelvis at the time of surgery helps determine the stage and guides follow-up. This is recommended even when the tumor appears confined to the ovary.
Observation and follow-up — Chemotherapy is generally not used for serous 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 a blood test called CA-125, is used to watch for recurrence.

Because serous borderline tumor can return many years after the original diagnosis, long-term follow-up with a gynecologist or gynecologic oncologist is recommended.

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?
Did my tumor show a micropapillary pattern or microinvasion?
Were any implants found, and if so, were they confirmed to be non-invasive?
Were lymph nodes examined, and did any contain tumor cells?
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?