Borderline Brenner Tumor: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
May 25, 2026

A borderline Brenner tumor of the ovary is an uncommon 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. A borderline Brenner tumor is more complex than a benign Brenner tumor, but it does not grow into the surrounding tissue the way a cancer does. It is made up of cells that look more crowded and more abnormal than those of a benign Brenner tumor, while still remaining within the surface layer of the tumor.

Borderline Brenner tumor is also called atypical proliferative Brenner tumor, and you may see that term on your pathology report. It is the least common of the borderline ovarian tumors, and only a small number of cases have been described. It is almost always found in one ovary, and it most often occurs in women after menopause. The outlook is very good, 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 a borderline Brenner tumor?

The exact cause of a borderline Brenner tumor is not well understood. Brenner tumors are made up of a type of cell that resembles the cells lining the bladder and urinary tract, and they are thought to develop from small groups of these cells found near the ovary and fallopian tube. The process that leads a Brenner tumor to develop the more crowded, more abnormal features of a borderline tumor is not known. Unlike some ovarian cancers, borderline Brenner tumors do not have a well-defined set of risk factors, such as specific inherited gene changes or a family history. It is not caused by an infection and is not contagious.

What are the symptoms?

Many borderline Brenner tumors cause no symptoms, particularly when they are small, and are discovered during an imaging test or surgery 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, pressure, or an increase in abdominal size.
Abnormal vaginal bleeding — In rare cases, changes in the menstrual cycle or vaginal bleeding may occur.

Because these symptoms are common and can have many causes, they are not specific to a borderline Brenner tumor. Any persistent abdominal or pelvic symptom should be evaluated by a doctor.

How is the diagnosis made?

The diagnosis of a borderline Brenner tumor is made after the tumor is surgically removed and examined under the microscope by a pathologist. Imaging studies such as ultrasound, CT, or MRI may show an ovarian mass, but they cannot reliably tell a borderline tumor apart from a benign or a cancerous tumor. The diagnosis can only be confirmed by examining the tissue.

Under the microscope, the pathologist looks for the crowded, more abnormal-looking cells of a borderline Brenner tumor and, importantly, confirms that these cells do not grow into the surrounding tissue. To support the diagnosis and tell this tumor apart from other types of ovarian tumors, the pathologist may perform a test called immunohistochemistry, which detects specific proteins inside the tumor cells.

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

When examined under the microscope, a borderline Brenner tumor shows several characteristic features:

Transitional-type cells — The tumor is composed of cells that look very similar to those that normally line the bladder and urinary tract. These are called transitional cells or urothelial-type cells.
Finger-like (papillary) growth — The tumor cells often form long, finger-like projections of tissue. The appearance closely resembles a low-grade, non-invasive tumor of the bladder lining.
More crowding and abnormality than a benign Brenner tumor — Compared with a benign Brenner tumor, the cells are more crowded and look more abnormal, a feature pathologists call atypia. Dividing cells, called mitotic figures, may also be seen.
No invasion — Importantly, the tumor cells stay within the surface layer and do not grow into the supporting tissue of the ovary. This absence of invasion is what separates a borderline Brenner tumor from a malignant (cancerous) Brenner tumor.

A borderline Brenner tumor is often found alongside an area of benign Brenner tumor, and in some cases a mucinous tumor is present in the same ovary.

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 found on the surface of the ovary, raises the pathologic stage. For a borderline Brenner tumor, however, the capsule is usually intact and the tumor is confined to the ovary.

Pathologic stage

Even though a borderline Brenner 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. Almost all borderline Brenner tumors 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.

Stages II, III, and IV are very rare for a borderline Brenner tumor. When a Brenner tumor appears to involve tissue beyond the ovary, the pathologist carefully reconsiders whether the tumor may instead be a malignant Brenner tumor.

What is the prognosis?

The prognosis for a borderline Brenner tumor is very good. Almost all of these tumors are confined to one ovary at the time of diagnosis, and surgical removal alone is curative for the great majority of patients. Because the tumor does not invade the surrounding tissue, it does not behave like cancer. Recurrence after complete removal is very rare; only isolated cases have been reported in the medical literature. The most important factor for a good outcome is complete surgical removal of the tumor.

What happens after this diagnosis?

Surgery is the main treatment for a borderline Brenner 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 borderline Brenner 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, or who are past menopause, 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. Most borderline Brenner tumors, however, occur in women after menopause.
Observation and follow-up — Chemotherapy is not used for a borderline Brenner tumor, because the tumor is not cancerous. After surgery, regular follow-up with examinations and sometimes imaging is used to confirm recovery and watch for the rare possibility of recurrence.

After treatment, 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?
Was the tumor confirmed to be borderline, rather than benign or malignant?
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?