Mitotically Active Cellular Fibroma of the Ovary: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
August 29, 2026

A mitotically active cellular fibroma is a noncancerous (benign) tumor of the ovary. It grows from fibroblasts, the cells that make the firm supporting tissue found inside the ovary. It belongs to a family of tumors called sex cord-stromal tumors.

The long name describes two things the pathologist saw. “Cellular” means the tumor cells are packed tightly together, with less collagen between them than in an ordinary ovarian fibroma. “Mitotically active” means the cells are dividing more often than usual. Neither finding makes this tumor a cancer. What matters just as much is what the pathologist did not see: clearly abnormal cells.

These tumors occur most often around age 40, somewhat younger than other fibrous ovarian tumors. They range in size from under 1 cm to more than 20 cm, and almost all involve one ovary only. 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 mitotically active cellular fibroma?

The cause of a mitotically active cellular fibroma is not known. In most cases, there is no identifiable reason why one person develops this tumor.

A small number of ovarian fibromas of all types occur as part of an inherited condition called nevoid basal cell carcinoma syndrome, also known as Gorlin syndrome. This syndrome is caused by a change in a gene called PTCH1, which normally helps limit how much cells grow. Fibromas linked to the syndrome tend to occur at a younger age, involve both ovaries, form multiple separate nodules, and contain calcium deposits. Most of these tumors have nothing to do with Gorlin syndrome, and a single tumor in one ovary is not a reason to suspect it.

What are the symptoms?

Many mitotically active cellular fibromas cause no symptoms. Smaller tumors are often found by chance, either during an imaging test performed for another reason or when an ovary is removed for another reason. When symptoms do occur, they usually come from tumor size.

Abdominal or pelvic pain — Discomfort or pressure low in the abdomen or pelvis.
Abdominal swelling — A large tumor can make the abdomen feel full or look enlarged.
Sudden, severe pain — A large ovarian tumor can twist on its blood supply, a problem called torsion. This causes sudden severe pain and needs urgent medical attention.
Fluid in the abdomen — Some fibrous ovarian tumors, particularly large ones, cause fluid to collect in the abdomen. This is called ascites.

A small number of people with a fibrous ovarian tumor develop fluid in both the abdomen and the chest. That combination is called Meigs syndrome. It closely resembles advanced ovarian cancer, and the blood test CA-125 can be raised as well. Many people in this situation are investigated for cancer before the diagnosis is known. The fluid clears on its own once the tumor is removed.

How is the diagnosis made?

A mitotically active cellular fibroma is diagnosed after the tumor is removed surgically and examined under the microscope by a pathologist. The surgery usually removes the whole ovary, sometimes along with the fallopian tube on the same side. Imaging tests such as ultrasound, CT, or MRI show a firm, solid mass in the ovary, but they cannot separate this tumor from other solid ovarian tumors, including cancers.

During the operation, the surgeon may request an intraoperative consultation, also called a frozen section. The pathologist examines a piece of the tumor while the patient is still in the operating room and gives a preliminary diagnosis within minutes. A frozen section cannot settle this diagnosis because counting dividing cells and assessing their appearance requires a full examination. The final diagnosis is made later.

Reaching this diagnosis takes more work than most. The pathologist counts dividing cells across several areas of the tumor and assesses how abnormal the cells look. Because both findings can vary from one part of a tumor to another, the pathologist samples the tumor thoroughly. A second pathologist often reviews these cases, or they are sent to a specialist center, because distinguishing them from a rare cancer called fibrosarcoma depends on these judgments. Additional stains and genetic testing may also be performed, as described below.

What does a mitotically active cellular fibroma look like under the microscope?

A mitotically active cellular fibroma is a benign ovarian tumor made of densely packed fibroblasts that are dividing more often than usual. To the naked eye, it is usually a firm, solid, white or pale tumor. Under the microscope, the pathologist looks for the following features.

Densely packed spindle cells — The tumor is made of long, thin cells called spindle cells, named for their tapered shape. They are usually arranged in bundles, called fascicles, that cross one another.
Four or more dividing cells — A cell caught in the act of dividing is called a mitotic figure. This diagnosis requires 4 or more per 10 high-power fields, the standard microscope area pathologists use for counting. Counts well above that are reported and do not change the diagnosis on their own.
Bland nuclei — This is the finding the diagnosis depends on. The cells look uniform, without the marked variation in size and shape that pathologists call nuclear atypia. Mild atypia may be present, but it is not widespread or severe.
No abnormal dividing cells — The dividing cells themselves look normal in shape. Misshapen dividing cells, called atypical mitotic figures, would suggest cancer.
Degenerative changes — Large or long-standing tumors may show bleeding into the tumor and areas of tissue death called infarct-type necrosis. This happens when the tumor outgrows its blood supply, and it is not a sign of cancer.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside cells. For a mitotically active cellular fibroma, these stains confirm that the spindle cells come from the supporting tissue of the ovary. They also separate this tumor from other spindle cell tumors that can look similar. If the tests were performed, the results appear in your report as a list of protein names with the word positive or negative beside each one.

Inhibin and calretinin. Often positive, but usually only in scattered areas. These proteins are made by the ovary’s hormone-producing and supporting cells. Patchy staining supports a fibrous tumor. Strong staining throughout the tumor points instead toward a thecoma, a related tumor.
WT1. Usually positive. WT1 is commonly present in tumors that arise from ovarian supporting tissue.
Estrogen receptor and progesterone receptor. Often positive. The estrogen receptor and progesterone receptor are proteins that respond to female hormones. A positive result is expected and does not mean the tumor is hormone-driven.
Ki-67. Often raised. Ki-67 estimates the proportion of cells preparing to divide, and it can be high in this tumor because the cells are dividing frequently. A high Ki-67 result here does not mean the tumor is a cancer.
Other markers. Stains such as desmin, S100, and cytokeratin are usually negative. Negative results help rule out a muscle tumor, a nerve sheath tumor, or a tumor that spread from elsewhere.

Two further tests are commonly used for this diagnosis. A reticulin stain outlines the fibers around individual cells, and the pattern differs between a fibrous tumor and an adult granulosa cell tumor. Testing for a change in a gene called FOXL2 can also help. That change is found in most adult granulosa cell tumors and is absent in these fibrous tumors, so a negative result supports the benign diagnosis.

How is this different from a fibrosarcoma?

This is the question a mitotically active cellular fibroma answers. A fibrosarcoma is a rare ovarian cancer made of the same kind of spindle cells. Under current World Health Organization criteria, a fibrosarcoma diagnosis requires two findings together: frequent cell division and cells that look clearly abnormal throughout the tumor. Frequent cell division alone is not enough.

The history matters here. Until the 1980s and 1990s, the mitotic count alone often decided the diagnosis. A fibrous ovarian tumor with 4 or more dividing cells per 10 high-power fields was often called a fibrosarcoma. Patients were treated for a cancer on the strength of a mitotic count alone. A 2006 study gathered a large series of these tumors with frequent division but bland cells and showed they behaved benignly. The category “mitotically active cellular fibroma” was created to describe them, and the World Health Organization has recognized it since 2014.

If your report carries this diagnosis, the pathologist counted frequently dividing cells, looked carefully for abnormal-looking cells, and found none. That is why the report says fibroma rather than sarcoma.

How does this differ from a fibroma and a cellular fibroma?

A mitotically active cellular fibroma sits within a group of related ovarian tumors made of the same spindle cells. What separates them is how tightly the cells are packed and how often they divide. Your report may use any of the following terms, and all three are noncancerous.

Fibroma — Cells spread apart in abundant collagen, with rare dividing cells.
Cellular fibroma — Cells packed tightly together, with fewer than 4 dividing cells per 10 high power fields. A cellular fibroma is benign.
Mitotically active cellular fibroma — Cells packed tightly together and dividing at 4 or more per 10 high power fields, without marked nuclear abnormality.

The practical difference between these three is small. All are removed surgically, none requires chemotherapy or radiation, and the outlook after complete removal is good in each case. The main difference is that follow-up is more often recommended for this diagnosis.

What other findings may be described in the report?

Along with the diagnosis of mitotically active cellular fibroma, your pathology report may describe other features of the tumor and the surrounding tissue.

Mitotic count — The report gives the number of dividing cells per 10 high-power fields. This number underpins the diagnosis, so it is worth knowing what yours was.
Degree of nuclear atypia — The report may state that atypia was absent or mild. This finding separates the tumor from a fibrosarcoma, so the report often states it explicitly.
Tumor size — The report gives the greatest dimension in centimeters. Size does not change the diagnosis, but it does influence the type of surgery performed.
One or both ovaries — Almost all of these tumors involve only one ovary. Involvement of both ovaries prompts a pathologist to consider Gorlin syndrome, particularly in a younger patient.
Adhesions and rupture — The report may note whether the tumor was stuck to nearby tissue or had ruptured. The rare tumors in this group that recurred were usually adherent or ruptured, so these findings guide how closely you are followed.
Tissue outside the ovary — Occasionally a small amount of tumor is found attached to nearby tissue. This has been reported in these tumors without any later recurrence, and it does not by itself mean the tumor is a cancer.

What happens after this diagnosis?

A mitotically active cellular fibroma is a benign ovarian tumor. In the reported series, these tumors have behaved benignly in almost every case, including cases where a small amount of tumor was found outside the ovary. Complete surgical removal is the treatment. Because the tumor is not cancer, it is not given a grade or assigned a stage.

What you and your gynecologic team discuss next depends on your report findings, your age, and your overall situation. Points the team may raise include:

No chemotherapy or radiation — These treatments are not used for this tumor. This holds even when the tumor was large, adherent, or ruptured, and even when the mitotic count was high.
Follow-up — Because this category is relatively new and reported cases are few, most teams recommend a period of clinical review and imaging after surgery. A small number of these tumors have come back in the pelvis, usually several years later.
A second pathology opinion — The line between this diagnosis and a fibrosarcoma rests on judgment about how the cells look. Many centers review these cases with a second pathologist, and you can ask whether they did that in your case.
Fertility — Removing one ovary does not usually prevent future pregnancy, since the other ovary continues to work. Complete removal of the tumor is possible while preserving fertility in many cases, and your team can explain what your surgery means for you.
Testing for Gorlin syndrome — If tumors involved both ovaries, formed multiple nodules, or occurred at a young age, your doctor may discuss referral for genetic assessment. This matters for blood relatives as well as for you.

Your doctor will tell you what follow-up is recommended in your situation and for how long it should continue.

Questions to ask your doctor

How many dividing cells were counted in my tumor?
Did the tumor cells show any abnormal features under the microscope?
Did you consider fibrosarcoma, and what ruled it out?
Did a second pathologist review my case, or was it sent for expert review?
Were special stains or FOXL2 testing performed, and what did they show?
How large was the tumor?
Was the tumor in one ovary only, or were both ovaries involved?
Was the tumor stuck to nearby tissue, or did it rupture during surgery?
Was any tumor found outside the ovary?
Was the whole tumor removed?
My report mentions necrosis or a high Ki-67. Does either mean anything worrying?
Should I be assessed for Gorlin syndrome, and does this affect my family?
What follow-up do you recommend, and for how long?
What symptoms should prompt me to contact you?