Mitotically Active Leiomyoma of the Uterus: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
September 2, 2026

A mitotically active leiomyoma is a noncancerous (benign) tumor that starts in the muscular wall of the uterus. It is made of smooth muscle cells, the same cells that make up the uterine wall, called the myometrium. In the uterus, leiomyoma and fibroid mean the same thing, so a mitotically active leiomyoma is a type of fibroid.

“Mitotically active” means the tumor cells are dividing more often than usual. A cell caught in the act of dividing is called a mitotic figure, and the pathologist counts them. Finding more than expected is what earns this tumor its name.

Dividing cells sound alarming, and this is the point of the article: in this tumor, they do not mean cancer. What matters just as much is what the pathologist did not see. The cells look normal, and there is no tumor cell death like what is seen in cancer. 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 leiomyoma?

The cause of fibroids in general is not fully understood. They grow from a single smooth muscle cell that begins dividing more than it should, and estrogen and progesterone drive their growth. That hormone dependence explains most of what is known about this particular type.

Hormone levels — Mitotically active leiomyomas are found more often in people who have not gone through menopause, and particularly in the second half of the menstrual cycle when progesterone is high.
Pregnancy — The high hormone levels of pregnancy increase cell division in fibroids, and a fibroid removed during or shortly after pregnancy is more likely to be reported this way.
Hormonal medications — Progestin-containing medications and other hormone treatments can have the same effect.

In other words, an increased number of dividing cells here often reflects the person’s hormonal state rather than anything unusual about the tumor.

What are the symptoms?

A mitotically active leiomyoma causes the same symptoms as any other fibroid, and small ones usually cause none. Symptoms depend on the size of the tumor and where it sits in the wall of the uterus.

Heavy or prolonged periods — The most common symptom, particularly when the fibroid bulges into the cavity of the uterus.
Pelvic pressure or a feeling of fullness — Caused by the bulk of a larger fibroid.
Pelvic or abdominal pain — Sometimes constant, sometimes worse around a period.
Pressure on nearby organs — A large fibroid can press on the bladder or bowel, changing how often you need to urinate or how your bowels work.
Difficulty becoming pregnant — Fibroids that distort the cavity of the uterus can interfere with pregnancy.

How is the diagnosis made?

Fibroids are usually seen first on an ultrasound or MRI, but no imaging test can tell a mitotically active leiomyoma from any other fibroid. The distinction is made only under the microscope, after the tumor has been removed.

The tissue reaches the laboratory after a myomectomy, in which the fibroid alone is removed, or after a hysterectomy. In many cases, the uterus was removed for another reason, and the fibroid was an incidental finding.

A pathologist then counts dividing cells in the busiest part of the tumor and assesses how the cells look. Two practical points shape that assessment. Dividing cells are counted away from areas of tissue breakdown, because those areas contain changes that can be mistaken for cell division. Any part of the tumor that looks unusual to the naked eye is also sampled specifically. The features that would change the diagnosis may be present in only one area.

What does a mitotically active leiomyoma look like under the microscope?

A mitotically active leiomyoma is a benign smooth muscle tumor of the uterus in which the cells are dividing more often than usual but look entirely normal. Under the microscope, the pathologist looks for the following features.

Spindle cells in bundles — The tumor is made of long, thin cells called spindle cells, arranged in interlacing bundles called fascicles. This is the usual appearance of any fibroid.
An increased number of dividing cells — The count is 5 or more per 10 high-power fields, the standard microscope area used for counting, and usually falls between 5 and 15. Some reports give the figure per square millimeter instead, where the equivalent is roughly 2 or more.
Normal-looking cells — The cells are uniform, without the marked variation in size and shape that pathologists call nuclear atypia. This finding supports the benign diagnosis.
No tumor cell necrosis — There is no tumor cell death of the type associated with cancer, explained further below.
Thick-walled blood vessels — A normal feature of fibroids.
Cystic spaces — Open, fluid-filled spaces described as cysts are common, particularly in larger tumors.

How is this different from a leiomyosarcoma?

This is what the diagnosis is for. A uterine leiomyosarcoma is a cancer made of the same kind of smooth muscle cells. Pathologists separate the two using three features, not one.

Cell division — How many tumor cells are dividing.
Nuclear atypia — Whether the cells look abnormal.
Tumor cell necrosis — Whether tumor cells are dying in the pattern seen in cancer.

A diagnosis of leiomyosarcoma generally requires a combination of these, not a high count alone. A mitotically active leiomyoma has only the first, and the other two are absent.

This distinction rests on real evidence, not convention. Series following patients with these tumors, some for over ten years, looked at counts between 5 and 15 dividing cells per 10 high-power fields. Those counts did not predict cancer-like behavior when the cells looked normal, and there was no tumor cell necrosis. None of the patients in those series developed recurrence or spread. That work is why this category exists and why your report lists leiomyoma rather than sarcoma.

What do degenerative changes and infarct-like necrosis mean?

Fibroids commonly outgrow their blood supply, and the tissue then breaks down. Pathologists call this degenerative change, and it is very common in larger fibroids and in those that have been present for a long time. It is also expected after treatment with hormone medication or after uterine artery embolization, a procedure that deliberately cuts off the blood supply to a fibroid.

When an area of the tumor dies because blood could not reach it, the result is called infarct-like necrosis, or infarct-type necrosis. Necrosis means tissue death.

The word necrosis appears in the criteria for leiomyosarcoma, so seeing it in a benign report is understandably confusing. The two are not the same thing. Infarct-like necrosis follows loss of blood supply and has a characteristic appearance, with a zone of healing tissue between the dead and living areas. Tumor cell necrosis, the kind that counts toward a diagnosis of cancer, has an abrupt border with no healing zone. Distinguishing them is a routine part of the pathologist’s assessment. If your report describes infarct-like or infarct-type necrosis in a leiomyoma, that is the benign kind.

How does this differ from other types of leiomyoma?

Fibroids come in several forms that differ in appearance under the microscope. Your report may use any of the following terms, and the first three are all benign.

Leiomyoma — The usual form. Spindle cells in bundles, few dividing cells, normal-looking cells.
Cellular leiomyoma — The cells are more tightly packed than usual. A cellular leiomyoma is benign and is treated the same way.
Mitotically active leiomyoma — An increased number of dividing cells, with normal-looking cells and no tumor cell necrosis.
Leiomyoma with bizarre nuclei — Also called atypical or symplastic leiomyoma. The cells look strikingly abnormal, but dividing cells are few, and there is no tumor cell necrosis. These behave benignly in almost all cases.
Smooth muscle tumor of uncertain malignant potential — A tumor with a combination of features that fits neither a benign leiomyoma nor a leiomyosarcoma. A STUMP is not cancer, but its behavior is less predictable, and it is followed more closely.

What other findings may be described in the report?

Your report will describe the tumor and the rest of the uterus. Findings commonly listed include:

Tumor size and number — Fibroids are often multiple, and each is measured. Size does not change the fact that the tumor is benign.
Location in the wall — Whether the fibroid sits within the wall, bulges into the cavity, or projects from the outer surface. This is what best explains the symptoms.
The mitotic count — The number of dividing cells counted, which is the figure the diagnosis rests on.
The lining of the uterus — If a hysterectomy was performed, the endometrium is examined and reported separately. An endometrial polyp or endometrial hyperplasia may be reported alongside the fibroid.
Adenomyosis — Endometrial-type tissue within the muscular wall, a common accompanying finding that causes similar symptoms.

What happens after this diagnosis?

A mitotically active leiomyoma is a benign fibroid, and it is managed exactly like any other fibroid. It is not given a grade or assigned a stage because grading and staging describe cancers. Points your doctor may raise include:

No cancer treatment — Chemotherapy and radiation are not used for this tumor.
No special follow-up — The increased number of dividing cells does not call for extra monitoring. Follow-up is the same as for an ordinary fibroid.
If the fibroid alone was removed — New fibroids can develop in the remaining uterus over time, and existing ones can grow. This is not a recurrence of anything worrying.
If the whole uterus was removed — Fibroids cannot come back.
Fertility — Removing a fibroid that was distorting the cavity of the uterus can improve the chance of pregnancy. Your surgeon can explain what your operation means for future pregnancies.

Most people need no further treatment. Your doctor will tell you whether any follow-up is recommended in your situation.

Questions to ask your doctor

Was this tumor confirmed as benign?
How many dividing cells were counted?
Did the tumor cells show any abnormal features under the microscope?
Did you consider leiomyosarcoma, and what ruled it out?
My report mentions necrosis. Is that the benign kind?
How large was the fibroid, and was there more than one?
Where in the wall of the uterus was it?
Was the fibroid alone removed, or the whole uterus?
Could this explain the bleeding or pressure I was having?
Could new fibroids develop, and would I need surgery again?
Does this affect my fertility or a future pregnancy?
Do I need any follow-up imaging or appointments?
What symptoms should prompt me to contact you?