Uterine Leiomyosarcoma: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
September 3, 2026

Uterine leiomyosarcoma is a cancer that starts in the smooth muscle cells of the uterus. These cells make up the uterus’s muscular wall, called the myometrium, and they are the same cells that form a leiomyoma, or fibroid. A sarcoma is a cancer arising from muscle or connective tissue rather than from the lining cells that give rise to most cancers of the uterus.

This is an uncommon cancer. Most people are over 50 at the time of diagnosis, and most learn of it after surgery that was planned for something else. This article will help you understand what this diagnosis means on your pathology report, what each section of the report describes, and why it matters for your care.

What causes uterine leiomyosarcoma?

For most people, the cause is not known. Previous radiation therapy to the pelvis is the one established risk factor, and it accounts for only a small proportion of cases.

These tumors involve complex genetic changes across many genes, rather than a single defining alteration like those found in some other uterine sarcomas. The changes arise within the tumor during a person’s life and are not passed to children. In a small number of cases, the tumor is linked to an inherited condition called hereditary leiomyomatosis and renal cell cancer. That condition is caused by a change in a gene called fumarate hydratase.

Does a fibroid turn into a leiomyosarcoma?

This is the question most people ask after this diagnosis, particularly those who were told for years that they had fibroids. Evidence indicates that leiomyosarcomas arise on their own rather than developing from an existing fibroid.

The numbers help put this in proportion. Around 70 in every 100 women develop fibroids at some point in life, while uterine leiomyosarcoma affects roughly 6 people in every million each year. Having fibroids does not put you at risk of this cancer, and having had one does not mean a fibroid changed into it.

What does happen is that a leiomyosarcoma can be mistaken for a fibroid before surgery, because the two cannot be told apart on imaging. That is why many people receive this diagnosis unexpectedly after an operation planned for fibroids.

What are the symptoms?

The symptoms overlap with those of fibroids, which is part of why the diagnosis is often unexpected.

Abnormal vaginal bleeding — The most common symptom, including bleeding after menopause.
Abdominal or pelvic pain and pressure — Often described as an ache or a feeling of fullness.
Bloating or an enlarging abdomen — Sometimes the first thing noticed.
Rapid change — A mass that grows quickly, particularly after menopause, is more likely to be investigated further.

How is the diagnosis made?

The diagnosis is usually made after the uterus has been removed in an operation called a hysterectomy. The ovaries, fallopian tubes, and cervix may be removed at the same time. Most of these tumors arise in the body of the uterus, though some start in the wall of the cervix.

An endometrial biopsy is often unhelpful here because the tumor sits within the muscular wall rather than the lining, and a lining sample may miss it entirely. Imaging cannot distinguish this tumor from a fibroid with certainty.

Once the tissue reaches the laboratory, a pathologist examines it and assesses three specific features, described in the next section.

What does uterine leiomyosarcoma look like under the microscope?

Uterine leiomyosarcoma is a cancer made of smooth muscle cells. Most are made of long, thin cells called spindle cells, arranged in interlacing bundles. Because a fibroid is made of the same cells, the diagnosis rests on three findings assessed together.

Frequent cell division — A cell caught in the act of dividing is called a mitotic figure. For the usual spindle cell type, 10 or more per 10 high-power fields counts toward this diagnosis. Fibroids have far fewer.
Abnormal-looking cells — The cells vary considerably in size, shape, and staining. Pathologists call this nuclear atypia, and it needs to be present throughout the tumor rather than in one small area.
Tumor cell necrosis — Necrosis means tissue death. The type that counts here is tumor cell necrosis, which has an abrupt border between dead and living cells. It is different from the infarct-type necrosis common in fibroids, which shows a zone of healing tissue in between.

A diagnosis of leiomyosarcoma generally requires a combination of these findings rather than any one alone. This is why a fibroid with many dividing cells but normal-looking cells is not a cancer, and is reported instead as a mitotically active leiomyoma. When the combination fits neither category cleanly, the tumor is reported as a smooth muscle tumor of uncertain malignant potential.

Two less common forms exist, and the same three features are assessed for both, using different thresholds.

Epithelioid leiomyosarcoma — Made of rounded cells that resemble the lining cells found on tissue surfaces.
Myxoid leiomyosarcoma — Made of scattered tumor cells within a gelatinous background. These can look deceptively bland, and fewer dividing cells are required for the diagnosis.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside cells. Here it confirms that the tumor is made of smooth muscle rather than another tissue type.

Desmin, smooth muscle actin, and h-caldesmon. Usually positive. These proteins are found in smooth muscle cells.
CD10. Usually negative or patchy. A strongly positive result would raise the possibility of an endometrial stromal tumor instead.
Estrogen receptor and progesterone receptor. Variable. Some of these tumors are positive, which occasionally opens up hormone-blocking treatment.

An important limitation applies. These same muscle proteins are also positive in an ordinary fibroid. Immunohistochemistry confirms what the tumor is made of, but it does not by itself separate cancer from a benign tumor. That separation depends on the three microscopic features described above.

Tumor extension

Several organs sit directly against or very close to the uterus, including the ovaries, fallopian tubes, vagina, bladder, and rectum. The fallopian tubes, ovaries, and the ligaments attached to the uterus are together called the adnexa.

As the tumor grows, it can extend into any of these, and surgeons may remove parts of them in the same operation. The pathologist examines each and reports whether tumor cells are present. Growth beyond the uterus raises the stage and is associated with a higher risk of the cancer returning.

Margins

A margin is the edge of the tissue removed during surgery. The pathologist examines the margins to determine whether the whole tumor was taken out. Margins are reported mainly when the tumor extends into the cervix or into the tissue surrounding the uterus.

Negative margin — No tumor cells at the cut edge. This is the goal of surgery.
Positive margin — Tumor cells present at the cut edge, meaning some cancer may remain. This raises the risk of the cancer returning in the same area and may lead to further treatment.

Lymphovascular invasion

Lymphovascular invasion means tumor cells have been seen inside a blood vessel or a lymphatic vessel. Blood vessels carry blood around the body, and lymphatic vessels carry a fluid called lymph toward lymph nodes.

This finding matters because these vessels are how tumor cells travel elsewhere. Uterine leiomyosarcoma spreads mainly through the bloodstream, and the lungs are the most common site. Your report will describe lymphovascular invasion as present or absent.

Lymph nodes

Lymph nodes are small immune organs found throughout the body. Cancer cells can travel to them and form a metastasis, meaning a deposit of cancer in a new location.

Lymph nodes are not routinely removed during surgery for this cancer. Spread to lymph nodes is uncommon, occurring in fewer than 3 in 100 people whose tumor is confined to the uterus. Studies have not shown that removing nodes improves survival. Nodes are generally taken only when they look abnormal or when the cancer has clearly spread beyond the uterus.

When lymph nodes are removed, the report states how many were examined and how many contained cancer. Unlike some other cancers, uterine sarcoma staging does not divide nodal involvement into size categories. Nodes either contain cancer or they do not.

Pathologic stage

Stage describes how far the cancer has spread. Two systems are used for uterine sarcomas, and they align closely. The American Joint Committee on Cancer system describes the tumor, nodes, and distant spread separately as T, N, and M. The International Federation of Gynecology and Obstetrics uses Roman numerals.

Tumor stage (pT), with the matching FIGO stage:

T1a, stage IA — The tumor involves only the uterus and measures 5 cm or less.
T1b, stage IB — The tumor involves only the uterus and measures more than 5 cm.
T2a, stage IIA — The tumor has spread into the adnexa.
T2b, stage IIB — The tumor has spread to other tissue in the pelvis.
T3a, stage IIIA — The tumor has spread to one site in the abdomen.
T3b, stage IIIB — The tumor has spread to more than one site in the abdomen.
T4, stage IVA — The tumor has grown directly into the bladder or the rectum.

Nodal stage (pN): N0 means no tumor cells were found in the lymph nodes examined. N1 means tumor cells were found in at least one lymph node, which corresponds to FIGO stage IIIC. NX means no lymph nodes were sent for examination, which is common for this cancer.

Metastatic stage (pM): M1, or FIGO stage IVB, means the cancer has spread to a distant site such as the lungs. This can only be assigned when tissue from that site has been examined, so most reports list it as MX or leave it blank. Doctors use imaging instead to look for distant spread.

How is uterine leiomyosarcoma treated?

Surgery is the main treatment. What follows depends on the stage and on your situation.

Hysterectomy — Removal of the whole uterus, usually with the cervix. This is the standard operation.
Removing the ovaries — Often done, particularly after menopause. In younger patients, the decision is individual, since removing them brings on early menopause and the benefit is less clear for this cancer than for some others.
Avoiding morcellation — Morcellation cuts a tumor into pieces so it can be removed through small incisions. If the tumor turns out to be a leiomyosarcoma, this can spread tumor fragments within the abdomen and has been associated with a higher rate of the cancer returning. This is why the technique is used cautiously when removing a fibroid.
Chemotherapy — Used for advanced or recurrent disease. Whether it helps after surgery for early-stage disease is still debated, and practice varies between centers.
Radiation therapy — Used in selected situations, mainly to reduce the chance of the cancer returning in the pelvis.
Hormone-blocking treatment — Considered when the tumor is positive for estrogen and progesterone receptors.

Because this cancer is uncommon, care at a center experienced with sarcomas is generally recommended, and asking for that referral is reasonable.

What is the prognosis?

Stage is the strongest factor. For tumors confined to the uterus, reported five-year survival is roughly 50 to 60 out of every 100 people. For tumors that have spread beyond the uterus, the figures are lower.

Other features that influence the outlook include tumor size, how many cells were dividing, whether the tumor was removed whole, and whether the margins were clear. Recurrence most often appears in the lungs or pelvis, and follow-up usually includes chest imaging for that reason.

Published figures come from groups of patients and cannot tell you what will happen in your case. They do not account for your stage, your surgery, or the treatment you receive. Your own team, and particularly a sarcoma specialist, is far better placed to discuss what the outlook means for you.

Questions to ask your doctor

What stage is my cancer?
How large was the tumor, and how many dividing cells were counted?
Were the margins negative, and was the tumor removed whole?
Was morcellation used during my surgery?
Was lymphovascular invasion present?
Were lymph nodes removed, and did any contain cancer?
Were the estrogen and progesterone receptors tested?
Did a pathologist who specializes in gynecologic or sarcoma pathology review my case?
Should I be referred to a sarcoma center?
Do you recommend chemotherapy or radiation therapy, and why?
What will follow-up involve, and how often?
Are there clinical trials I should consider?
What symptoms should prompt me to contact you between appointments?