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.
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.
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.
The symptoms overlap with those of fibroids, which is part of why the diagnosis is often unexpected.
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.
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.
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.
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.
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.
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.
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.
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 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.
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:
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.
Surgery is the main treatment. What follows depends on the stage and on your situation.
Because this cancer is uncommon, care at a center experienced with sarcomas is generally recommended, and asking for that referral is reasonable.
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.