Leiomyosarcoma is a rare cancer, accounting for 10% to 20% of all soft tissue sarcomas. Soft tissue sarcomas themselves make up roughly 1% of adult cancers, which places leiomyosarcoma in a very small slice of overall cancer diagnoses. For the uterine form specifically, the annual incidence is just 0.35 to 0.64 per 100,000 women in the United States.
Leiomyosarcoma by the Numbers
Leiomyosarcoma (LMS) develops from smooth muscle cells, the type of muscle found in the walls of organs, blood vessels, and the digestive tract. Because smooth muscle exists throughout the body, LMS can appear in many locations. The most common sites are the uterus, the retroperitoneum (the deep space behind the abdominal cavity), and the extremities (arms and legs).
Among patients with non-uterine LMS, about 60% of tumors arise in the extremities and 32% in the abdomen, with smaller numbers in the head, neck, and chest. The median age at diagnosis for uterine LMS is 52 years, and the cancer is exceptionally rare in children and adolescents. In adults, it remains uncommon enough that many oncologists outside of specialized sarcoma centers may see only a handful of cases in their careers.
Why It’s Hard to Detect Early
One of the most challenging aspects of LMS is that it can look like something harmless. In the uterus, LMS is nearly impossible to distinguish from fibroids (benign smooth muscle growths) on imaging alone. Fibroids are extremely common, affecting up to 70% of women by age 50, while LMS is vanishingly rare by comparison. This creates a needle-in-a-haystack problem for doctors.
A large California-based study found that among women who underwent hysterectomy or myomectomy for presumed fibroids, roughly 0.3% had an unsuspected uterine sarcoma. The FDA has cited a similar estimate of about 1 in 352. That risk is not evenly distributed, though. Women under 50 had the lowest rates, between 0.08% and 0.13%. Women over 60 had a significantly higher prevalence, ranging from 0.36% to 1.53%. This age-dependent pattern means a rapidly growing uterine mass in a postmenopausal woman warrants more suspicion than the same finding in a younger woman.
Because there is no reliable biopsy or imaging method that consistently distinguishes a fibroid from LMS before surgery, the diagnosis is often made only after tissue is removed and examined under a microscope. This is one reason the cancer can be caught at a more advanced stage.
Where LMS Occurs in the Body
The uterus is the single most studied site, but LMS is not exclusively a gynecologic cancer. It develops anywhere smooth muscle is present. In broad terms, there are two major categories: uterine LMS and non-uterine LMS. These behave somewhat differently and carry different prognoses.
For non-uterine cases, the limbs are the most frequent location, followed by abdominal and retroperitoneal tumors. Retroperitoneal LMS is particularly difficult to catch early because the tumor can grow to a large size in a spacious area of the body before causing noticeable symptoms. Tumors arising from blood vessel walls, especially the inferior vena cava (the large vein returning blood to the heart), are a recognized but rare subtype.
How Rare Compared to Other Cancers
To put LMS in context, consider that about 13,000 new soft tissue sarcomas are diagnosed in the U.S. each year. LMS makes up 10% to 20% of those, translating to roughly 1,300 to 2,600 new LMS cases annually across all body sites. Compare that to breast cancer (around 300,000 new cases per year) or lung cancer (around 230,000), and the scale difference is enormous.
This rarity creates real challenges. Clinical trials are harder to fill, which means treatment advances come more slowly than for common cancers. Finding a surgeon or oncologist with deep LMS experience often requires traveling to a major academic medical center or a sarcoma-specific program. Patient communities and advocacy organizations, such as the Leiomyosarcoma Support & Direct Research Foundation, exist in part to bridge this gap by connecting patients with specialized care.
What Rarity Means for Patients
Being diagnosed with a rare cancer can feel isolating, and the practical effects are significant. Because most general oncologists encounter LMS infrequently, getting a second opinion from a sarcoma specialist is widely recommended. Pathology review is especially important. Sarcomas are notoriously difficult to classify under the microscope, and misdiagnosis or subtype confusion can lead to the wrong treatment approach.
The rarity also means that symptoms are easy to dismiss. A growing mass in the abdomen or a lump in the thigh may be attributed to more common conditions long before LMS is considered. For uterine LMS, symptoms like abnormal bleeding or pelvic pain overlap with dozens of benign gynecologic conditions. There is no widely available screening test, so most cases are found either incidentally during surgery for another condition or after symptoms become persistent enough to prompt imaging and biopsy.