Do Uterine Fibroids Become Cancerous or Raise Cancer Risk?

If you have been diagnosed with uterine fibroids, one of your biggest fears might be that they will turn into cancer. The short answer is reassuring: typical benign uterine fibroids (medically known as leiomyomas) are not considered precancerous lesions, and they are not generally believed to transform into cancer ^[1].

When people ask this question, they are usually worried about a specific, rare type of uterine cancer called leiomyosarcoma. (Note that the most common type of uterine cancer is endometrial cancer, which starts in the lining of the uterus, not the muscle). Current scientific evidence generally supports that most leiomyosarcomas develop independently from the start, rather than slowly changing from a benign fibroid into a cancer ^[1].

However, because these two types of tumors can cause identical symptoms and look very similar on imaging, a rare cancer can sometimes be mistaken for a fibroid until it is removed and tested in a lab ^[2][3]. While most people with fibroid symptoms do not have cancer, it is important to understand the biological difference and know what signs warrant prompt medical review.

The Difference Between a Fibroid and a Sarcoma

Fibroids (leiomyomas) are common, non-cancerous (benign) tumors made of smooth muscle tissue in the wall of the uterus. Uterine leiomyosarcomas are rare, cancerous (malignant) tumors that grow in the same smooth muscle tissue.

When pathologists look at the tumor tissue under a microscope, they look for specific cellular signs to tell the difference. Diagnosis requires careful expert review of several features, including cytologic atypia (highly abnormal and chaotic cells), high mitotic activity (rapidly dividing cells), and a specific type of cell death called coagulative tumor necrosis ^[4]. Pathologists must carefully distinguish this dangerous necrosis from the common, harmless tissue degeneration that happens when a benign fibroid outgrows its blood supply.

In some rare cases, the cells do not clearly look like an ordinary benign fibroid, but they also don’t meet the strict criteria for cancer. These borderline cases are classified as a STUMP (smooth muscle tumor of uncertain malignant potential) ^[5][6]. A STUMP is not a confirmed cancer, but its future behavior can be difficult to predict. If you receive a STUMP diagnosis, it typically requires specialist review and individualized long-term follow-up ^[4].

The Myth of Fibroids “Turning Into” Cancer

You may have heard statistics about cancer being found unexpectedly during fibroid surgeries, which can cause significant anxiety. Studies estimate that an unexpected leiomyosarcoma is found in roughly 1 in 500 to 1 in 1,000 surgeries performed for what doctors presumed were benign fibroids ^[2][7].

It is important to put this number in context. This is the rate of unexpected cancer found in a select group of people already undergoing surgery for a problematic mass—it is not the risk for everyone walking around with fibroids. The risk also varies by age and study method, and is generally higher in postmenopausal women ^[2]. Crucially, this does not mean the fibroid turned into cancer; it means the tumor was likely a sarcoma all along, but standard tests couldn’t definitively tell the difference before surgery ^[1].

When Should You Seek Medical Review?

Most people with fibroid symptoms like bleeding, pressure, or a growing abdomen do not have a sarcoma. However, your doctor will look for certain “red flag” symptoms that require clinical reassessment.

You should seek prompt medical evaluation if you experience:

• Postmenopausal growth: Fibroids usually shrink after menopause because they rely on reproductive hormones like estrogen to grow. A new or growing uterine mass after menopause is a significant warning sign that needs careful evaluation ^[8][9].
• Postmenopausal bleeding: Any vaginal bleeding after menopause requires prompt investigation. While most causes are not cancer, it is a key symptom that should never be ignored ^[8][10].
• Persistent abnormal bleeding: Heavy, irregular, or persistent bleeding that does not improve with typical treatments should be evaluated further ^[8].
• Rapid enlargement: Benign fibroids can grow quickly, so rapid growth alone is not a reliable indicator of cancer (especially before menopause) ^[11]. However, it is a reason for your doctor to take a closer look and potentially reassess your treatment plan.

(Note: If you experience bleeding heavy enough to soak a pad in an hour, fainting, severe weakness, or sudden severe pelvic pain, you should seek urgent medical care, regardless of cancer concerns).

Diagnosis and Safe Tissue Removal

Imaging tools like ultrasound or MRI can help assess a mass and identify unusual features, but they cannot definitively rule a sarcoma in or out ^[12].

A definitive diagnosis requires a specialized doctor to examine the actual tumor tissue under a microscope ^[4][13]. You might wonder if a simple biopsy can do this. While an endometrial or needle biopsy can sometimes be helpful, it can easily miss a tumor located deep in the uterine muscle ^[13]. Therefore, a negative biopsy does not guarantee a mass is benign.

If surgery is required to remove a suspicious mass, the surgical technique matters immensely. Cutting a tumor into smaller pieces to remove it through a small incision (a process called uncontained power morcellation) can accidentally spread hidden cancer cells throughout the abdomen. If you are having surgery, you should discuss whether intact (en-bloc) removal or “contained” morcellation in a surgical bag is possible ^[14]. If there is a high suspicion of cancer based on your age, symptoms, or imaging, your care should be managed by a gynecologic oncologist (a specialist in female reproductive cancers) ^[14].