How Are Desmoid Tumors Diagnosed?

Medically reviewed by Richard F. Riedel, M.D.

Written by Emily Wagner, M.S.

Updated on October 21, 2026

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Key Takeaways

If you are experiencing desmoid tumor symptoms, your doctor will use a combination of medical history, physical exams, imaging tests, and biopsies to make a diagnosis.
Imaging tests like MRI, CT scans, and ultrasound help doctors locate and study tumors, while a biopsy is the most reliable way to confirm a desmoid tumor diagnosis. Genetic testing may also be done to check for gene mutations linked to desmoid tumors, which can provide useful information for both you and your family members.
Because desmoid tumors are rare and can sometimes be misdiagnosed, it may be helpful to ask your doctor about getting a second opinion from a soft tissue pathologist or sarcoma specialist who can make sure you have the most accurate diagnosis and access to the latest treatment options.

View all takeaways

If you’re experiencing desmoid tumor symptoms, your doctor can run tests to make a diagnosis. Ultimately, imaging will help identify and characterize a suspected tumor, but a biopsy is generally used to make the definitive diagnosis.

Your healthcare provider may also run genetic testing to see if you have an inherited condition associated with desmoid tumors.

Here’s what you need to know about the different tests used to diagnose desmoid tumors and what they tell your doctor.

History

Your doctor will first ask about your medical history, including how long you’ve had desmoid tumor symptoms. Desmoid tumors are usually slow-growing, and some people have symptoms for a long time before seeing a doctor.

They’ll also ask about your history of trauma, pregnancy, previous surgeries, and family history of cancer. Thinking through your family history prior to your appointment and making notes can help you provide a clear picture for your healthcare provider.

Physical Exam

Your doctor will continue the diagnostic process by performing a physical exam. During the exam, they’ll ask more about any symptoms you may be experiencing. Be sure to tell them if you have any pain or swelling in a specific area.

In some cases, you may even be able to feel a lump or mass under your skin. Common places for extra-abdominal tumors (tumors outside the abdomen) include the shoulders, arms, and legs.

Sometimes, people develop intra-abdominal desmoid tumors — desmoid tumors that occur in the tissue around the intestines. Intra-abdominal tumors can cause digestive issues and weight loss. Some people may also experience nausea, cramps, and vomiting.

Using the information you provide, your doctor will take a closer look at your body for any signs of desmoid tumors. If your doctor finds a lump or wants to investigate your symptoms more, they’ll likely order some imaging tests.

Imaging Tests

Desmoid tumors are noncancerous growths that form in the body’s connective tissues. To see these growths, your doctor can order imaging tests. Examples include magnetic resonance imaging (MRI), computed tomography (CT) scans, and ultrasound.

Magnetic Resonance Imaging

An MRI scan uses powerful magnetic fields to take detailed pictures of your body for measuring tumors. An MRI machine takes images of your body in “slices” — similar to a loaf of bread. When all of the images are put together, they create a 3D scan.

Doctors often use MRIs to look for signs of extra-abdominal desmoid tumors and may use CT scans to spot intra-abdominal desmoid tumors. These same scans can also be used to monitor your tumor after diagnosis.

In general, doctors prefer MRIs to other imaging methods. This is because MRIs are often more sensitive than CT scans when looking at soft tissues, like muscle.

Before an MRI, you’ll need to remove any metal from your body, such as rings, earrings, and necklaces. If you have a pacemaker or metal implant, let your doctor know.

Depending on the technology and materials used for your device, an MRI may not be ideal for you. However, if you have a newer device, it may be MRI-conditional, meaning an MRI is possible under certain conditions.

On the day of your scan, you may receive an intravenous (into a vein) injection with a special dye called contrast. This dye helps the doctor better see what’s happening inside your body.

Computed Tomography

A CT scan uses X-rays to take detailed images of your soft tissues (fat, muscle, and organs) and bones. The machine collects the images from several different angles. Computer software then uses them to create 3D pictures of your body.

Doctors use CT scans to find desmoid tumors or look for other abnormalities. If your doctor thinks you have an intra-abdominal desmoid tumor, they may order a CT scan with contrast.

Before your scan, you’ll either have an IV injection or drink a beverage containing contrast medium, which — like the dye you may receive prior to an MRI — helps your doctor see your body’s tissues in more detail.

Ultrasound

Ultrasound uses sound waves to create pictures of the body’s tissues. If you have a noticeable lump close to the surface of your skin, your doctor may use an ultrasound to take a closer look.

Biopsies

Your doctor can confirm you have a desmoid tumor by doing a biopsy. This test involves removing small tissue samples to look at under a microscope. A pathologist (tissue specialist) is trained to look at cells for abnormalities and make a diagnosis.

There are several types of biopsies that may be performed. Your doctor will choose the best technique based on your tumor’s location. The most common option is a core-needle biopsy. This biopsy uses a wide, hollow needle to take a tissue sample.

Fine-needle aspirations use thinner needles but aren’t used as often as core-needle biopsies for desmoid tumors. This is because they can miss the tumor, and you want to have enough tissue to make the diagnosis.

Before your core-needle biopsy, your doctor will likely numb the area with a local anesthetic around the tissue they’re going to biopsy. Your imaging test results will help guide what tissues need to be biopsied.

After your biopsy, your doctor will cover the site with a bandage. You’ll be able to go home the same day. You may experience pain and minimal bleeding, so be sure to rest for the remainder of the day.

It may take a few days to get your biopsy results back from the pathologist.

Genetic Testing

Around 85 percent of spontaneous desmoid tumors are caused by mutations (changes) in the catenin beta 1 (CTNNB1) gene. This mutation is not passed down from parent to child.

CTNNB1 provides instructions for beta-catenin — a protein that sends growth signals to cells. Too much beta-catenin leads to uncontrolled cell growth and desmoid tumor formation.

About 5 percent to 10 percent of desmoid tumors are associated with familial adenomatous polyposis (FAP). This inherited condition causes hundreds of polyps (growths) in the intestines.

People with FAP are more likely to develop desmoid tumors compared to the general population. FAP is also associated with a high risk of colon cancer. FAP is specifically caused by mutations in the adenomatous polyposis coli (APC) gene. These gene changes are passed down through family members.

If you’ve been diagnosed with desmoid tumors, your doctor may offer genetic testing to find out if you have mutations in the CTNNB1 or APC genes. But CTNNB1 and APC mutations are mutually exclusive, meaning if you have one, you won’t have the other.

To check for CTNNB1 mutations, doctors can test a tumor tissue sample, while APC mutations can be detected with a blood test. Overall, APC mutations are found in around 80 percent of people with FAP.

How Is Genetic Testing Used?

The results of your genetic testing may help inform your doctor’s treatment recommendations.

Genetic testing can also help other family members be proactive about their health risks. If you have the genetic mutation associated with FAP, your healthcare team may recommend that family members undergo genetic testing. This can be lifesaving, because people with FAP can develop colon cancer as early as age 20.

Staging

Tumor staging is commonly used for cancer to describe its size, location, and spread. But desmoid tumors aren’t cancer.

Doctors don’t use any official staging system for these tumors. Instead, they base their treatments on your tumor’s size and what (if any) symptoms you’re experiencing.

Getting a Second Opinion

Desmoid tumor is rare, and only about 1,000 new cases are diagnosed every year in the United States. This means that many pathologists have never seen desmoid tumor tissue samples before.

The Desmoid Tumor Research Foundation notes that it’s possible to receive a misdiagnosis from a pathologist. Consider asking your doctor about getting a second opinion from a soft tissue pathologist or other sarcoma specialist.

These doctors specialize in diagnosing sarcomas, or cancers that affect the soft tissues and bones. Desmoid tumor samples require specific tests for an accurate diagnosis, and it’s best if a soft tissue pathologist performs them.

If your doctor confirms you have a desmoid tumor, look for a sarcoma specialist. They’ll have all the latest information on which treatment options are best for you. Many sarcoma specialists work in teams with oncologists who focus on surgery, radiation, and medical treatments.

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