If you’ve been diagnosed with tenosynovial giant cell tumor (TGCT), it’s likely that you had not previously heard of this joint disease. That’s because this type of tumor, which occurs in the joint and tendon sheaths, is rare, with only about 600 to 1,200 cases diagnosed each year in the United States, according to Thomas J. Scharschmidt, MD, a member of the Molecular Biology and Cancer Genetics Program at The Ohio State University Comprehensive Cancer Center in Columbus, Ohio.

Here are key facts about TGCT that can help you understand this little-known condition and get the treatment you need.

1. There are two main types of TGCT.

Doctors today categorize TGCTs into two types: localized (or nodular) and diffuse, says Dr. Scharschmidt. Localized means the tumor is present in a smaller, defined area; diffuse means the tumor is widespread. (Diffuse-type TGCT was formerly known as pigmented villonodular synovitis, or PVNS.)

Both diffuse and localized TGCT can be “intra,” meaning inside the joint or “extra,” meaning outside a joint, in the tendon sheath, says Scharschmidt.

2. TGCTs affect the joints.

These tumors grow in the synovium (the membrane lining the inside of a joint), bursae (fluid-filled sacs that cushion bones, tendons, and muscles around the joints), or tendon sheath (a membrane covering a tendon), according to the National Organization for Rare Disorders (NORD).

According to a study published in February 2017 in the journal Orthopaedics and Traumatology: Surgery and Research, localized TGCTs most often affect the hands, specifically the fingers or the wrist, but they can also form in the foot, ankle, knee, hip, or other joints. This study notes that diffuse forms mainly involve the large joints such as the knee, hip, ankle, and elbow, although a Dutch study published in August 2017 in the journal Acta Orthopaedica found that both localized and diffuse TGCTs were most often found in the knee.

3. A chromosomal change causes the tumor.

According to NORD, researchers have found that some of the cells inside these tumors have a chromosomal translocation, which in simple terms means a portion of a chromosome breaks and reattaches to a different chromosome. Researchers are investigating why this happens but do not yet have clear answers. “We know there is a neoplastic change in the joint [which means that a tumor forms] and this is not just an inflammatory disease,” says Scharschmidt. Too many copies of a protein called colony-stimulating factor-1 (CSF-1) are produced and that causes other cells in the body to grow the tumor but, he says, “We don’t know what causes [this process] to go haywire.”

4. TGCTs seem to affect certain people more than others.

Doctors aren’t sure why but TGCT is most often diagnosed in younger adults. According to NORD, the disease often occurs in people 25 to 40, and the median age of those diagnosed is 30.

There may also be a gender difference: According to NORD, some studies have shown these tumors may occur slightly more often in women than in men. “It affects females more than males, though we don’t know why that might be,” says Scharschmidt. In one study of 2,087 TGCT patients published in October 2017 in The Journal of Rheumatology, 61 percent of patients with localized tumors were women. The diffuse cases, though, were just about split evenly between women and men.

5. It can take years to be diagnosed.

People with TGCT often experience pain, swelling, or a catching or locking sensation in the affected joint, says Scharschmidt. But since these symptoms are similar to those of a host of other conditions from arthritis to regular wear and tear, he notes that it can take months — even years — before an individual receives a TGCT diagnosis, which may involve consulting several doctors, including a primary care physician, a rheumatologist, and an orthopedic doctor.

To diagnose TGCT, your doctor will likely perform a thorough physical exam and order imaging scans, such as a computerized tomography (CT) scan or magnetic resonance imaging (MRI), to look at the affected joint and the surrounding area, says Scharschmidt. In some cases, a biopsy may also be done to confirm the diagnosis.

6. TGCTs are benign but not necessarily harmless.

For many people, the word “tumor” invokes cancer. But TGCTs are typically benign, which means they are not cancerous and do not spread, or metastasize, to other areas of the body, notes NORD. Just because they’re benign doesn’t mean TGCTs can’t cause problems. “These tumors can be locally invasive and destructive to the joint,” says Scharschmidt. That’s why it’s so important to follow the treatment recommended by your doctor. There is a lot of inflammation in the joint in these types of tumors; left untreated, that inflammation “wears down the cartilage, causing the joint to become arthritic and chronically painful,” he says.

7. Surgery can be a successful treatment option for many people with TGCT.

For localized TGCT, which is more common than diffuse TGCT, according to the August 2017 study, surgery is the gold standard of care. “Around ninety percent of the time, surgery is curative, and the tumor never grows back,” says Scharschmidt.

For diffuse tumors, which can coat the entire inside of the joint, surgery may be less effective. While surgery is still routinely performed, says Scharschmidt, there’s about a 50 percent chance the tumor will grow back again, and people often require multiple surgeries over time.

In 2019, the Food and Drug Administration approved a new drug called pexidartinib for treatment of diffuse TGCT, and while doctors have yet to determine the best application for this drug, says Scharschmidt, it may be recommended for people whose tumor has grown back multiple times or cannot be safely or effectively removed. Medications called colony-stimulating factor-1 receptor (CSF-1R) inhibitors are currently being investigated as a potential treatment option for diffuse TGCT.