According to the National Organization for Rare Disorders, (NORD), both forms of TGCT affect the tissues of the joints, specifically the:

Bursae Small, fluid-filled sacs that cushion the bones, tendons, and muscles near your jointsTendon sheaths The membrane that covers a tendon, the fibrous tissue that connects muscle to boneSynovium Thin layer of connective tissue that lines the inside of the joint spaces of the bursae and tendon sheaths

Localized TGCT, as the name implies, features the growth of an abnormal mass or nodule that usually stays limited to a specific area of the joint, says Arun Singh, MD, an assistant professor in the division of hematology and oncology at the David Geffen School of Medicine at the University of California Los Angeles (UCLA). Diffuse TGCT, in contrast, is widespread and can affect the entire joint.

Where Diffuse and Localized Types of TGCT Tend to Strike

While localized TGCT can appear anywhere, it tends to develop more commonly in the hands and feet, according to Dr. Singh. An article published in February 2017 in the journal Orthopaedics and Traumatology: Surgery and Research estimates that as many as 85 percent of localized TGCTs occur in the fingers and wrist. Diffuse TGCT most often affects larger joints, such as the knee or hip, as well as the ankle, elbow, or shoulder, according to NORD. In rare cases, the two joints that connect the jawbones to the skull (temporomandibular joints) or the joints that connect vertebrae (spinal facet joints) can also develop diffuse TGCT, per NORD. Given the aggressiveness of diffuse TGCT, it is very fortunate that it is much less common than localized TGCT. “Epidemiologically, it’s 10 to one that it is the [local] type over the diffuse type,” says Bernthal. There doesn’t seem to be a difference in what ages are affected by localized vs. diffuse. The typical person with TGCT is about 30 years old, says, Bernthal, although he notes that he’s treated patients ranging in age from teens to 70s. According to NORD, TGCT most commonly affects people 25 to 40. There may be a small gender difference between the two types. Localized TGCT, but not diffuse TGCT, seems to affect women at a slightly higher rate than men, though doctors are not yet sure why. In a study of 2,087 TGCT patients published in October 2017 in The Journal of Rheumatology, researchers found that 61 percent of patients with localized tumors were women, but diffuse TGCT seemed to affect men and women equally.

How Diffuse TGCT and Localized TGCT Are Diagnosed

Both diffuse TGCT and localized TGCT have similar symptoms, says Bernthal, and they are diagnosed the same way. Typically, a person with TGCT may experience pain, stiffness, swelling, and limited range of motion in a joint that’s affected by TGCT, says Bernthal. They may also feel an unpleasant “locking” or “popping” sensation in the joint. These symptoms, notes Bernthal, will usually be disproportionate to the amount of activity they engaged in (experiencing pain as if they’d worked out strenuously when they hadn’t, for instance). Because these symptoms are so similar to those of other joint conditions that are much more common, such as arthritis, someone with TGCT may see several doctors before they get the right diagnosis. They may starting with their primary care physician and then perhaps be evaluated by a sports doctor and eventually an orthopedic specialist, says Bernthal. A doctor will usually perform a physical exam, take a full medical history, and order scans, such as computed tomography (CT) or magnetic resonance imaging (MRI), says Bernthal. A radiologist may be able to diagnose TGCT through a scan, but in many cases, a biopsy will also be performed to confirm the diagnosis. The fascinating thing about TGCT is that the two types look very different to the naked eye but are the same microscopically. When doctors examine a scan of the affected area, the difference between the types is immediately apparent, says Bernthal. While localized TGCT is a clearly defined tumor, diffuse TGCT is spread out so that “you can’t draw a circle around it,” says Bernthal. “With the localized type, you can see where it begins and where it ends, whereas with the diffuse type, it looks like the roots of a tree — it just kind of goes into things,” he notes. But when biopsied cells of the two types of TGCT are examined under a microscope, they look identical, says Bernthal. For both types of TGCT, the biopsy can confirm that the mass is not cancerous or malignant, and can also reveal that it’s not just an inflammatory condition.

How Doctors Treat Localized Type TGCT vs. Diffuse Type TGCT

Surgery has traditionally been the main treatment for both localized and diffuse TGCT, but the success rates have been very different. For localized TGCT, surgery to remove the tumor has been able to cure the disease in most patients, says Singh. According to the February 2017 article in Orthopaedics and Traumatology: Surgery and Research, as many as 73 to 91 percent of patients show no recurrence of disease at 5 years after surgery, meaning that the tumor doesn’t grow back in the majority of cases. Bernthal estimates that the cure rate could be higher, as much as 95 percent. “For [localized] disease, surgery is the mainstay of treatment and should stay that way,” says Bernthal. But the outcomes for diffuse TGCT have been a very different story. Because this type of TGCT affects a larger area of the joint, it has been more difficult to treat successfully with surgery, says Singh. Diffuse TGCT, he explains, could wrap around a joint, increasing the risk that an operation to remove the tumor may damage the joint and cause pain. In addition, diffuse TGCT has a high likelihood of eventually growing back. Depending on the location and extent of the disease, diffuse TGCT may be treated with a partial synovectomy (removing the diseased area of the synovium) or a complete synovectomy. In severe cases, total joint replacement may be necessary, according to NORD. In short, says Bernthal, surgery as a treatment has not been that successful for some people with diffuse TGCT. “The published reported rates of curing people with diffuse type with surgery is about 50 percent, and I think most of us who see a lot of this would argue that that’s optimistic,” says Bernthal. “Because a lot of people simply do not get more MRIs or come back to you, you’re counting them as cured; the truth is, they probably still have residual symptoms of the disease.” Another treatment that has been used for diffuse TGCT is radiation, but it is not ideal, as it has some side effects, such as stiffening of the joint, and also increases risk of cancer, says Bernthal. In 2019 the Food and Drug Administration approved the drug pexidartinib as a treatment option for people with diffuse TGCT who are not good candidates for surgery. Research has shown pexidartinib to be effective at inhibiting tumor growth and it may reduce or even eliminate the need for surgery in patients who would be at high risk for a poor outcome, says Singh.