“Localized TGCT is usually around the joint space on smaller joints like the fingers and toes,” says Arun Singh, MD, an assistant professor of hematology/oncology at the David Geffen School of Medicine at UCLA in Los Angeles. “Diffuse TGCT is usually around larger joints, like the knee or hip. It’s more like a blob, as opposed to a tumor that’s circular." TGCT can cause joint stiffness and pain and impact mobility, and it will need to be treated, according to the National Organization for Rare Disorders (NORD). Localized TGCT can often be treated successfully with surgery alone. Diffuse tumors are more widespread and difficult to treat, so they may require additional interventions, such as medication, according to the American Academy of Orthopaedic Surgeons. TGCT is not common, but it can have serious, life-changing consequences for people who have it, especially those with the diffuse form. A study published in August 2017 in the journal Acta Orthopaedica estimated worldwide diagnosis rates in a given year at approximately 10 in one million for localized and four in a million for diffuse TGCT. If you’ve been diagnosed with TGCT or your doctor suspects you may have this condition, you probably want to learn as much as you can. Take this quiz to find out where your knowledge gaps are so you can focus your research and learn what to ask your doctor.