Synovial Sarcoma, from the CureRays guide library.

Synovial sarcoma is a soft-tissue cancer that most often appears as a deep mass near a joint in a younger adult; it is defined by a specific gene fusion, and it is treated with limb-sparing surgery and radiation, with chemotherapy playing a larger role than in many other sarcomas.

What it is.

Synovial sarcoma is a soft-tissue sarcoma — a cancer of the body's connective tissues — that tends to occur in adolescents and younger adults, an age group in which most cancers are uncommon. Despite its name, it does not actually come from the synovium (the lining of joints); the name reflects how the cells looked to early pathologists, but the tumor can arise anywhere in the soft tissues. It most often appears near a large joint, especially around the knee, thigh, or other parts of the arms and legs, as a deep, slowly growing mass that may be present for a long time and is sometimes mistaken for a benign cyst or a sports injury. Because it can grow quietly, a deep soft-tissue lump that persists, grows, or is larger than a few centimeters — particularly in a young person — should be imaged and biopsied at a sarcoma center rather than assumed to be harmless. Synovial sarcoma is defined at the molecular level by a characteristic gene fusion (an SS18 rearrangement), and finding that fusion confirms the diagnosis. It is generally considered a higher-grade, aggressive cancer that can spread, most often to the lungs, sometimes years after the original tumor — which is why long-term follow-up is important. Treatment combines complete surgical removal of the tumor with radiation to treat the microscopic disease that extends beyond the visible mass, so the limb can be preserved and the chance of local recurrence kept low. Chemotherapy plays a larger role in synovial sarcoma than in many other adult soft-tissue sarcomas, because this tumor is relatively chemotherapy-sensitive and tends to affect younger, otherwise-healthy patients who can tolerate it; it is considered especially for larger, higher-risk tumors and for disease that has spread.

How radiation treats it.

Radiation therapy treats synovial sarcoma by delivering focused beams of energy that damage the DNA inside tumor cells so they can no longer grow and divide. As with other soft-tissue sarcomas, the reason radiation pairs so naturally with surgery comes from how the tumor grows: rather than staying within the firm, visible mass, it pushes microscopic fingers of tumor into the surrounding tissue beyond what can be seen or felt. If a surgeon removed only the visible tumor, those microscopic extensions could be left behind and seed a recurrence. Radiation treats a wider zone around the tumor, sterilizing the microscopic disease so the limb can be preserved and the cancer is far less likely to come back locally. Because synovial sarcoma so often arises near a joint in a young person with many active years ahead, preserving the limb and its function is a central goal, and combining a smaller operation with radiation achieves local control as good as more radical surgery once did. Radiation can be given before or after surgery: beforehand it uses a smaller treatment area and a lower dose and is associated with better long-term limb function, though it requires extra attention to wound healing; afterward it treats the tumor bed at a higher dose over a larger area. Modern techniques such as intensity-modulated radiation shape the dose tightly around the target to spare the surrounding muscle, bone, and joint — an especially important consideration in young patients who will live with the treated limb for decades. Radiation also has a role beyond the original site: when synovial sarcoma spreads to a limited number of spots in the lungs, focused high-dose radiation (stereotactic body radiation) can ablate those deposits and provide durable control without surgery. Throughout, radiation works alongside surgery and, more often than in many adult sarcomas, chemotherapy, because this tumor is relatively chemotherapy-sensitive and tends to affect younger, otherwise-healthy patients.

The ways we can treat it.

Preoperative (neoadjuvant) radiation. Radiation given before surgery treats the rim of microscopic disease using a smaller field and lower dose, which can make a limb-sparing operation safer and is associated with less long-term stiffness, at the cost of more wound-healing care.

Postoperative (adjuvant) radiation. Radiation to the tumor bed after surgery, used when preoperative radiation wasn't given, treats microscopic disease left behind and lowers the chance of local recurrence, at a higher dose over a larger area.

Intensity-modulated radiation (IMRT). Shaping the beams tightly around the target spares surrounding muscle, bone, and the nearby joint, helping preserve limb function — particularly valuable in young patients with many active years ahead.

Stereotactic body radiation (SBRT) for metastases. Focused, high-dose radiation can ablate a limited number of lung metastases, offering durable control of oligometastatic disease without surgery.

Questions we hear often.

I'm young and healthy — why did I get this cancer? Synovial sarcoma is one of the few cancers that disproportionately affects adolescents and younger adults, and in nearly all cases it is not inherited and not caused by anything you did. It arises from a specific genetic change — an SS18 gene fusion — that occurs in a cell by chance and is found in the tumor, not in the rest of your body. There is rarely an identifiable cause or anything that could have prevented it. What matters now is getting treatment at a sarcoma center, where the combination of surgery, radiation, and chemotherapy is tailored to give the best chance of cure while preserving your limb and function.

Why is chemotherapy used more for synovial sarcoma than for some other sarcomas? Two reasons. First, synovial sarcoma is relatively chemotherapy-sensitive compared with many other adult soft-tissue sarcomas, so chemotherapy is more likely to help. Second, it tends to affect younger, otherwise-healthy people who can tolerate chemotherapy well. For these reasons, chemotherapy is weighed more heavily here — considered especially for larger or higher-risk tumors and used for disease that has spread — alongside the surgery and radiation that control the tumor where it started.

Why do I need follow-up for so many years? Synovial sarcoma can spread to the lungs, and that spread sometimes appears years after the original tumor was treated. Because of this, follow-up — including periodic imaging of the chest — continues for a long time, so that any spread can be found early when it is most treatable. If a limited number of lung deposits appear, they can often be removed surgically or treated with focused, high-dose radiation. Staying on schedule with follow-up visits is an important part of long-term care.

This guide is informational only. It is not medical advice — please confirm anything here with your care team.