Synovial sarcoma is a rare and slow-growing cancer that affects the soft tissues of the body, often appearing as a painless lump near large joints like the knee or ankle, though it can develop almost anywhere in the body.
This type of cancer gets its name because the cancer cells look similar to cells found in the synovial joints, which are the spaces where bones meet, like in your elbows, hips, and shoulders. However, despite its name, synovial sarcoma doesn’t always start in these joint areas. It can form in muscles, ligaments (tissues that connect bones), tendons (tissues that connect muscles to bones), and other soft tissues throughout the body, including the lungs and abdomen.[1]
One of the challenging aspects of synovial sarcoma is that it grows very slowly and often doesn’t cause pain at first. This means it can go unnoticed for months or even years, sometimes growing for up to two years before anyone realizes something is wrong. During this time, the tumor may be quietly developing deep within the tissue, making early detection difficult.[1]
The disease typically shows itself as a swelling or lump that gradually becomes large enough to see or feel. Because the symptoms can be vague and similar to more common problems like arthritis or bursitis (inflammation of the cushioning pads around joints), doctors sometimes initially misdiagnose it as something less serious. This delay in correct diagnosis can be frustrating for patients and may lead to the cancer being more advanced when it is finally identified.[2]
Epidemiology
Synovial sarcoma is an uncommon cancer. In the United States, approximately 800 to 1,000 new cases are diagnosed each year, which translates to about one to three people per million being affected annually. According to detailed analysis of national cancer data, the age-adjusted incidence rate is approximately 0.177 per 100,000 people, with a prevalence rate of 0.65 per 100,000.[4][1]
This cancer can affect people of any age, but it shows a clear pattern in who it most commonly strikes. It is most frequently seen in adolescents and young adults, particularly those under the age of 30. In fact, synovial sarcoma is considered the most common type of sarcoma (cancer of connective tissues) in the adolescent age group. This means it often affects people at pivotal points in their lives, when they are building careers, starting families, or pursuing education.[4][3]
Men are affected more often than women, with the ratio being approximately 1.2 males for every female patient. One third of all patients with synovial sarcoma are diagnosed when they are younger than 30 years old. While the cancer can technically develop at any age, from children to older adults, the concentration in younger people is a distinctive characteristic of this disease.[6][7]
Synovial sarcoma represents about 5 to 10 percent of all soft tissue sarcomas, making it a significant portion of this broader cancer category. Despite being rare overall, it is relatively common within the specific group of soft tissue cancers, which themselves are uncommon forms of cancer.[4][6]
Causes
The exact cause of synovial sarcoma remains unclear to researchers, but they have identified specific changes in the body’s genetic material that are always present when this cancer develops. These changes involve abnormalities in chromosomes, which are the structures inside cells that contain all of our genetic information organized into genes.[1]
In synovial sarcoma, something unusual happens with chromosomes. Parts of chromosome 18 and chromosome X break apart and then rejoin in an abnormal way. This is called a translocation, specifically labeled as t(X;18). To understand this, imagine your chromosomes as instruction manuals and your genes as individual instructions or recipes. When the translocation occurs, it’s as if pages from two different manuals get torn out and then taped together in the wrong order.[4]
This chromosomal rearrangement causes a gene called SS18 (previously called SYT) from chromosome 18 to join inappropriately with one of several genes called SSX (usually SSX1, SSX2, or rarely SSX4) from chromosome X. When these genes merge incorrectly, they create what scientists call a fusion protein. This fusion protein is found in more than 90 to 95 percent of people with synovial sarcoma, making it a hallmark feature of the disease.[4][7]
The fusion protein causes cells to stop working properly. It interferes with normal cell functions and leads to the uncontrolled cell growth that defines cancer. However, scientists still don’t understand why this translocation happens in the first place. It appears to occur randomly, and there are no known environmental triggers or lifestyle factors that cause it.[1]
Risk Factors
Unlike many other types of cancer, synovial sarcoma does not have clearly identified risk factors related to lifestyle, environment, or family history. The disease appears to develop randomly due to the chromosomal changes that occur spontaneously within cells. There are no known behaviors, habits, exposures, or inherited genetic conditions that increase a person’s likelihood of developing synovial sarcoma.[4]
The main demographic patterns show that being young (especially under 30 years old) and being male are associated with slightly higher occurrence rates, but these are observations about who gets the disease rather than modifiable risk factors. Young people in their teens and twenties, particularly males, represent the group most commonly affected, but this doesn’t mean they did anything to cause the disease or could have done anything to prevent it.[4]
Because there are no identifiable risk factors, there is currently no way to predict who will develop synovial sarcoma, and no screening programs exist for this cancer in the general population. The random nature of the genetic changes that cause the disease means it can potentially affect anyone, regardless of their health habits or background.[6]
Symptoms
The symptoms of synovial sarcoma depend largely on where in the body the tumor is growing. The most common symptom is a lump or swelling that gradually increases in size over time. This lump is often painless at first, which is one reason why people may not seek medical attention right away. The lump typically appears deep in the tissue rather than on the surface of the skin, and it may feel firm when touched.[2][1]
When synovial sarcoma develops near a nerve, it can cause additional symptoms as the tumor presses against the nerve tissue. These symptoms might include pain in the affected area, numbness, or tingling sensations. Some people experience weakness in the limb where the tumor is growing. If the tumor is near a joint, it might cause stiffness or difficulty moving the joint normally.[2]
The most common locations for synovial sarcoma are in the arms and legs, particularly around large joints like the knee, ankle, wrist, elbow, hip, or shoulder. When the tumor grows in these locations, people might notice a bump under the skin or experience symptoms that they initially mistake for a sports injury or arthritis. The lump often grows slowly over many months, which can make it seem less urgent than it actually is.[3][5]
In less common cases where synovial sarcoma develops in the head or neck region, symptoms can be quite different. People might experience difficulty breathing, trouble swallowing, or changes in their voice. These symptoms occur because the tumor interferes with the normal function of structures in the throat and neck area.[2]
One of the challenging aspects of diagnosing synovial sarcoma is that its symptoms often mimic those of much more common and less serious conditions. Swelling near a joint might be dismissed as bursitis, pain might be attributed to tendonitis, and a lump might be thought to be a harmless cyst or lipoma (fatty lump). This similarity to other conditions can lead to delays in diagnosis, sometimes lasting months or even up to two years from when symptoms first appear.[16][1]
Prevention
Currently, there is no known way to prevent synovial sarcoma. Because the disease is caused by random chromosomal changes that occur spontaneously and are not linked to lifestyle factors, environmental exposures, or inherited genetic conditions, there are no preventive measures that can reduce the risk of developing this cancer.[4]
No dietary changes, exercise routines, supplements, or lifestyle modifications have been shown to prevent synovial sarcoma. Similarly, there are no vaccines or medications that can protect against this disease. The random nature of the genetic mutation that causes synovial sarcoma means that it cannot be predicted or prevented with current medical knowledge.[6]
However, early detection remains important. While the disease itself cannot be prevented, recognizing symptoms early can lead to earlier diagnosis and treatment, which may improve outcomes. Anyone who notices a persistent lump, especially one that is growing, or experiences unexplained swelling, pain, or numbness that doesn’t resolve should see a healthcare provider promptly. Being aware of unusual changes in the body and seeking medical attention for concerning symptoms is the best approach to catching synovial sarcoma as early as possible.[1]
Pathophysiology
The development of synovial sarcoma involves complex changes in how cells function and grow. At the molecular level, the disease is driven by the abnormal fusion of genetic material that creates new, harmful instructions for cells. When chromosome 18 and chromosome X break and rejoin incorrectly, they produce fusion proteins known as SS18:SSX1, SS18:SSX2, or rarely SS18:SSX4. These fusion proteins are found in 95 percent of synovial sarcoma cases and are essential for confirming the diagnosis.[4]
These fusion proteins interfere with normal cellular machinery, particularly with complexes called BAF complexes (BRG1- or HBRM-associated factors), which normally help regulate which genes are turned on or off in a cell. When the fusion protein binds to these complexes, it displaces normal components and causes widespread changes in gene activity. This leads to abnormal cell growth and the loss of normal growth controls that usually prevent cells from dividing uncontrollably.[4]
The name “synovial” sarcoma is actually a misnomer, meaning the name is misleading. The tumor doesn’t actually originate from synovial tissue found in joints. Instead, it can develop anywhere in the body from various soft tissues. The cancer cells just happen to look similar to synovial cells under a microscope, which is how the disease got its name early in the twentieth century. The actual cell of origin remains unknown.[4][7]
Under the microscope, synovial sarcoma can show two different types of cells. One type looks like spindle-shaped cells (called sarcomatous cells) that are relatively small and uniform, often appearing in sheets. The other type looks like epithelial cells, which are cells that typically line surfaces in the body. When both cell types are present, it’s called biphasic synovial sarcoma. Some tumors contain only the spindle cells, called monophasic fibrous type, or rarely only the epithelial-appearing cells. The tumor may also appear poorly differentiated, meaning the cells look very abnormal and immature.[7]
Synovial sarcoma is considered an aggressive malignancy with high potential for metastasis, which means it can spread to other parts of the body. The most common site for metastasis is the lungs, though it can spread to regional lymph nodes and other organs. Distant spread occurs in approximately half of all cases, sometimes months to years after initial diagnosis. The tumor’s ability to spread makes it a serious disease that requires comprehensive treatment.[4][8]
Recent research has identified additional molecular features of synovial sarcoma cells that may be important for understanding the disease. Scientists have discovered that a protein called WDR5 interacts with the SS18:SSX fusion protein at cancer-related genes, promoting changes that support cancer growth. This discovery is providing new insights into potential treatment approaches that could target these specific molecular vulnerabilities.[14]
