Synovial Sarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
September 16, 2026

Synovial sarcoma is a cancer that develops in the body’s soft tissues, most often deep in the arms or legs. It is a type of sarcoma, a cancer that begins in the body’s connective tissues. Synovial sarcoma makes up about 5% to 10% of all soft tissue sarcomas.

Despite its name, synovial sarcoma does not start from the synovium, the thin lining of the joints. The name comes from early descriptions of tumors found near joints such as the knee. The exact type of normal cell that synovial sarcoma comes from is still unknown.

Synovial sarcoma most often affects adolescents and young adults, although it can occur at any age. It is the second most common cancerous soft tissue tumor in children. Most tumors start in the arms or legs, often near a joint. Others start in the trunk, the head and neck, or, less often, internal organs such as the lung.

This article explains how doctors diagnose synovial sarcoma and what the fusion test, type, grade, margins, and stage in your synovial sarcoma pathology report mean.

What causes synovial sarcoma?

Synovial sarcoma is caused by a genetic change that develops in the tumor cells during a person’s lifetime. Pieces of chromosome X and chromosome 18 break and join together incorrectly. This joins a gene called SS18 to one of the SSX genes, creating a new fusion gene.

Nearly all synovial sarcomas have one of three fusions: SS18::SSX1, SS18::SSX2, or, less often, SS18::SSX4. The fusion protein changes how DNA is packaged inside the cell, which switches large groups of genes on or off. This keeps the cells in an immature state and drives them to grow. Synovial sarcoma usually has very few other genetic changes.

The SS18::SSX fusion is found only in the tumor cells. It is not inherited and cannot be passed on to children. In rare cases, synovial sarcoma has developed in an area of the body treated with radiation years earlier.

What are the symptoms of synovial sarcoma?

The most common symptom of synovial sarcoma is a lump or swelling that grows slowly and may or may not be painful. Because it can grow slowly and look well-defined on imaging, it is sometimes mistaken at first for a noncancerous growth. A tumor near a joint can cause stiffness or limited movement. A tumor in the chest may cause a cough or trouble breathing.

How is the diagnosis made?

The diagnosis of synovial sarcoma is made after a pathologist examines a sample of the tumor under the microscope. The sample is usually obtained by a core needle biopsy, which removes small pieces of the tumor with a needle.

Under the microscope, synovial sarcoma is usually made of closely packed spindle cells, which are long, thin cells arranged in sheets or bundles. Some tumors also contain epithelial cells that form small glands. Some tumors contain calcium deposits and may also show them on imaging.

Because synovial sarcoma can look like several other tumors, the pathologist confirms the diagnosis with immunohistochemistry, molecular testing, or both. Molecular tests such as next-generation sequencing (NGS), RT-PCR, or FISH look for the SS18::SSX fusion or a break in the SS18 gene. Finding the fusion confirms the diagnosis.

Once synovial sarcoma is confirmed, imaging tests look for spread. A chest CT scan is usually done because the lungs are the most common site of spread. The next section describes the immunohistochemistry tests used to confirm the diagnosis.

Immunohistochemistry

Immunohistochemistry uses antibodies to show which proteins tumor cells make. For synovial sarcoma, newer stains can detect the fusion protein directly and can often confirm the diagnosis without a separate molecular test. Your report may include some of the following:

SS18-SSX. This stain detects the fusion protein itself. It is positive in about 9 out of 10 synovial sarcomas, and other tumors do not stain, so a positive result essentially confirms the diagnosis. A negative result does not completely rule out synovial sarcoma, especially in a small biopsy or in tissue treated to remove calcium.
SSX. This stain, sometimes listed as SSX C-terminus, is positive in nearly all synovial sarcomas. Some other tumors are also positive, so a negative result is most useful for ruling out synovial sarcoma.
TLE1. Strong staining in the nucleus of the tumor cells is seen in most synovial sarcomas. Some other tumors are also positive, so TLE1 supports the diagnosis but does not confirm it.
Keratins and EMA. These proteins are normally found in epithelial cells. They are often positive in at least some cells of synovial sarcoma, even when the tumor is made only of spindle cells.
CD99 and BCL2. These proteins are often positive in synovial sarcoma, but they are also seen in many other tumors.

Not every case of synovial sarcoma needs every stain. The pathologist chooses tests based on how the tumor looks, where it started, and which other tumors need to be ruled out.

Histologic type

The histologic type describes how synovial sarcoma cells look under the microscope. The World Health Organization (WHO) classification of soft tissue and bone tumors, 5th edition (2020), recognizes three types. All three share the same SS18::SSX fusion.

Monophasic synovial sarcoma. This type is made almost entirely of spindle cells. It is the most common type.
Biphasic synovial sarcoma. This type contains both spindle cells and epithelial cells that form glands or small nests.
Poorly differentiated synovial sarcoma. This type consists of densely packed, highly abnormal cells that are often round rather than spindle-shaped. Many of the cells are dividing, and this type is associated with a less favorable outcome.

Histologic grade

Histologic grade describes how abnormal the cells of a synovial sarcoma look and how quickly they appear to be growing. Pathologists grade synovial sarcoma using the FNCLCC system, which adds together scores for three features seen under the microscope:

Differentiation. This describes how closely the tumor cells resemble normal tissue. Synovial sarcoma always receives the highest score of 3, because its cells do not resemble any normal tissue.
Mitotic count. This is the number of dividing cells in 10 high-power fields, which are areas seen through the microscope at high magnification. Fewer than 10 dividing cells scores 1, 10 to 19 scores 2, and 20 or more scores 3.
Necrosis. This is the amount of dead tumor tissue. No necrosis scores 0, less than 50% scores 1, and 50% or more scores 2.

The total score gives the final grade:

Grade 1. Total score of 2 or 3.
Grade 2. Total score of 4 or 5.
Grade 3. Total score of 6 to 8.

Because synovial sarcoma always starts with a differentiation score of 3, it is always grade 2 or grade 3. Both grades are considered high grade. Grade 3 tumors are more likely to come back after treatment and to spread to other parts of the body.

Tumor size

Tumor size is the greatest dimension of the synovial sarcoma, measured in centimeters (cm). The final measurement comes from the tumor removed at surgery rather than from a biopsy. For most body sites, size is used to determine the tumor stage (pT).

Synovial sarcomas 5 cm or smaller are less likely to spread and are associated with a more favorable outcome. Larger tumors have a higher risk of spreading to other parts of the body.

Tumor extension

Tumor extension describes whether synovial sarcoma has grown beyond the tissue where it started into nearby structures such as bone, blood vessels, nerves, or organs. The pathologist examines the tissue removed with the tumor and reports which structures contain tumor cells.

For tumors in the head and neck, the orbit (the space around the eye), and internal organs, growth into nearby structures raises the tumor stage. For tumors of the trunk, arms, legs, and retroperitoneum, the stage depends on size alone. Extension into nearby structures still affects how surgeons and radiation oncologists plan treatment.

Treatment effect

Some people with synovial sarcoma receive chemotherapy, radiation therapy, or both before surgery. This is called neoadjuvant or pre-operative treatment. When this happens, the pathologist estimates what percentage of the removed tumor is non-viable (dead) and what percentage is still viable (alive).

A tumor that is 90% or more non-viable is often considered a strong response to pre-operative treatment. For soft tissue sarcomas, including synovial sarcoma, experts have not agreed on a single cut-off that predicts outcome. Your doctors interpret the percentage together with the other findings in your report.

Treatment changes how tumor cells look under the microscope. For this reason, the grade is usually taken from the biopsy done before treatment. If no treatment was given before surgery, the report may say there was no known presurgical therapy.

Lymphovascular invasion

Lymphovascular invasion means that cells from the synovial sarcoma are seen inside a small blood vessel or lymphatic channel. These vessels give cancer cells a route to other parts of the body. Current reports may list this finding as “lymphatic and/or vascular invasion.”

Present. Tumor cells were seen inside a vessel. This finding is associated with a higher risk of tumor spread.
Not identified. No tumor cells were seen inside vessels in the tissue examined.

Perineural invasion

Perineural invasion means tumor cells are growing around or along a nerve. It is not a standard item in soft tissue sarcoma reports, but a pathologist may mention it when it is seen in synovial sarcoma. When present, it suggests the tumor may extend beyond its visible edge and may raise the risk of the tumor coming back in the same place.

Surgical margins

A margin is the edge of tissue cut by the surgeon to remove a synovial sarcoma. The pathologist examines each margin to see whether tumor cells reach it. For soft tissue sarcomas, margin status is the most important predictor of whether the tumor will come back in the same place.

Negative margin. No tumor cells are seen at the cut edge. The report usually names the closest margin and gives its distance from the tumor.
Close margin. Tumor cells are near the cut edge but do not reach it. Reports often list every margin that is less than 0.5 cm from the tumor.
Positive margin. Tumor cells are present at the cut edge. This means some tumor may remain in the body and raises the risk of the tumor coming back. Further surgery or radiation therapy may be considered.

Lymph nodes

Lymph nodes are small immune organs that filter fluid from the tissues. Spread of synovial sarcoma to lymph nodes is uncommon. For this reason, lymph nodes are usually removed only if they look enlarged or suspicious on imaging.

If lymph nodes are examined, the report states how many were examined and how many contain tumor cells. Tumor cells in a lymph node change the nodal stage to pN1 and are associated with a less favorable outcome.

Biomarker testing

Biomarker testing looks for tumor features that guide treatment, predict outcome, or point to an inherited condition. For synovial sarcoma, the SS18::SSX fusion confirms the diagnosis, as described above, but no approved drug targets the fusion itself. When synovial sarcoma cannot be removed or has spread, other tests can show whether a person may be eligible for a type of cell therapy:

MAGE-A4. MAGE-A4 is a protein made by many synovial sarcomas. It is tested by immunohistochemistry, often on tumor tissue stored from an earlier biopsy or surgery. In large screening studies, about 70% of synovial sarcomas were positive. A positive result is required for the cell therapy afamitresgene autoleucel (afami-cel).
HLA-A*02 typing. HLA types are inherited markers on the surface of your cells, similar to the tissue typing done for organ transplants. This is a blood test, not a test of the tumor. Afami-cel can only be used in people with certain HLA-A*02 types, which about 45% of screened patients had.
NY-ESO-1. NY-ESO-1 is another protein made by many synovial sarcomas. Testing for it helps identify people who may be eligible for clinical trials of a similar cell therapy.

A negative MAGE-A4 result, or an HLA type that does not match, does not change the diagnosis of synovial sarcoma. It means that this specific treatment is not an option, and other treatments remain available. You can learn more about tumor testing in our Biomarkers and Genetic Testing section.

Pathologic stage (pTNM)

The pathologic stage for synovial sarcoma is assigned using the TNM system from the American Joint Committee on Cancer (AJCC), 8th edition. The tumor stage (pT) is based on the tissue removed at surgery, and the nodal stage (pN) describes the lymph nodes. The metastasis stage (M) is usually determined by imaging and is often not included in the pathology report.

If you received treatment before surgery, the stage may begin with the letter “y,” as in ypT2. A stage beginning with “r” describes a tumor that has come back after treatment.

The tumor stage (pT) for synovial sarcoma depends on where the tumor started in the body. in the body

Trunk and extremities (chest, back, abdominal wall, arms, and legs):

pT1. The tumor is 5 cm or smaller.
pT2. The tumor is larger than 5 cm but not larger than 10 cm.
pT3. The tumor is larger than 10 cm but not larger than 15 cm.
pT4. The tumor is larger than 15 cm.

Retroperitoneum (the space at the back of the abdomen):

pT1. The tumor is 5 cm or smaller.
pT2. The tumor is larger than 5 cm but not larger than 10 cm.
pT3. The tumor is larger than 10 cm but not larger than 15 cm.
pT4. The tumor is larger than 15 cm.

Head and neck:

pT1. The tumor is 2 cm or smaller.
pT2. The tumor is larger than 2 cm but not larger than 4 cm.
pT3. The tumor is larger than 4 cm.
pT4a. The tumor has grown into the eye socket, the bones of the face, or the base of the skull and its lining. It may instead involve the organs in the center of the neck or the chewing muscles called the pterygoid muscles.
pT4b. The tumor has grown into the brain, surrounds the carotid artery, or has grown into the muscles in front of the spine. A tumor that has spread along a nerve into the brain or spinal cord is also pT4b.

Abdominal and thoracic visceral organs (internal organs such as the stomach, intestines, and lungs):

pT1. The tumor is confined to the organ where it started.
pT2a. The tumor has grown into the organ’s thin outer lining.
pT2b. The tumor has grown beyond the outer lining into the surrounding tissue.
pT3. The tumor has grown into another organ or a nearby structure such as the diaphragm or abdominal wall.
pT4a. Tumor is found in 2 separate sites.
pT4b. Tumor is found in 3 to 5 separate sites.
pT4c. Tumor is found in more than 5 separate sites.

Orbit (the space around the eye):

pT1. The tumor is 2 cm or smaller.
pT2. The tumor is larger than 2 cm and has not grown into the bony walls of the orbit or the eye.
pT3. The tumor, of any size, has grown into the bony walls of the orbit.
pT4. The tumor has grown into the eye or nearby structures such as the eyelid, sinuses, or brain.

A stage of pT0 means no tumor was found in the tissue removed, which can happen after treatment before surgery. If the tumor cannot be assessed, for example because it was removed in many pieces, the report may say that pT was not assigned.

The nodal stage (pN) for synovial sarcoma describes whether tumor cells were found in the lymph nodes:

pN0. No tumor cells were found in the lymph nodes examined.
pN1. Tumor cells were found in at least one nearby lymph node.

If no lymph nodes were removed, which is common for synovial sarcoma, the report will usually say that pN was not assigned. Older reports may show pNX, but current reporting standards no longer use this term for soft tissue sarcomas.

What is the prognosis?

The outlook for a person with synovial sarcoma depends mainly on whether the cancer has spread, the size of the tumor, and whether it can be completely removed. More than 80% of people are diagnosed before the cancer has spread. Among these people, about 70% to 80% are alive five years after diagnosis.

Synovial sarcoma can come back or spread many years after treatment. Up to half of people whose cancer had not spread at diagnosis later develop spread, most often to the lungs. In one long-term study, spread appeared at a median of about 4 to 5 years after diagnosis, and some cases appeared more than 15 years later.

When synovial sarcoma cannot be removed or has spread widely, fewer than 10% of people are alive five years later. Newer treatments, including cell therapy, are being used for some of these people.

Features associated with the outcome of synovial sarcoma include:

Spread at diagnosis. Cancer that has already spread when it is found is the strongest predictor of a less favorable outcome.
Tumor size. Tumors 5 cm or smaller are associated with a more favorable outcome than larger tumors.
Age. Children tend to do better than adults, and outcomes become less favorable with increasing age.
Tumor location. Tumors of the arms and legs tend to have a more favorable outcome than tumors in other locations.
Histologic grade and type. Grade 3 tumors and poorly differentiated synovial sarcoma are associated with a less favorable outcome.
Completeness of surgery. Complete removal of the tumor with negative margins is associated with a more favorable outcome.

What happens after the diagnosis?

After synovial sarcoma is confirmed, care is usually planned by a team at a center experienced in treating sarcoma. The team often includes surgeons, radiation oncologists, and medical oncologists. The findings in your report, including the grade, size, margins, and stage, help the team decide which options to consider.

Surgery. Complete removal of the tumor with negative margins is the main treatment for synovial sarcoma that has not spread.
Radiation therapy. Radiation may be given before or after surgery, often for larger or deep tumors. It may also be considered when margins are close or positive.
Chemotherapy. Ifosfamide and doxorubicin are the chemotherapy drugs most often used for synovial sarcoma. Chemotherapy may be recommended for larger, high-grade tumors or when the cancer has spread. Its benefit for tumors that have not spread is still debated, so recommendations depend on factors such as age, size, and grade.
Cell therapy. Afami-cel is made from a person’s own immune cells, which are changed in a laboratory to recognize MAGE-A4 on cancer cells. The FDA granted it full approval in June 2026 for people 12 years and older with synovial sarcoma that cannot be removed or has spread. It is used after chemotherapy, only in people with a positive MAGE-A4 result and a matching HLA-A*02 type.
Other drugs. Pazopanib, a drug that blocks signals tumors use to grow blood vessels, may be used when synovial sarcoma grows after chemotherapy.
Clinical trials. Researchers are studying new treatments, including other cell therapies. Your oncologist can tell you whether a trial is open to you.

In the study that supported the full approval of afami-cel, tumors shrank in about 44% of people treated. Approval and access to these treatments differ by country, and your care team can explain what applies where you live.

Because synovial sarcoma can come back many years after treatment, follow-up usually continues for more than 10 years. It typically includes regular imaging of the chest and of the area where the tumor started.

Questions to ask your doctor

How was the diagnosis of synovial sarcoma confirmed: by SS18-SSX staining, a molecular test, or both?
Which type of synovial sarcoma do I have: monophasic, biphasic, or poorly differentiated?
Is my tumor grade 2 or grade 3?
How large was the tumor, and did it grow into nearby structures?
Were all the margins negative? How close was the closest margin?
If I had treatment before surgery, what percentage of the tumor was non-viable?
What is my pathologic stage, and has the cancer spread to my lungs or elsewhere?
Will chemotherapy or radiation therapy be considered in my case?
If the cancer cannot be removed or has spread, should my tumor be tested for MAGE-A4?
Should I have HLA typing to see whether cell therapy could be an option?
Is there a clinical trial I could join?
How often will I need imaging, and for how many years?