by Bibianna Purgina, MD FRCPC
May 3, 2022
Synovial sarcoma is a type of cancer called a sarcoma. It is characterized by a genetic change that combines the gene SS18 with either SS1, SSX2 or SSX4 genes. Synovial sarcoma can develop at any age but are most common in adults. Typical locations include the arms and legs but the tumour can develop anywhere in the body.
Because the earliest reports of synovial sarcoma described tumours around joints such as the knee, the tumour was thought to develop from the tissue around the joint called synovium. We know now that the tumour does not actually develop from the synovium, however, the original name “synovial” still remains.
There are two main types of synovial sarcoma: biphasic synovial sarcoma and monophasic synovial sarcoma. Other types of synovial sarcoma exist, including poorly differentiated synovial sarcoma, however, these are very rare.
The first diagnosis of synovial sarcoma is usually made after a small sample of the tumour is removed in a procedure called a biopsy. The biopsy tissue is then sent to a pathologist who examines it under a microscope. Additional tests such as immunohistochemistry and fluorescence in situ hybridization (FISH) may also be performed to confirm the diagnosis.
Under the microscope, biphasic synovial sarcomas are made of two types of cancer cells: epithelial tumour cells and spindle tumour cells. Monophasic synovial sarcomas are made of only one of these types of cancer cells, usually the spindle tumour cells.
The microscopic appearance of a biphasic synovial sarcoma.
Synovial sarcoma contains a genetic change called a translocation. This genetic change combines the gene SS18 with either SS1, SSX2 or SSX4 genes. Tumours with the SS18-SSX1 translocation are associated with a worse prognosis compared to tumours with translocations involving other genes. Pathologists test for these molecular changes by performing either fluorescence in situ hybridization (FISH) or next-generation sequencing (NGS) on a piece of the tissue from the tumour. This type of testing is can be done on the biopsy specimen or when the tumour has been surgically removed.
Pathologists divide synovial sarcoma into three grades based on a system created by the French Federation of Cancer Centers Sarcoma Group (FNCLCC). This system uses three microscopic features to determine the tumour grade: differentiation, mitotic count, and necrosis. These features are explained in more detail below. The grade can only be determined after a sample of the tumour has been examined under the microscope.
Points (from 0 to 3) are assigned for each of the microscopic features (0 to 3) and the total number of points determines the final grade of the tumour. According to this system, synovial sarcomas may be either low or high-grade tumours. High-grade tumours (grades 2 and 3) are associated with a worse prognosis.
Points associated with each grade:
Microscopic features used to determine the grade:
Tumour size is important because tumours less than 5 cm are less likely to spread to other parts of the body and are associated with a better prognosis. Tumour size is also used to determine the pathologic tumour stage (see Pathologic stage below).
Most synovial sarcomas tend to occur in the extremity and are well defined, but the tumour may grow into or around nearby organs and bones. This is called tumour extension. Your pathologist will examine samples of the surrounding organs and tissues under the microscope to look for tumour cells. Any surrounding organs or tissue that contains tumour cells will be described in your report.
If you received chemotherapy and/or radiation therapy before the operation to remove the tumour, your pathologist will examine all the tissue sent to pathology to see how much of the tumour was still alive at the time it was removed from the body. Pathologists use the term viable to describe tissue that was still alive at the time it was removed from the body. In contrast, pathologists use the term non-viable to describe tissue that was not alive at the time it was removed from the body. Most commonly, your pathologist will describe the percentage of tumours that is non-viable.
Nerves are like long wires made up of groups of cells called neurons. Nerves are found all over the body and they are responsible for sending information (such as temperature, pressure, and pain) between your body and your brain. Perineural invasion is a term pathologists use to describe tumour cells attached to a nerve. Perineural invasion is important because tumour cells that have become attached to a nerve can grow along the nerve and into surrounding tissues. This increases the risk that the tumour will re-grow after treatment.
Blood moves around the body through long thin tubes called blood vessels. Another type of fluid called lymph which contains waste and immune cells moves around the body through lymphatic channels. The term lymphovascular invasion is used to describe tumour cells that are found inside a blood vessel or lymphatic channel. Lymphovascular invasion is important because once the tumour cells are inside a blood vessel or lymphatic channel they are able to metastasize (spread) to other parts of the body such as lymph nodes or the lungs.
A margin is any tissue that was cut by the surgeon to remove the tumour from your body. Depending on the type of surgery you have had, the margins can include bones, muscles, blood vessels, and nerves that were cut to remove the tumour from your body. Margins will only be described in your report after the entire tumour has been removed.
A negative margin means that no tumour cells were seen at any of the cut edges of tissue. A margin is called positive when there are tumour cells at the very edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will recur in the same site after treatment.
Lymph nodes are small immune organs located throughout the body. Cancer cells can travel from the tumour to a lymph node through lymphatic channels located in and around the tumour (see Lymphovascular invasion above). The movement of cancer cells from the tumour to a lymph node is called metastasis.
Many cancers can spread to the lymph nodes, but synovial sarcoma does this very rarely. If lymph nodes were part of the surgery to remove your tumour, your pathologist will assess them under the microscope and report whether they are involved by tumour.
The pathologic stage for synovial sarcoma is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer. This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M) to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means more advanced disease and a worse prognosis.
The tumour stage for synovial sarcoma varies based on the body part involved. For example, a 5-centimetre tumour that starts in the head will be given a different tumour stage than a tumour that starts deep in the back of the abdomen (the retroperitoneum). However, in most body sites, the tumour stage includes the tumour size and whether the tumour has grown into surrounding body parts.
Synovial sarcoma is given a nodal stage of 0 or 1 based on the presence or absence of tumour cells in one or more lymph nodes. If no tumour cells are seen in any lymph nodes, the nodal stage is N0. If no lymph nodes are sent for pathological examination, the nodal stage cannot be determined, and the nodal stage is listed as NX. If tumour cells are found in any lymph nodes, then the nodal stage is listed as N1.
Synovial sarcoma is given a metastatic stage of 0 or 1 based on the presence of tumour cells at a distant site in the body (for example the lungs). The metastatic stage can only be assigned if tissue from a distant site is submitted for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined and is listed as MX.