by Bibianna Purgina, MD FRCPC
March 29, 2023
Synovial sarcoma is a type of cancer called a sarcoma. There are two types of synovial sarcoma. Monophasic synovial sarcoma is made up entirely of long thin spindle cells. Biphasic synovial sarcoma is made up of both spindle cells and round epithelioid or glandular cells. Synovial sarcoma can develop at any age but are most common in adults.
Typical locations for synovial sarcoma 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.
Like other types of cancer, synovial sarcoma is able to spread beyond the original tumour to other parts of the body. The term metastatic synovial sarcoma means that cancer cells were found in another part of the body. When synovial sarcoma metastasized, the cells commonly spread to the lungs and lymph nodes.
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.
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.
Perineural invasion is a term pathologists use to describe cancer cells attached to or inside a nerve. A similar term, intraneural invasion, is used to describe cancer cells inside a nerve. 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 important because the cancer cells can use the nerve to spread into surrounding organs and tissues. This increases the risk that the tumour will regrow after surgery.
Lymphovascular invasion means that cancer cells were seen inside a blood vessel or lymphatic vessel. Blood vessels are long thin tubes that carry blood around the body. Lymphatic vessels are similar to small blood vessels except that they carry a fluid called lymph instead of blood. The lymphatic vessels connect with small immune organs called lymph nodes that are found throughout the body. Lymphovascular invasion is important because cancer cells can use blood vessels or lymphatic vessels to spread to other parts of the body such as lymph nodes or the lungs.
In pathology, a margin is the edge of a tissue that is cut when removing a tumour from the body. The margins described in a pathology report are very important because they tell you if the entire tumour was removed or if some of the tumour was left behind. The margin status will determine what (if any) additional treatment you may require.
Most pathology reports only describe margins after a surgical procedure called an excision or resection has been performed for the purpose of removing the entire tumour. For this reason, margins are not usually described after a procedure called a biopsy is performed for the purpose of removing only part of the tumour. The number of margins described in a pathology report depends on the types of tissues removed and the location of the tumour. The size of the margin (the amount of normal tissue between the tumour and the cut edge) depends on the type of tumour being removed and the location of the tumour.
Pathologists carefully examine the margins to look for tumour cells at the cut edge of the tissue. If tumour cells are seen at the cut edge of the tissue, the margin will be described as positive. If no tumour cells are seen at the cut edge of the tissue, a margin will be described as negative. Even if all of the margins are negative, some pathology reports will also provide a measurement of the closest tumour cells to the cut edge of the tissue.
A positive (or very close) margin is important because it means that tumour cells may have been left behind in your body when the tumour was surgically removed. For this reason, patients who have a positive margin may be offered another surgery to remove the rest of the tumour or radiation therapy to the area of the body with the positive margin. The decision to offer additional treatment and the type of treatment options offered will depend on a variety of factors including the type of tumour removed and the area of the body involved.
Lymph nodes are small immune organs found throughout the body. Cancer cells can spread from a tumour to lymph nodes through small vessels called lymphatics. For this reason, lymph nodes are commonly removed and examined under a microscope to look for cancer cells. The movement of cancer cells from the tumour to another part of the body such as a lymph node is called a metastasis.
Cancer cells typically spread first to lymph nodes close to the tumour although lymph nodes far away from the tumour can also be involved. For this reason, the first lymph nodes removed are usually close to the tumour. Lymph nodes further away from the tumour are only typically removed if they are enlarged and there is a high clinical suspicion that there may be cancer cells in the lymph node.
If any lymph nodes were removed from your body, they will be examined under the microscope by a pathologist and the results of this examination will be described in your report. Most reports will include the total number of lymph nodes examined, where in the body the lymph nodes were found, and the number (if any) that contain cancer cells. If cancer cells were seen in a lymph node, the size of the largest group of cancer cells (often described as “focus” or “deposit”) may also be included.
The examination of lymph nodes is important for two reasons. First, this information is used to determine the pathologic nodal stage (pN). Second, finding cancer cells in a lymph node increases the risk that cancer cells will be found in other parts of the body in the future. As a result, your doctor will use this information when deciding if additional treatment such as chemotherapy, radiation therapy, or immunotherapy is required.
Pathologists often use the term “positive” to describe a lymph node that contains cancer cells. For example, a lymph node that contains cancer cells may be called “positive for malignancy”.
Pathologists often use the term “negative” to describe a lymph node that does not contain any cancer cells. For example, a lymph node that does not contain cancer cells may be called “negative for malignancy”.
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 a 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.