by Bibianna Purgina, MD FRCPC, updated on December 2, 2018
The human body is made up of many different types of tissue. Mesenchymal tissues include tissues such as fibrous tissue, nerves, fat, muscle, tendons, ligaments, bone and cartilage. Solitary fibrous tumour is a type of tumour that develops from fibrous mesenchymal tissue.
What is a solitary fibrous tumour?
Solitary fibrous tumours typically occur in adults and they can develop anywhere in the body. Most solitary fibrous tumours behave as non-cancerous tumours but some can behave in an aggressive manner that is more similar to a cancer.
Your pathologist will carefully examine your tissue sample to look for features that will help predict how the tumour will behave.
All solitary fibrous tumours have the potential to re-grow (recur) at the original site or spread to distant body sites. However, some features that can be associated with aggressive behaviour include:
These features will be discussed in more detail in the sections below.
How do pathologists make this diagnosis?
Under the microscope, the tumour cells of solitary fibrous tumour look similar to the cells that make up normal fibrous tissue. These cells are called fibroblasts. The fibroblasts in a solitary fibrous tumour are arranged in what pathologist describe as a “patternless pattern”. Lots of small branching small blood vessels are also seen in the tumour. Pathologists sometimes describe these small blood vessels as having a “hemangiopericytoma-like” or “staghorn” vascular pattern.
The diagnosis of a solitary fibrous tumour 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 or molecular testing may also be performed to confirm the diagnosis.
Solitary fibrous tumours are removed by surgery after a diagnosis is made on biopsy. If your pathologist sees any features that have been shown to be associated with aggressive behavior, you may receive additional treatments, such as radiation or chemotherapy after surgery.
After the tumour has been removed completely, your pathologist will examine the tumour under the microscope and provide your surgeon and oncologist with critical information required for your subsequent treatment.
Information included in your report will include tumour size and margin status, as well as any features associated with aggressive behaviour described above.
If you are diagnosed with a cancerous (malignant) solitary fibrous tumour, your pathologist will then provide additional information including tumour staging, as described below.
Grade is a word pathologists use to describe how different the tumour cells look and behave compared to normal cells in the same location. If your tumour shows features of a cancerous solitary fibrous tumour, your pathologist may attempt to provide a grade using an internationally recognized system created by the French Federation of Cancer Centers Sarcoma Group (FNCLCC).
This system uses three microscopic features assessed by your pathologist:
Your pathologist will give each feature a certain number of points (from 0 to 3) and the total number of points determines the final grade of the tumour.
Low grade tumours have a grade of 1. High grade tumours have a grade of either 2 or 3.
Why is this important? Compared to low grade tumours (grade 1), high grade tumours (grade 2 or 3) are associated with more aggressive behavior and worse prognosis.
After the tumour is completely removed your pathologist will measure it in three dimensions but only the largest dimension is typically included in your report. For example, if the tumour measures 5.0 cm by 3.2 cm by 1.1 cm, the report may describe the tumour size as 5.0 cm in greatest dimension.
Why is this important? Tumours less than 5 cm are associated with better prognosis.
Necrosis is a form of cell death and it commonly occurs in cancers (malignant tumours). Your pathologist will closely examine the tumour for evidence of necrosis. Necrosis is used to determine the tumour grade (see Histologic grade above).
Why is this important? Finding necrosis in a tumour is important, especially if you received pre-surgery chemotherapy or radiation therapy, as this can be an indication of treatment response. It can also be associated with worse prognosis in tumours that have not been treated yet with chemotherapy or radiation therapy.
Solitary fibrous tumours are usually well-defined tumours but tumours with cancerous (malignant) features can grow into or around neighbouring muscles, bone and blood vessels.
Your pathologist will examine samples of the surrounding tissues under the microscope to look for tumour cells. Any surrounding organs or tissues that contain tumour cells will be described in your report.
If you received chemotherapy and/or radiation therapy before the operation to remove your tumour, your pathologist will examine all the tissue sent to pathology to see how much of the tumour is still alive (viable). Most commonly, your pathologist will describe the percentage of tumour that is dead.
Nerves are like long wires made up of groups of cells called neurons. Nerves transmit information (such as temperature, pressure, and pain) between your brain and your body. Perineural invasion is a term pathologists use to describe tumour cells attached to a nerve.
Why is this important? Perineural invasion is important because tumour cells that have attached to a nerve can use the nerve to travel into tissue outside of the original tumour. For this reason, perineural invasion is associated with a higher risk that the tumour will come back in the same area of the body (local recurrence) 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.
Cancer cells can use blood vessels and lymphatics to travel away from the tumour to other parts of the body. The movement of cancer cells from the tumour to another part of the body is called metastasis.
Before cancer cells can metastasize, they need to enter a blood vessel or lymphatic. This is called lymphovascular invasion.
Why is this important? Lymphovascular invasion increases the risk that cancer cells will be found in a lymph node or a distant part of the body such as 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.
All margins will be very closely examined under the microscope by your pathologist to determine the margin status. Specifically, a margin is called negative when there are no tumour cells at the edge of the cut tissue. Alternatively, a margin is called positive when there are tumour cells at the edge of the cut tissue.
Why is this important? A positive margin is associated with a higher risk that the tumour will recur in the same site after treatment (local recurrence).
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 a metastasis.
Lymph nodes on the same side as the tumour are called ipsilateral while those on the opposite side of the tumour are called contralateral.
Your pathologist will carefully examine each lymph node for cancer cells. Lymph nodes that contain cancer cells are often called positive while those that do not contain any cancer cells are called negative. Most reports include the total number of lymph nodes examined and the number, if any, that contain cancer cells.
Only solitary fibrous tumours with cancerous (malignant) features receive a pathologic stage 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 worse prognosis.
The pathologic stage will only be included in your report after the entire tumour has been removed from your body. It will not be included after a biopsy.
Tumour stage (pT) for solitary fibrous tumour
The tumour stage for cancerous (malignant) solitary fibrous tumour varies based on the body part involved. For example, a 5 centimeter 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.
Tumour stage for tumours starting in the head and neck:
Tumour stage for tumours starting on the outside of the chest, back, or stomach and the arms or legs (trunk and extremities):
Tumour stage for tumours starting in the abdomen and organs inside the chest (thoracic visceral organs):
Tumour stage for tumours starting in the space at the very back of the abdominal cavity (retroperitoneum):
Tumour stage for tumours starting in the space around the eye (orbit):
If after microscopic examination, no tumour is seen in the resection specimen sent to pathology for examination, it is given the tumour stage pT0 which means there is no evidence of primary tumour.
If your pathologist cannot reliable evaluate the tumor size or the extent of growth, it is given the tumour stage pTX (primary tumour cannot be assessed). This may happen if the tumour is received as multiple small fragments.
Nodal stage (pN) for solitary fibrous tumour
Cancerous solitary fibrous tumours are given an nodal stage of 0 or 1 based on the presence or absence of cancer cells in one or more lymph nodes.
If no cancer 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 cancer cells are found in any lymph nodes, then the nodal stage is listed as N1.
Metastasis stage (pM) for solitary fibrous tumour
Cancerous (malignant) solitary fibrous tumours are given an metastatic stage of 0 or 1 based on the presence of cancer 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.
Each cell in your body contains a set of instructions that tell the cell how to behave. These instructions are written in a language called DNA and the instructions are stored on 46 chromosomes in each cell. Because the instructions are very long, they are broken up into sections called genes and each gene tells the cell how to produce piece of the machine called a protein.
Sometimes, a piece of DNA falls off one chromosome and becomes attached to a different chromosome. This is called a translocation and it can result in the cell making a new and abnormal protein. If the new protein allows the cell to live longer than other cells or spread to other parts of the body, the cell can become a cancer (a malignant tumour).
Why is this important? Solitary fibrous tumour often contains a translocation that combines the gene NAB2 with the STAT6 gene.
Pathologists test for this translocation by performing immunohistochemistry, 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 your tumor has been surgically removed.