Soft tissue -

Malignant peripheral nerve sheath tumour

This article was last reviewed and updated on December 11, 2018
by Bibianna Purgina, MD FRCPC

Quick facts:

  • A malignant peripheral nerve sheath tumour (MPNST) is a cancer that develops from a nerve.

  • It is a special type of cancer called a sarcoma.

  • Most malignant peripheral nerve sheath tumours occur in adults.

  • People with the genetic syndrome neurofibromatosis type 1 are at higher risk for developing these tumours.

 

Learn more

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. Nerves are found throughout your body. Some nerves are very small (like those just under the surface of the skin) while other are very large (like those that go to muscles).

A malignant peripheral nerve sheath tumour is a cancer (a malignant tumour) that develops from a nerve.  A malignant peripheral nerve sheath tumour is a special type of cancer called a sarcoma. Sarcomas are cancers that develop from mesenchymal tissues including nerves, fat, muscle, tendons, ligaments, bone and cartilage.

   

Most malignant peripheral nerve sheath tumours occur in adults.  The most common locations include the neck, arm, leg and buttock.  About half of the cases of malignant peripheral nerve sheath tumour develop in patients with a genetic syndrome known as neurofibromatosis type 1. In some cases the malignant peripheral nerve sheath tumour develops from an existing tumour called a neurofibroma.

  

Malignant peripheral nerve sheath tumours do not have any characteristic genetic changes that can be tested for at the present time.  If a test called immunohistochemistry is performed, malignant peripheral nerve sheath tumour show only rare or patchy staining with markers normally found in nerves, such as S100 protein.

 

As a result, it can be difficult for your pathologist to make a definite diagnosis of malignant peripheral nerve sheath tumour on biopsy tissue, but may suggest the diagnosis as a possibility in your pathology report.

 

Some malignant peripheral nerve sheath tumours can have areas that start to look like other types of mesenchymal tissue such muscle. Malignant peripheral nerve sheath tumour with skeletal muscle are sometimes called malignant Triton tumor and are more common in patients with neurofibromatosis type 1.

 

After a pathologist makes a diagnosis of malignant peripheral nerve sheath tumour or suggests the diagnosis as a possibility, the patient is usually treated first with surgery to remove the tumour. 

 

Sometimes, the patient may receive chemotherapy and/or radiation therapy before surgery, but usually these treatments are done after surgery, if recommended by your surgeon or oncologist. When the tumour is surgically removed as a resection specimen, it is sent to pathology for examination.

After surgical resection, your pathologist examines the tumour under the microscope and provides your surgeon and oncologist with critical information required for your subsequent treatment.

Histologic grade

Grade is a word pathologists use to describe how different the cancer cells look and behave compared to normal cells in the same location. Usually tumours that look high grade under the microscope have a worse prognosis than tumours that look low grade under the microscope. However, some types of sarcoma are not graded because research has shown that grade does not accurately predict how the tumour will behave. 

 

Currently, there is debate whether malignant peripheral nerve sheath tumour should be graded. Your pathologist may grade your malignant peripheral nerve sheath tumour using an internationally recognized system created by the French Federation of Cancer Centers Sarcoma Group (FNCLCC) or may choose to not include a grade in your pathology report.

 

If a grade has been given to your malignant peripheral nerve sheath tumour, your pathologist will use the FNCLCC grading system and look for three microscopic features: tumour differentiation, mitotic count, and necrosis. 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.

  • Tumour differentiation – Tumour differentiation describes how closely the cancer cells look like normal cells. Tumours that look very similar to normal cells are given are given 1 point while those that look very different from normal cells are given 2 or 3 points. All malignant peripheral nerve sheath tumour receive a score of either 2 or 3 points for this category.

  • Mitotic count – A cell that is in the process of dividing to create two new cells is called a mitotic figure. Tumours that are growing fast tend to have more mitotic figures than tumours that are growing slowly. Your pathologist will determine the mitotic count by counting the number of mitotic figures in ten areas of the tumour while looking through the microscope. Tumours with no mitotic figure or very few mitotic figures are given 1 point while those with 10 to 20 mitotic figures are given 2 points and those with more than 20 mitotic figures are given 3 points.

  • Necrosis - Necrosis is a type of cell death. Tumours that are growing fast tend to have more necrosis than tumours that are growing slowly. If your pathologist sees no necrosis, the tumour will be given 0 points. The tumour will be given 1 point if necrosis is seen but it makes up less than 50% of the tumour or 2 points if necrosis makes more than 50% of the tumour.

 

The final grade is based on the total number of points given to the tumour:

  • Grade 1 – 2 or 3 points.

  • Grade 2 – 4 or 5 points.

  • Grade 3 – 6 to 8 points.

Low grade sarcomas are grade 1.  High grade sarcomas have a grade of either 2 or 3.

Tumour size

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.


Tumours less than 5 cm are associated with better prognosis.

Necrosis

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).

 

The presence of necrosis is important, especially if you have 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.

Tumour extension

Malignant peripheral nerve sheath tumours can grow into or around organs and bone.  Your pathologist will examine samples of the surrounding organs and tissues under the microscope to look for cancer cells. Any surrounding organs or tissue that contains cancer cells will be described in your report.

Treatment effect

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. 

Perineural invasion

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 cancer cells attached to a nerve.

 

Perineural invasion is important because cancer 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.

Lymphovascular invasion

Lymphatics and blood vessels are long tubes that allow fluid (lymph and blood, respectively) and cells to travel around the body. When cancer cells enter a lymphatic or blood vessel it is called lymphovascular invasion and is associated with a higher risk that cancer cells will travel (metastasize) to a lymph node or a distant site such as the lungs.

Margins

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 considered ‘negative’ when there are no cancer cells at the edge of the cut tissue. Alternatively, a margin is considered ‘positive’ when there are cancer cells at the 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 (local recurrence).

Margins will only be described in your report after the entire tumour has been removed.

Lymph nodes

Metastatic disease describes the process where cancer cells escape the main tumour and travel to another part of the body. Lymph nodes are small immune organs located throughout the body.

 

Many cancers can spread to the lymph nodes, but malignant peripheral nerve sheath tumour 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.

If lymph nodes were removed as part of your surgery, then your report will include the total number of lymph nodes examined and the number that contain cancer cells.

Pathologic stage

​The pathologic stage for malignant peripheral nerve sheath tumour 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 worse prognosis.

Pathologic stage is not reported on a biopsy specimen. It is only reported when the entire tumour has been removed in an excision or resection specimen.

Tumour stage (pT) for malignant peripheral nerve sheath tumour

The tumour stage for malignant peripheral nerve sheath 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:
T1 – The tumour is no greater than 2 centimeters in size.
T2 – The tumour is between 2 and 4 centimeters in size.
T3 – The tumour is greater than 4 centimeters in size.
T4 – The tumour has grown into surrounding tissues such as the bones of the face or skull, the eye, the larger blood vessels in the neck, or the brain.

 

Tumour stage for tumours starting on the outside of the chest, back, or stomach and the arms or legs (trunk and extremities):
T1 – The tumour is no greater than 5 centimeters in size.
T2 – The tumour is between 5 and 10 centimeters in size.
T3 – The tumour is between 10 and 15 centimeters in size.
T4 – The tumour is greater than 15 centimeters in size.

Tumour stage for tumours starting in the abdomen and organs inside the chest (thoracic visceral organs):
T1 – The tumour is only seen in one organ.
T2 – The tumour has grown into the connective tissue that surrounds the organ from which is started.
T3 – The tumour has grown into at least one other organ.
T4 – Multiple tumours are found.

 

Tumour stage for tumours starting in the space at the very back of the abdominal cavity (retroperitoneum):
T1 – The tumour is no greater than 5 centimeters in size.
T2 – The tumour is between 5 and 10 centimeters in size.
T3 – The tumour is between 10 and 15 centimeters in size.
T4 – The tumour is greater than 15 centimeters in size.

 

Tumour stage for tumours starting in the space around the eye (orbit):
T1 – The tumour is no greater than 2 centimeters in size.
T2 – The tumour is greater than 2 centimeters in size but has not grown into the bones surrounding the eye.
T3 – The tumour has grown into the bones surrounding the eye or other bones of the skull.
T4 – The tumour has grown into the eye (the globe) or the surrounding tissues such as the eyelids, sinuses, or brain.

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 malignant peripheral nerve sheath tumour

Malignant peripheral nerve sheath tumour is given an nodal stage between 0 and 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 malignant peripheral nerve sheath tumour

Malignant peripheral nerve sheath tumour is given an metastatic stage between 0 and 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.

The metastatic stage can only be given if tissue from a distant site is sent for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined, and it is typically not included in your report. 

Molecular tests

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.

Some sarcomas have characteristic molecular changes to the tumour DNA that can be found using molecular tests. Malignant peripheral nerve sheath tumour does not have any known characteristic molecular changes.

 

Sometimes, your pathologist will perform molecular tests on your tumour to make sure it is not another type of sarcomasA negative molecular test (for example, a molecular test without an identified translocation or amplification) is compatible with a malignant peripheral nerve sheath tumour.

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 more often done on the biopsy specimen. If your pathologist is certain that your tumor is a malignant peripheral nerve sheath tumour, then no molecular testing may be done.

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