A malignant peripheral nerve sheath tumor (MPNST) is a cancer that starts from the cells that surround and protect a nerve. It is a type of sarcoma, which is a cancer that begins in the body’s connective tissues. Older pathology reports may use the names malignant schwannoma, neurofibrosarcoma, or neurogenic sarcoma for the same tumor.
Malignant peripheral nerve sheath tumor usually develops along a medium-sized or large nerve deep in the body. Common locations include the trunk, the arms and legs, and the head and neck. It is a rare cancer, making up roughly 2% to 10% of soft tissue sarcomas. Most people diagnosed are adults, and people with neurofibromatosis type 1 (NF1) tend to be diagnosed at a younger age.
This article explains how pathologists diagnose malignant peripheral nerve sheath tumor and what the grade, size, margins, and stage in your MPNST pathology report mean. Our articles on neurofibroma and schwannoma describe the noncancerous nerve sheath tumors that MPNST must be told apart from.
Malignant peripheral nerve sheath tumor develops when the support cells lining a nerve accumulate changes in their genes. These changes allow the cells to grow without the normal controls. MPNST arises in three main settings:
The NF1 gene makes a protein called neurofibromin, which acts as a brake on signals that tell cells to grow. When both copies of the gene stop working in a nerve sheath cell, that brake is lost. Changes in the NF1 gene are found in most MPNSTs, including many sporadic tumors. Additional changes, most often in genes called CDKN2A, SUZ12, and EED, usually occur as a noncancerous neurofibroma becomes a cancer.
This change from a noncancerous tumor to a cancer often happens in steps. Between a plexiform neurofibroma and MPNST, pathologists recognize an in-between stage called an atypical neurofibromatous neoplasm of uncertain biologic potential (ANNUBP). A person with NF1 may have had a biopsy showing ANNUBP before a diagnosis of MPNST.
The most common symptom of a malignant peripheral nerve sheath tumor is a growing lump, which may be painful. Because the tumor starts in a nerve, it can also cause numbness, tingling, or weakness in the part of the body that nerve supplies. Tumors deep in the body may not cause symptoms until they are large.
For people with NF1, a change in an existing neurofibroma can be the first sign of MPNST. Warning signs include rapid growth, new pain that does not go away, and new weakness or numbness.
The diagnosis of malignant peripheral nerve sheath tumor 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. In people with NF1, a PET scan may first be used to find the most active part of a large neurofibroma so the biopsy can target that area. Sometimes doctors make the diagnosis only after the whole tumor has been removed.
Under the microscope, MPNST is usually made of long, thin cells arranged in bundles. The tumor cells often look very abnormal, many are dividing, and some areas may show dead tumor tissue called necrosis. Sometimes the pathologist can also see the neurofibroma that the tumor developed from. Because MPNST can look like several other tumors, a small biopsy may not give a final answer. In that case, the report may describe a spindle cell sarcoma and suggest a complete diagnosis after surgery.
Unlike many other sarcomas, MPNST does not have a gene fusion, which is a joining of two separate genes. For this reason, pathologists may use molecular tests such as next-generation sequencing (NGS) to look for fusions. Finding a fusion would point to a different diagnosis, such as synovial sarcoma. A result showing no fusion supports the diagnosis of MPNST when the other findings fit.
Once malignant peripheral nerve sheath tumor is confirmed, imaging tests such as MRI and CT scans show the full size of the tumor and whether it has spread. The lungs are the most common site of MPNST spread, so a chest CT scan is usually part of this assessment. The next section describes the immunohistochemistry tests used to confirm the diagnosis.
Immunohistochemistry uses antibodies to show which proteins tumor cells make. For malignant peripheral nerve sheath tumor, the test helps confirm that the tumor came from nerve sheath cells. It also helps rule out tumors that can look similar, such as melanoma, synovial sarcoma, and schwannoma. No single stain proves the diagnosis, so the pathologist interprets the results as a whole. Your report may include some of the following:
Not every case of malignant peripheral nerve sheath tumor needs every stain. The pathologist chooses tests based on how the tumor looks and where it is located.
Several types of malignant peripheral nerve sheath tumor are recognized in the World Health Organization (WHO) classification of soft tissue and bone tumors, 5th edition (2020). The type is included in your report because some types behave differently or occur in different groups of people.
Histologic grade describes how abnormal the cells of a malignant peripheral nerve sheath tumor look and how quickly they appear to be growing. Most soft tissue sarcomas are graded using the FNCLCC system. Grading MPNST this way is less reliable. The College of American Pathologists (CAP) soft tissue protocol, version 4.2 (2024), states that the value of FNCLCC grade in MPNST is unclear.
For this reason, many pathology reports describe MPNST simply as low-grade or high-grade. Some reports do not include a grade at all, and this is expected.
If your report does include an FNCLCC grade, the pathologist has added scores for tumor differentiation, the number of dividing cells, and the amount of necrosis. The total score gives the final grade.
For staging purposes, grade 2 and grade 3 tumors are both treated as high grade. Higher-grade tumors are more likely to come back after treatment and to spread to other parts of the body.
Tumor size is the greatest dimension of the malignant peripheral nerve sheath tumor, measured in centimeters (cm). The final size comes from the tumor removed at surgery rather than from a biopsy. If the tumor was removed in pieces, the size may be taken from imaging instead.
For MPNSTs of the trunk, arms, legs, and retroperitoneum (the space behind the abdomen), size determines the tumor stage (pT). Tumors larger than 5 cm are associated with a higher risk of spread and a less favorable outcome. MPNSTs are often large when they are found, especially in people with NF1.
Tumor extension describes whether a malignant peripheral nerve sheath tumor has grown beyond the nerve where it started and into nearby tissues. MPNST can grow along the nerve and into surrounding fat, muscle, bone, or blood vessels. 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, and legs, the stage is based on size alone. Even so, growth into bone, major blood vessels, or other large nerves affects how doctors plan surgery and radiation.
Some people with malignant peripheral nerve sheath tumor receive radiation therapy, chemotherapy, 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 MPNST, 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 means that cells from the malignant peripheral nerve sheath tumor 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.”
Perineural invasion describes cancer cells growing around a nerve. For most cancers, it is a separate finding that suggests a higher risk of the cancer coming back. Malignant peripheral nerve sheath tumor is different, because it starts inside a nerve. For this reason, perineural invasion is usually not reported as a separate finding for MPNST.
For MPNST, what matters more is how far the tumor has spread along the nerve. The tumor can extend within the nerve beyond the edge of the lump seen on imaging or at surgery. The surgeon may remove a length of nerve on each side of the tumor, and the pathologist checks the cut ends for tumor cells. These nerve margins are described in the next section.
A margin is the edge of tissue cut by the surgeon to remove a malignant peripheral nerve sheath tumor. 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.
Lymph nodes are small immune organs that filter fluid from the tissues. Malignant peripheral nerve sheath tumor rarely spreads to lymph nodes. 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. Finding tumor cells in a lymph node is uncommon in MPNST but is associated with a less favorable outcome.
Biomarker testing looks for tumor features that can guide treatment, predict outcomes, or point to an inherited condition. For most people with malignant peripheral nerve sheath tumor, no biomarker test currently selects a specific targeted drug. As of 2026, the FDA has not approved any drug specifically for MPNST. The tests used to confirm the diagnosis, including H3K27me3 staining and fusion testing, are described above.
When MPNST has come back, has spread, or cannot be removed, the tumor may be tested with a broad NGS panel. This test commonly finds changes in NF1, in CDKN2A, and in genes that help control which other genes are switched on. These results do not currently match an approved targeted drug for MPNST, but they may make a person eligible for a clinical trial. Rarely, a panel finds a change targeted by a drug that is approved across many cancer types.
Genetic testing for NF1 is a separate question from tumor testing. A change in the NF1 gene found in the tumor does not by itself mean the change is inherited, because sporadic MPNSTs also have NF1 changes. Features of NF1 include light brown skin patches called café-au-lait spots and multiple neurofibromas. If you have these features or a family history of NF1, your doctor may suggest genetic counseling and a blood test.
Our article What is a hereditary cancer syndrome? explains how inherited gene changes are tested. You can learn more about tumor testing in our Biomarkers and Genetic Testing section.
The pathologic stage for malignant peripheral nerve sheath tumor 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 MPNST depends on where the tumor started in the body.
Trunk and extremities (chest, back, abdominal wall, arms, and legs):
Retroperitoneum (the space at the back of the abdomen):
Head and neck:
Abdominal and thoracic visceral organs (internal organs such as the stomach, intestines, and lungs):
Orbit (the space around the eye):
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 MPNST describes whether tumor cells were found in the lymph nodes:
If no lymph nodes were removed, which is common for MPNST, 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.
The outlook for a person with malignant peripheral nerve sheath tumor depends mostly on whether the tumor has spread and whether it can be completely removed. Across published studies, about half of people are alive five years after diagnosis. Reported five-year survival ranges widely, from about 15% to about 66%. These differences reflect the size, location, and stage of the tumors included in each study.
MPNST comes back in the same place in about 4 in 10 people after treatment. When MPNST has spread or cannot be removed, survival is much shorter. In reported series of these patients, half lived longer than about 13 to 15 months and half lived less.
Features associated with the outcome of MPNST include:
After a malignant peripheral nerve sheath tumor is confirmed, a team at a sarcoma center usually plans care. The team often includes surgeons, radiation oncologists, and medical oncologists who specialize in sarcoma. The findings in your pathology report, including the grade, size, margins, and stage, help this team decide which options to consider.
After treatment for MPNST, follow-up usually includes regular imaging of the area where the tumor started and of the chest. People with NF1 also need ongoing monitoring of their other neurofibromas. Genetic counseling can help relatives understand whether they should be tested for NF1.