Low-Grade Fibromyxoid Sarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
September 16, 2026

Low-grade fibromyxoid sarcoma (LGFMS) is a rare, slow-growing cancer that develops in the body’s deep soft tissues. It is a type of sarcoma, a cancer that begins in the body’s connective tissues. It is also called Evans tumor, after the pathologist who first described it in 1987. LGFMS makes up less than 1% of all soft tissue sarcomas.

The name describes how the tumor looks under the microscope. “Low-grade” means the cells look only mildly abnormal and few of them are dividing. “Fibromyxoid” means the tumor contains a mix of firm, fibrous tissue and soft, gel-like tissue called myxoid tissue. Although it looks calm under the microscope, LGFMS can come back or spread years after surgery.

Low-grade fibromyxoid sarcoma most often affects adults, with a median age at diagnosis of about 35, and it affects men and women equally. It can also occur in children and older adults. Most tumors start deep in the soft tissues, often in the thigh, the hip and pelvis area, or the trunk.

This article explains how doctors diagnose low-grade fibromyxoid sarcoma and what the fusion test, margins, and stage in your LGFMS pathology report mean.

What causes low-grade fibromyxoid sarcoma?

Low-grade fibromyxoid sarcoma is caused by a genetic change that develops in the tumor cells during a person’s lifetime. In most tumors, pieces of chromosome 7 and chromosome 16 break and join together incorrectly. This joins a gene called FUS to a gene called CREB3L2, creating a new fusion gene written as FUS::CREB3L2.

Less often, LGFMS has a similar fusion called FUS::CREB3L1 or EWSR1::CREB3L1. The fusion protein acts like a switch, turning on genes that should normally be off and driving tumor cell growth.

The fusion in low-grade fibromyxoid sarcoma occurs only in tumor cells. It is not inherited and cannot be passed on to children. No lifestyle or environmental causes of LGFMS are known.

What are the symptoms of low-grade fibromyxoid sarcoma?

The most common symptom of low-grade fibromyxoid sarcoma is a painless lump that grows slowly. Because it grows slowly and usually doesn’t hurt, the lump may be present for a long time before it is noticed. A larger tumor can cause pain or limit movement if it presses on nearby nerves or muscles.

How is the diagnosis made?

The diagnosis of low-grade fibromyxoid sarcoma 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. Sometimes the diagnosis is made only after the whole tumor has been removed.

Under the microscope, LGFMS is made of long, thin spindle cells that look only mildly abnormal. The cells are often arranged in a swirling pattern, with firm fibrous areas next to soft myxoid areas. Very few of the cells are dividing. Because the tumor looks so calm, it can be mistaken for a noncancerous growth such as a neurofibroma unless special tests are done.

Some tumors contain round clusters of tumor cells arranged around a central collagen core, called giant rosettes. Tumors with this feature were once called hyalinizing spindle cell tumor with giant rosettes, but they are now known to be the same disease. Other tumors contain areas that look like a related cancer called sclerosing epithelioid fibrosarcoma. The pathologist may mention these areas because sclerosing epithelioid fibrosarcoma tends to behave less favorably than LGFMS.

Molecular testing can confirm low-grade fibromyxoid sarcoma by finding the fusion. Pathologists may use next-generation sequencing (NGS), RT-PCR, or FISH. Finding an FUS::CREB3L2 fusion confirms the diagnosis when the other findings fit. A negative FUS test does not completely rule out LGFMS because some tumors have an EWSR1 fusion instead.

Once imaging confirms low-grade fibromyxoid sarcoma, doctors look for spread. A chest CT scan is usually done because the lungs are the most common site of spread. The next section describes the immunohistochemistry tests used to confirm the diagnosis.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins tumor cells make. For low-grade fibromyxoid sarcoma, it is especially important because it helps separate this cancer from noncancerous tumors that look similar. Your report may include some of the following:

MUC4. MUC4 is the most useful stain for LGFMS and is positive in about 96% of cases. It is usually negative in the noncancerous look-alike tumors. A positive result strongly supports the diagnosis, although MUC4 is also positive in the related sclerosing epithelioid fibrosarcoma.
EMA. EMA is sometimes positive in scattered tumor cells.
SMA. SMA is a muscle-type protein that is occasionally positive in LGFMS.
S100. S100 is usually negative. A strongly positive result would suggest a nerve sheath tumor such as neurofibroma instead.
Beta-catenin. Beta-catenin does not build up in the nucleus of LGFMS cells. Nuclear staining would suggest desmoid fibromatosis, a noncancerous tumor that can look similar.
CD34 and desmin. These stains are usually negative in LGFMS. They are positive in several other spindle cell tumors and help rule those tumors out.

Not every case of low-grade fibromyxoid sarcoma needs every stain. The pathologist chooses tests based on how the tumor looks and where it started.

Histologic grade

Pathologists do not assign an FNCLCC grade to low-grade fibromyxoid sarcoma. The College of American Pathologists (CAP) soft tissue protocol, version 4.2 (2024), lists LGFMS among sarcomas not graded this way. The FNCLCC system relies on dividing cells and dead tumor tissue, and LGFMS usually has very few of either.

A score based on these features would suggest a low-risk tumor. That would be misleading, because LGFMS can come back or spread many years later. For this reason, your report will usually not include a grade number, or it may state that grading does not apply. Some older reports list an FNCLCC grade of 1 for this tumor.

Tumor size

Tumor size is the greatest dimension of the low-grade fibromyxoid sarcoma, measured in centimeters (cm). The final measurement comes from the tumor removed at surgery rather than from a biopsy. For most body sites, size is used to determine the tumor stage (pT). Size also helps the surgical team plan how much surrounding tissue to remove.

Tumor extension

Tumor extension describes whether low-grade fibromyxoid sarcoma has grown beyond the tissue where it started into nearby structures such as bone, blood vessels, nerves, or organs. 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, legs, and retroperitoneum, the stage depends on size alone. Extension into nearby structures still affects how surgeons and radiation oncologists plan treatment.

Treatment effect

Most people with low-grade fibromyxoid sarcoma have surgery without any treatment beforehand. Occasionally, radiation therapy or chemotherapy is given before surgery, for example when a tumor would be difficult to remove.

When treatment is given before surgery, the pathologist estimates what percentage of the tumor is non-viable (dead) and what percentage is still viable (alive). No cut-off has been established for LGFMS, so your doctors interpret this percentage alongside the rest of the report. If no treatment was given before surgery, the report may say there was no known presurgical therapy.

Lymphovascular invasion

Lymphovascular invasion means that cells from the low-grade fibromyxoid sarcoma 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.”

Present. Tumor cells were seen inside a vessel. This finding is associated with a higher risk of tumor spread.
Not identified. No tumor cells were seen inside vessels in the tissue examined.

Perineural invasion

Perineural invasion means tumor cells are growing around or along a nerve. It is not a standard item in soft tissue sarcoma reports, but a pathologist may mention it when it is seen in low-grade fibromyxoid sarcoma. When present, it suggests the tumor may extend beyond its visible edge and may raise the risk of the tumor coming back in the same place.

Surgical margins

A margin is the edge of tissue cut by the surgeon to remove a low-grade fibromyxoid sarcoma. The pathologist examines each margin to see whether tumor cells reach it. For LGFMS, removal with negative margins has been linked to a lower chance of the tumor coming back than removal with a positive margin.

Negative margin. No tumor cells are seen at the cut edge. The report usually names the closest margin and gives its distance from the tumor.
Close margin. Tumor cells are near the cut edge but do not reach it. Reports often list every margin that is less than 0.5 cm from the tumor.
Positive margin. Tumor cells are present at the cut edge. This means some tumor may remain in the body and raises the risk of the tumor coming back. Further surgery may be considered.

Lymph nodes

Lymph nodes are small immune organs that filter fluid from the tissues. Low-grade fibromyxoid sarcoma very 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. Tumor cells in a lymph node change the nodal stage to pN1.

Pathologic stage (pTNM)

The pathologic stage for low-grade fibromyxoid sarcoma 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 LGFMS depends on where the tumor started in the body.

Trunk and extremities (chest, back, abdominal wall, arms, and legs):

pT1. The tumor is 5 cm or smaller.
pT2. The tumor is larger than 5 cm but not larger than 10 cm.
pT3. The tumor is larger than 10 cm but not larger than 15 cm.
pT4. The tumor is larger than 15 cm.

Retroperitoneum (the space at the back of the abdomen):

pT1. The tumor is 5 cm or smaller.
pT2. The tumor is larger than 5 cm but not larger than 10 cm.
pT3. The tumor is larger than 10 cm but not larger than 15 cm.
pT4. The tumor is larger than 15 cm.

Head and neck:

pT1. The tumor is 2 cm or smaller.
pT2. The tumor is larger than 2 cm but not larger than 4 cm.
pT3. The tumor is larger than 4 cm.
pT4a. The tumor has grown into the eye socket, the bones of the face, or the base of the skull and its lining. It may instead involve the organs in the center of the neck or the chewing muscles called the pterygoid muscles.
pT4b. The tumor has grown into the brain, surrounds the carotid artery, or has grown into the muscles in front of the spine. A tumor that has spread along a nerve into the brain or spinal cord is also pT4b.

Abdominal and thoracic visceral organs (internal organs such as the stomach, intestines, and lungs):

pT1. The tumor is confined to the organ where it started.
pT2a. The tumor has grown into the organ’s thin outer lining.
pT2b. The tumor has grown beyond the outer lining into the surrounding tissue.
pT3. The tumor has grown into another organ or a nearby structure such as the diaphragm or abdominal wall.
pT4a. Tumor is found in 2 separate sites.
pT4b. Tumor is found in 3 to 5 separate sites.
pT4c. Tumor is found in more than 5 separate sites.

Orbit (the space around the eye):

pT1. The tumor is 2 cm or smaller.
pT2. The tumor is larger than 2 cm and has not grown into the bony walls of the orbit or the eye.
pT3. The tumor, of any size, has grown into the bony walls of the orbit.
pT4. The tumor has grown into the eye or nearby structures such as the eyelid, sinuses, or brain.

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 LGFMS describes whether tumor cells were found in the lymph nodes:

pN0. No tumor cells were found in the lymph nodes examined.
pN1. Tumor cells were found in at least one nearby lymph node.

If no lymph nodes were removed, which is common for LGFMS, 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.

What is the prognosis?

Low-grade fibromyxoid sarcoma usually grows slowly, and the outlook right after surgery is generally good. The main concern is that LGFMS can come back in the same place or spread to other organs, sometimes many years after treatment. Spread most often involves the lungs.

In a 2026 pooled analysis of 773 people, about 1 in 5 had the tumor come back in the same place during a median follow-up of 3 years. A similar proportion developed spread during that time, while most people had neither. In an older series with much longer follow-up, local recurrence eventually reached 64%, and spread reached 46%. These figures show why the risk continues to grow with time.

The feature most clearly linked to a better outcome is complete tumor removal with negative margins. Tumors with areas resembling sclerosing epithelioid fibrosarcoma may carry a higher risk. Your care team can explain how the findings in your report apply to you.

What happens after the diagnosis?

After low-grade fibromyxoid sarcoma is confirmed, care is usually planned by a team at a center experienced in treating sarcoma. The findings in your report, especially margin status and whether the cancer has spread, help the team decide which options to consider.

Surgery. Complete removal of the tumor with negative margins is the main treatment for LGFMS. If margins are positive, further surgery may be considered.
Radiation therapy and chemotherapy. These treatments are used much less often for LGFMS. Pooled studies show limited benefit, but they may be considered when a tumor cannot be completely removed or has spread.
Clinical trials. Because LGFMS is rare, your oncologist may discuss clinical trials for sarcomas that have come back or spread.

No biomarker test currently selects a targeted treatment for low-grade fibromyxoid sarcoma. The fusion test confirms the diagnosis but does not select a specific drug.

Because LGFMS can come back or spread many years after surgery, follow-up usually continues for a long time. It typically includes regular imaging of the chest and of the area where the tumor started.

Questions to ask your doctor

How was the diagnosis of low-grade fibromyxoid sarcoma confirmed: by MUC4 staining, a fusion test, or both?
Which fusion was found in my tumor?
Did my tumor have giant rosettes or areas resembling sclerosing epithelioid fibrosarcoma?
How large was the tumor, and where did it start?
Were all the margins negative? How close was the closest margin?
What is my pathologic stage?
Has the tumor spread to my lungs or elsewhere?
Do I need any treatment after surgery?
How often will I need imaging, and for how many years?
What symptoms should I watch for between appointments?