Myxofibrosarcoma: Understanding Your Pathology Report

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

Myxofibrosarcoma is a cancer that develops from fibroblasts, the cells that make the connective tissue that supports and holds the body together. It is a type of sarcoma, a cancer that begins in the body’s connective tissues. Myxofibrosarcoma makes up about 5% of all soft tissue sarcomas and is one of the most common sarcomas in older adults.

The name describes how the tumor looks under the microscope. “Myxo” refers to the soft, gel-like myxoid tissue that fills much of the tumor. “Fibro” refers to the fibroblasts that make up the tumor cells. Older reports may call this tumor the myxoid type of malignant fibrous histiocytoma, a name that is no longer used.

Myxofibrosarcoma most often affects adults over 50, and it is slightly more common in men. It usually starts in the arms or legs, especially the legs. Many tumors start just under the skin, while others start deeper in the muscle.

This article explains how myxofibrosarcoma is diagnosed and what the grade, margins, and stage in your myxofibrosarcoma pathology report mean.

What causes myxofibrosarcoma?

Myxofibrosarcoma develops when fibroblasts collect many changes in their genes and chromosomes that allow them to grow without control. Unlike some sarcomas, myxofibrosarcoma does not have a single defining genetic change such as a gene fusion. Instead, tumor cells usually have many different changes, and these vary from person to person.

No lifestyle or environmental causes of myxofibrosarcoma are known. It is not inherited.

What are the symptoms of myxofibrosarcoma?

The most common symptom of myxofibrosarcoma is a painless lump that grows slowly. People often notice tumors just under the skin while they are still small. Deeper tumors may not be noticed until they are larger, and they can cause pain if they press on nearby nerves or muscles.

How is the diagnosis made?

The diagnosis of myxofibrosarcoma 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, myxofibrosarcoma is made of abnormal spindle-shaped cells scattered through myxoid tissue. The tumor often grows as several nodules and contains thin, curved blood vessels. In higher-grade tumors, the cells are larger, more crowded, and vary greatly in shape and size. Some cells contain small bubbles of myxoid material and are called pseudolipoblasts.

Myxofibrosarcoma tends to spread out into the surrounding tissue in thin strands, like the roots of a plant. These strands can extend well beyond the edge of the lump seen on imaging or felt at surgery. On MRI, this growth may show up as a “tail” of tissue extending along the thin layers that cover the muscles.

Because myxofibrosarcoma does not have a specific genetic change, the diagnosis is based mainly on how the tumor looks. The pathologist may use immunohistochemistry or molecular tests to rule out other tumors that look similar. Once myxofibrosarcoma is confirmed, doctors usually do a chest CT scan because the lungs are the most common site of spread.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins tumor cells make. For myxofibrosarcoma, no single stain confirms the diagnosis. Instead, the test shows that the tumor is not another type of cancer. Your report may include some of the following:

SMA and CD34. These proteins are sometimes weakly positive in some of the tumor cells. This result is consistent with myxofibrosarcoma but does not prove it.
MDM2 and CDK4. These proteins are usually negative. A positive result, or extra copies of the MDM2 gene found by FISH, would suggest liposarcoma instead.
S100 and SOX10. These proteins are negative in myxofibrosarcoma. A positive result would suggest melanoma or a nerve sheath tumor instead.
Keratins. Keratins are negative in myxofibrosarcoma. A positive result would suggest a carcinoma, a cancer that starts from epithelial cells.
Desmin. Desmin is usually negative. A positive result would suggest a muscle tumor.

Not every case of myxofibrosarcoma needs every stain. The pathologist chooses tests based on how the tumor looks, where it started, and which other tumors need to be ruled out.

Histologic grade

Histologic grade describes how abnormal the cells of a myxofibrosarcoma look and how quickly they appear to be growing. Grade is one of the most important findings in the report for myxofibrosarcoma, because it strongly predicts whether the tumor will spread. Most pathologists use the FNCLCC system, which adds together scores for three features:

Differentiation. This describes how closely the tumor cells resemble normal tissue. Myxofibrosarcoma usually receives a score of 2.
Mitotic count. This is the number of dividing cells in 10 high-power fields, which are areas seen through the microscope at high magnification. Fewer than 10 dividing cells score 1, 10 to 19 score 2, and 20 or more score 3.
Necrosis. This is the amount of dead tumor tissue. No necrosis scores 0, less than 50% scores 1, and 50% or more scores 2.

The total score gives the final grade:

Grade 1. Total score of 2 or 3. These tumors are called low grade.
Grade 2. Total score of 4 or 5. These tumors are considered high grade.
Grade 3. Total score of 6 to 8. These tumors are also considered high grade.

Grading myxofibrosarcoma can be difficult, and some pathologists also consider how crowded the cells are and how much of the tumor is solid rather than myxoid. Low-grade myxofibrosarcoma often comes back after surgery, but it rarely spreads to other organs. High-grade tumors are more likely to spread. A tumor that comes back can also be higher grade than the original tumor.

Some myxofibrosarcomas contain round, epithelial-like tumor cells. This type is called epithelioid myxofibrosarcoma, and it behaves like a high-grade tumor.

Tumor size

Tumor size is the greatest dimension of the myxofibrosarcoma, 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).

Myxofibrosarcomas larger than 5 cm are associated with a less favorable outcome than smaller tumors. Because the tumor spreads out in thin strands, the true extent of the tumor can be larger than the measured size of the main lump.

Tumor depth and extension

Tumor depth describes whether the myxofibrosarcoma is located in the tissue just under the skin or deep to the thin layer of tissue covering the muscles, called the fascia. Pathology reports for myxofibrosarcoma often describe depth, because superficial and deep tumors can behave differently. Deep tumors have been associated with a higher risk of spread to other organs.

Tumor extension describes whether the myxofibrosarcoma has grown into nearby structures such as the skin, muscle, bone, or blood vessels. 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.

Treatment effect

Some people with myxofibrosarcoma receive radiation therapy, and less often chemotherapy, 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 myxofibrosarcoma, 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

Lymphovascular invasion means that cells from the myxofibrosarcoma 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 myxofibrosarcoma. 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 myxofibrosarcoma. The pathologist examines each margin to see whether tumor cells reach it. Margins are especially important for myxofibrosarcoma because its thin strands of tumor cells make it harder to remove completely than most other sarcomas.

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 or radiation therapy may be considered.

A positive margin is one of the strongest predictors that myxofibrosarcoma will come back in the same place. During surgery, the pathologist may examine some margins right away using a rapid test called a frozen section. This helps the surgeon decide whether to remove more tissue.

Lymph nodes

Lymph nodes are small immune organs that filter fluid from the tissues. Myxofibrosarcoma 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 myxofibrosarcoma 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 myxofibrosarcoma 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 myxofibrosarcoma 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 myxofibrosarcoma, 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?

The outlook for a person with myxofibrosarcoma depends mainly on the grade of the tumor, its size, and whether it can be completely removed. In the largest study to date, which included 908 people, about 68% were alive five years after diagnosis. This figure includes deaths from all causes, and many people with myxofibrosarcoma are older adults.

Myxofibrosarcoma comes back in the same place more often than most other sarcomas. Reported rates of local recurrence range from about 16% to 61%, compared with around 10% for soft tissue sarcomas overall. In one detailed group from the large study, 39% of people had the tumor come back in the same place and 28% developed spread to other organs.

Features associated with the outcome of myxofibrosarcoma include:

Histologic grade. High-grade tumors are much more likely to spread than low-grade tumors, and grade 3 tumors are associated with the least favorable outcome.
Tumor size. Tumors larger than 5 cm are associated with a less favorable outcome.
Age. Several studies show that people older than 65 have a less favorable outcome.
Surgical margins. A positive margin greatly increases the chance of the tumor coming back in the same place.
Tumor depth. Deep tumors have been associated with a higher risk of spread than tumors just under the skin.

What happens after the diagnosis?

After myxofibrosarcoma is confirmed, a team at a center experienced in treating sarcoma usually plans care. In one study, complete removal with negative margins was achieved much more often when surgery was done at a sarcoma center. The findings in your report, especially the grade, size, and margin status, help the team decide which options to consider.

Surgery. Removal of the tumor with a rim of normal tissue around it is the main treatment for myxofibrosarcoma. Because the tumor spreads in thin strands, the surgeon often needs to remove more tissue than the lump’s size suggests. Reconstructive surgery may be needed to close the wound.
Radiation therapy. Radiation is often given before or after surgery to lower the risk of the tumor coming back in the same place. In one recent study, radiation before surgery followed by removal with negative margins controlled the tumor locally in 87% of people at five years.
Chemotherapy. Chemotherapy may be considered for some high-grade tumors or when the cancer has spread.
Clinical trials. Your oncologist may discuss clinical trials, especially if the tumor has come back or spread.

No biomarker test is currently used to choose a targeted treatment for myxofibrosarcoma. Because the tumor can come back in the same place, follow-up usually includes regular examination and imaging of the area where it started. Chest imaging is also usually part of follow-up.

Questions to ask your doctor

What is the grade of my myxofibrosarcoma?
Is my tumor low grade or high grade, and what does that mean for my risk of spread?
Was the tumor just under the skin, or was it deep?
How large was the tumor, and did it grow into nearby structures?
Were all the margins negative? How close was the closest margin?
Will I need more surgery or radiation therapy?
If I had treatment before surgery, what percentage of the tumor was non-viable?
What is my pathologic stage?
Has the tumor spread to my lungs or elsewhere?
How often will I need imaging, and for how long?