Alveolar Soft Part Sarcoma: Understanding Your Pathology Report

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

Alveolar soft part sarcoma (ASPS) is a rare cancer that develops in the body’s deep soft tissues, such as muscle. It is a type of sarcoma, a cancer that begins in the body’s connective tissues. ASPS makes up less than 1% of all soft tissue sarcomas.

Despite its name, alveolar soft part sarcoma does not start in the alveoli, the tiny air sacs of the lungs. The word “alveolar” describes how the tumor cells look under the microscope. The cells grow in small nests that often have open centers, creating a pattern that resembles those air sacs.

ASPS most often affects adolescents and young adults between about 15 and 35 years old. In adults, it usually starts deep in the thigh or buttock. In children, it more often starts in the head and neck, especially the tongue or the tissue around the eye. ASPS tends to grow slowly, but it often spreads to other parts of the body, most commonly the lungs.

This article explains how doctors diagnose alveolar soft part sarcoma and what the fusion test, grade, margins, and stage in your ASPS pathology report mean.

What causes alveolar soft part sarcoma?

Alveolar soft part sarcoma is caused by a genetic change that develops in the tumor cells during a person’s lifetime. Pieces of chromosome X and chromosome 17 break and join together incorrectly. This joins a gene called ASPSCR1 to a gene called TFE3, creating a new fusion gene written as ASPSCR1::TFE3.

The TFE3 gene normally makes a protein that controls when groups of other genes are switched on. The fusion protein keeps those genes switched on when they should be off, driving tumor cell growth. ASPS tumors also develop a very rich blood supply, which is one reason drugs that block blood vessel growth are used to treat them.

The ASPSCR1::TFE3 fusion is found only in the tumor cells. It is not inherited and cannot be passed on to children.

What are the symptoms of alveolar soft part sarcoma?

The most common symptom of alveolar soft part sarcoma is a painless lump that grows slowly. Because it usually does not hurt, the lump may grow large before you notice it. Other symptoms depend on where the tumor is located:

Arms, legs, and trunk. A deep lump or swelling, sometimes with pain if the tumor presses on nearby nerves or muscles.
Tongue and mouth. A firm lump in the tongue that is usually painless.
Around the eye. A bulging eye, trouble moving the eye, or vision changes.

In some people, the first sign of ASPS is a spot in the lung found on a chest scan, before anyone notices a lump. Less often, spread to the brain causes headaches or other neurological symptoms.

How is the diagnosis made?

The diagnosis of alveolar soft part 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 or by surgically removing part or all of the tumor.

Under the microscope, ASPS consists of large cells with abundant pink cytoplasm, the body of the cell. The cells grow in nests separated by thin walls of tissue that contain many small blood vessels. In the center of the nests, the cells often pull apart, creating the alveolar pattern. Very few of the cells are dividing, but this does not mean the tumor is low risk.

A special stain called PAS with diastase often shows small rod-shaped crystals inside the tumor cells. These crystals are seen in most cases of ASPS and strongly support the diagnosis.

Molecular testing confirms alveolar soft part sarcoma by finding the ASPSCR1::TFE3 fusion. Pathologists may use next-generation sequencing (NGS) or a test called RT-PCR to identify both genes in the fusion. FISH, a test that uses glowing probes, can show that TFE3 is rearranged but cannot name its partner gene. This matters because TFE3 rearrangements also occur in other tumors.

The same ASPSCR1::TFE3 fusion is also found in a rare type of kidney cancer. For this reason, the pathologist also considers where the tumor started before making the final diagnosis.

Once ASPS is confirmed, imaging tests look for spread. A chest CT scan is used because the lungs are the most common site of spread. An MRI of the brain may also be done, because ASPS spreads to the brain more often than other sarcomas. The next section describes the immunohistochemistry tests used to confirm the diagnosis.

Immunohistochemistry

Immunohistochemistry uses antibodies to show which proteins tumor cells make. For alveolar soft part sarcoma, it supports the diagnosis and helps rule out tumors that can look similar, such as kidney cancer, melanoma, and paraganglioma. Your report may include some of the following:

TFE3. The fusion causes TFE3 protein to accumulate in the nucleus of tumor cells. Strong nuclear staining for TFE3 is seen in nearly all cases of ASPS and is the most useful stain. It does not prove the diagnosis on its own, because other tumors with TFE3 changes stain the same way.
Cathepsin K. This protein is positive in ASPS and supports the diagnosis.
Desmin. This muscle protein is positive in some cases of ASPS, usually in only part of the tumor.
Markers of look-alike tumors. Stains such as cytokeratins, HMB-45, and synaptophysin are usually negative in ASPS. A positive result would suggest a different tumor, such as kidney cancer, melanoma, or paraganglioma.

Not every case of alveolar soft part 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 alveolar soft part sarcoma. The FNCLCC system gives points for dividing cells and dead tumor tissue, but ASPS often has very few dividing cells. A score based on these features would underestimate how ASPS behaves, because it can spread even when the cells look calm.

Instead, ASPS is considered a high-grade cancer by definition. Your report will therefore not include a grade number, or it may state that grading does not apply. This is expected for this diagnosis.

Tumor size

Tumor size is the greatest dimension of the alveolar soft part 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).

Larger tumors are more likely to have already spread when ASPS is found. In one national study, about 8 in 10 people whose tumors were larger than 5 cm had spread at diagnosis. Tumors larger than 10 cm have also been associated with a less favorable outcome.

Tumor extension

Tumor extension describes whether alveolar soft part sarcoma has grown beyond the tissue where it started into nearby structures such as bone, blood vessels, 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 doctors plan surgery and radiation.

Treatment effect

Most people with alveolar soft part sarcoma have surgery first, because standard chemotherapy works poorly for this tumor. Occasionally, a targeted drug or radiation therapy is given before surgery to shrink a tumor that would otherwise be hard 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 ASPS, 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 alveolar soft part sarcoma are seen inside a small blood vessel or lymphatic channel. Tumor cells inside blood vessels are common in ASPS. This fits with the tumor’s rich blood supply and its tendency to spread through the bloodstream. 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 alveolar soft part 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 an alveolar soft part sarcoma. 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.

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.

Lymph nodes

Lymph nodes are small immune organs that filter fluid from the tissues. Alveolar soft part sarcoma rarely spreads to lymph nodes. It much more often spreads through the bloodstream to the lungs, bones, or brain. For this reason, doctors usually remove lymph nodes 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.

Biomarker and molecular testing

Biomarker testing looks for tumor features that guide treatment, predict outcome, or point to an inherited condition. For alveolar soft part sarcoma, no biomarker test is currently needed to select treatment. The ASPSCR1::TFE3 fusion confirms the diagnosis, as described above, but no approved drug targets the fusion itself.

Treatment options for ASPS, including immunotherapy, are chosen based on the diagnosis and on whether the cancer has spread. They do not depend on a PD-L1 result or another biomarker test. These options are described under “What happens after the diagnosis?” below.

When ASPS has come back or spread, the tumor may be tested with a broad NGS panel. This test looks for changes that could make a person eligible for a clinical trial. Because the fusion is usually the main genetic change in ASPS, these panels often find no other targetable change. This result is common and does not change the diagnosis.

You can learn more about tumor testing in our Biomarkers and Genetic Testing section.

Pathologic stage (pTNM)

The pathologic stage for alveolar soft part 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.

For ASPS, whether the cancer has spread to the lungs or other organs affects the outlook more than the pT number does. 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 ASPS 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 (including the tongue):

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 ASPS 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 ASPS, 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 alveolar soft part sarcoma depends mainly on whether the cancer has spread when it is diagnosed. ASPS usually grows slowly, so many people live for years even after it has spread. It can also come back or spread many years after treatment, sometimes more than 10 years later.

When ASPS has not spread and is completely removed, roughly 85 in 100 people are alive five years later in reported series. Even so, some of these people later develop spread to the lungs, which is why long-term follow-up matters. In many published series, more than half of people already had spread to the lungs or other organs when ASPS was diagnosed.

When ASPS has spread at diagnosis, reported five-year survival ranges from about 20% in an older series to about 60% in a more recent national study. The more recent figures may partly reflect newer targeted treatments. Across all people with ASPS, about half are alive 10 years after diagnosis.

Features associated with the outcome of alveolar soft part sarcoma include:

Spread at diagnosis. Cancer that has already spread when it is found is the strongest predictor of a less favorable outcome.
Tumor size. Tumors larger than 10 cm are associated with a less favorable outcome, and larger tumors are more likely to have spread.
Age. Younger people, especially children, tend to do better than older adults.
Tumor location. Tumors that start in the trunk are associated with a less favorable outcome than tumors of the arms and legs.
Completeness of surgery. Complete removal of a tumor that has not spread offers the best chance of long-term control.

What happens after the diagnosis?

After a team at a sarcoma-experienced center confirms alveolar soft part sarcoma, they usually plan care. The team often includes surgeons, radiation oncologists, and medical oncologists. Your report findings, especially margin status and whether the cancer has spread, help the team decide which options to consider.

Surgery. Complete removal with negative margins is the main treatment for ASPS that has not spread. In selected people, surgery to remove a small number of lung or brain metastases may also be considered.
Radiation therapy. Radiation may be considered when a margin is close or positive, or when the tumor is in a place where wide surgery is difficult. It may also be used to treat spread to the brain.
Immunotherapy. Atezolizumab was approved by the FDA in December 2022 for adults and children 2 years and older with ASPS that cannot be removed or has spread. In the study that led to this approval, tumors shrank in about 1 in 4 people.
Targeted therapy. Pazopanib and sunitinib block signals that tumors use to build new blood vessels, and both are used for advanced ASPS. The 2025 NCCN guidelines also list pembrolizumab, with or without axitinib, as an option.
Standard chemotherapy. Drugs such as doxorubicin work poorly for ASPS and are not usually a preferred option.
Clinical trials. Researchers are studying new treatments, including combinations of immunotherapy and targeted drugs. Your oncologist can tell you whether a trial is open to you.

Approval and access to these drugs differ by country, and your care team can explain what applies where you live. Because ASPS can spread many years after treatment, follow-up usually continues for more than 10 years. It typically includes regular imaging of the chest and the area where the tumor started, and it may include imaging of the brain.

Questions to ask your doctor

Was the ASPSCR1::TFE3 fusion confirmed, and which test was used?
Where did my tumor start, and how large was it?
Has the cancer spread to my lungs, bones, or brain?
Do I need a brain MRI?
Were all the margins negative? How close was the closest margin?
Was lymphatic or vascular invasion found?
Were any lymph nodes examined, and did they contain tumor cells?
What is my pathologic stage?
Will radiation therapy be considered after my surgery?
If the cancer has spread, is immunotherapy such as atezolizumab an option for me?
Are targeted drugs such as pazopanib or sunitinib an option in my situation?
Is there a clinical trial I could join?
How often will I need imaging, and for how many years?