Your pathology report for alveolar soft part sarcoma

By Jason Wasserman MD PhD FRCPC and Bibianna Purgina MD FRCPC
December 8, 2025

Alveolar soft part sarcoma (ASPS) is a rare soft-tissue cancer that develops in the muscles, fat, or connective tissue of the body. Despite its name, it does not start in the alveoli of the lungs. Instead, the term “alveolar” refers to the way the tumor cells are arranged under the microscope. The cells tend to form small nests separated by thin walls, creating a pattern that resembles the tiny air sacs in the lung.

ASPS can occur almost anywhere in the body. It most commonly affects the legs and arms, but tumors can also develop in the head and neck, chest, abdomen, and pelvis. This type of sarcoma often grows slowly at first, but it has a strong tendency to spread (metastasize) to other parts of the body, including the lungs, brain, and bones.

What are the symptoms?

Symptoms of ASPS depend on the tumor’s size and location. Many people notice a painless, slow-growing lump. Because ASPS can remain painless for a long time, it may become large before being discovered.

Symptoms may include:

  • A painless lump or swelling in the arm, leg, trunk, or head and neck.

  • Pain or discomfort if the tumor presses on nerves, muscles, or organs.

  • Headaches, nasal obstruction, or vision changes for head and neck tumors.

  • Fullness or abdominal discomfort for tumors in the abdomen or retroperitoneum.

  • Difficulty walking or neurological symptoms if the tumor is near the spine.

Symptoms usually appear only once the tumor becomes large enough to affect surrounding structures.

What causes alveolar soft part sarcoma?

Alveolar soft part sarcoma is caused by a specific genetic change that develops in the tumour cells during a person’s lifetime. This change is called a translocation, which means that two chromosomes break and reattach in the wrong places. In ASPS, part of chromosome X (Xp11) fuses with part of chromosome 17 (17q25), creating a new fusion gene called ASPSCR1–TFE3.

This fusion gene produces an abnormal protein that acts like a “permanent switch,” keeping specific growth pathways turned on when they should normally be off. As a result, the affected cells grow and divide more than they should, eventually forming a tumour. This genetic change is not inherited, does not run in families, and is not caused by anything a person did or did not do. It appears to occur randomly in cells throughout the body and is considered the key driver of alveolar soft part sarcoma.

How is this diagnosis made?

Imaging

Most people undergo imaging tests before a biopsy. These may include:

  • MRI to show the tumor’s location and how it interacts with muscles, nerves, or blood vessels.

  • CT scans to look for spread to the lungs or other organs.

  • Ultrasound for superficial or small soft-tissue masses.

ASPS often appears as a well-defined mass with strong contrast enhancement due to its rich blood supply. Imaging helps plan the biopsy and surgery.

Biopsy

A biopsy is required to confirm the diagnosis. A small sample of the tumor is removed using a needle or surgical technique. This specimen is examined by a pathologist, who evaluates the tumor’s appearance, cell type, and other features.

If the tumor is later removed surgically, the entire specimen is evaluated to determine margins, tumor extension, and treatment effect.

Microscopic features

Under the microscope, ASPS has a distinctive appearance.

Features include:

  • Tumor cells arranged in nests or clusters separated by thin fibrous walls.

  • Tumor cells may look large with clear, pink, or granular cytoplasm (the body of the cell).

  • Round nuclei with prominent nucleoli, which are the darker dots within the nucleus.

  • Low mitotic activity, meaning relatively few dividing cells, even though the tumor is high grade.

  • The “alveolar” pattern is a key clue to this diagnosis.

Immunohistochemistry (IHC)

Immunohistochemistry is a test that uses antibodies linked to coloured dyes to highlight specific proteins inside tumour cells. Pathologists use this technique to help determine the type of tumour and to distinguish alveolar soft part sarcoma from other cancers that may look similar under the microscope. In ASPS, tumour cells almost always show strong nuclear staining for TFE3, a protein that becomes overproduced when the ASPSCR1–TFE3 fusion gene is present. This characteristic staining pattern supports the diagnosis and guides the selection of molecular tests. Other markers may be used to rule out tumours with overlapping features, but TFE3 is the most important finding in this tumour.

Molecular tests

Molecular testing confirms the specific genetic change that defines ASPS.

A translocation is a genetic event where two chromosomes break and rejoin in the wrong places, forming a new fusion gene. In ASPS, the fusion gene ASPSCR1–TFE3 is the hallmark of the tumor.

Molecular tests include:

  • FISH (fluorescence in situ hybridization): Uses fluorescent probes that bind to DNA. FISH can detect whether the TFE3 gene is rearranged or fused.

  • Next-generation sequencing (NGS): A detailed DNA test that can identify the exact fusion gene and detect other genetic changes.

  • RT-PCR: A test that identifies the specific ASPSCR1–TFE3 fusion transcript.

Identifying the fusion gene confirms the diagnosis and helps rule out other tumors that express TFE3.

Tumour grade

All alveolar soft part sarcomas are considered high-grade tumours because they behave aggressively and have a significant risk of metastasis. Unlike many other soft tissue sarcomas, pathologists do not assign a grade using the French Federation of Cancer Centres Sarcoma Group (FNCLCC) grading system because ASPS already meets the criteria for the highest-grade category based on its biological behaviour, regardless of how it looks under the microscope. Even though mitotic activity may appear low, the tumor’s clinical behaviour places it firmly in the high-grade group.

Tumor size

Tumor size is important in staging and prognosis. Tumors 5 cm or smaller generally have a lower risk of spread. Larger tumors—especially those deep in the body—are more complex to remove completely and more likely to metastasize.

Tumour extension

Tumour extension describes how far the cancer has grown beyond where it first started. Although alveolar soft part sarcoma typically begins in soft tissues such as muscle or fat, it can expand outward into nearby structures, including connective tissue, deeper muscle layers, bones, or organs. When the tumour grows into these surrounding tissues, the pathologist will document exactly where tumour cells are found. This information is important because extension into adjacent structures increases the tumour stage (pT) and may influence treatment planning, particularly decisions about surgery and radiation therapy.

Perineural invasion

Perineural invasion (PNI) means tumor cells are growing along or into a nerve. Nerves act as communication pathways, sending signals between the body and brain.

Tumor spread along nerves increases the risk of recurrence because it provides a route for cancer to travel beyond the original tumor site.

Lymphovascular invasion

Lymphovascular invasion (LVI) means that tumour cells have entered a blood vessel or a lymphatic channel. Blood vessels carry blood throughout the body, while lymphatic channels carry lymph, a fluid that drains into lymph nodes and participates in the immune response. When tumour cells are seen inside one of these vessels, it suggests that the cancer has gained access to pathways that allow it to spread to distant sites such as lymph nodes, lungs, bones, or other organs. Because alveolar soft part sarcoma has a strong tendency to metastasize, the presence of lymphovascular invasion provides important prognostic information and helps guide follow-up and treatment decisions.

Margins

A margin is the edge of the tissue removed during surgery. Margins are only assessed after the entire tumor has been surgically removed.

  • A negative margin means no cancer cells are present at the cut edge, suggesting the tumor was entirely removed.

  • A positive margin means cancer cells reach the cut surface, raising concern that some tumor remains.

Your report may also state how close tumor cells came to the nearest margin. Margin status guides decisions about radiation therapy or further surgery.

Margin

Lymph nodes

Lymph nodes are small immune system organs scattered throughout the body. They act as filters that trap infections, foreign material, and sometimes cancer cells.

ASPS can spread to lymph nodes, although lung and bone metastases are more common. If lymph nodes are removed during surgery, the pathologist will evaluate them for:

  • The presence or absence of tumor cells

  • The number of lymph nodes involved

  • The size of the largest tumor deposit

  • Whether tumor cells have broken through the lymph node capsule (extranodal extension)

Finding tumor cells in lymph nodes increases the pN stage and may influence treatment decisions.

Pathologic stage (pTNM)

Staging describes how advanced the cancer is and helps guide treatment and predict prognosis. The TNM system evaluates:

  • T (tumor): how large the tumor is and whether it has grown into nearby tissues

  • N (nodes): whether lymph nodes contain tumor cells

  • M (metastasis): whether cancer has spread to distant organs

Your pathologist assigns a number to each category. Higher numbers indicate more advanced disease.

Tumor stage (pT)

The T stage for ASPS varies by tumor location.

Head and neck

  • T1: Tumor ≤ 2 cm

  • T2: 2–4 cm

  • T3: > 4 cm

  • T4: Tumor grows into bones, brain, major vessels, or surrounding tissues

Trunk and extremities

  • T1: Tumor ≤ 5 cm

  • T2: 5–10 cm

  • T3: 10–15 cm

  • T4: > 15 cm

Thoracic visceral organs

  • T1: Tumor confined to one organ

  • T2: Tumor grows into the surrounding connective tissue

  • T3: Tumor grows into at least one additional organ

  • T4: Multiple tumors

Retroperitoneum

  • T1: ≤ 5 cm

  • T2: 5–10 cm

  • T3: 10–15 cm

  • T4: > 15 cm

Orbit

  • T1: ≤ 2 cm

  • T2: > 2 cm but no bone invasion

  • T3: Tumor grows into the orbital or skull bones

  • T4: Tumor grows into the globe or surrounding tissues

Nodal stage (pN)

  • N0: No tumor in lymph nodes

  • N1: Tumor present in one or more lymph nodes

  • NX: No lymph nodes were submitted for evaluation

Treatment effect

If you received chemotherapy and/or radiation therapy before surgery, your pathologist will evaluate how much of the tumor remains alive (viable) versus dead (non-viable). A tumor that is 90% or more non-viable is considered to have had an excellent response to therapy.

What happens after the diagnosis?

Treatment for ASPS typically involves surgery, but because this tumor frequently spreads, systemic therapy is also important. Unlike many other sarcomas, ASPS does not respond well to standard chemotherapy. Targeted therapies and immunotherapies may be used, depending on your clinical situation and whether the tumor has metastasized. Radiation therapy may be recommended when surgery cannot achieve negative margins or when the tumor is located in a sensitive area.

Regular follow-up with imaging is essential because ASPS can recur or spread months or years after the initial treatment. The follow-up plan often includes CT scans, MRI, and clinical examinations.

Questions to ask your doctor

  • Was the tumor completely removed, and were the margins negative?

  • Did molecular testing confirm the ASPSCR1–TFE3 fusion?

  • Has the tumor spread to lymph nodes, lungs, bones, or other organs?

  • What treatment options are recommended for me—surgery, radiation, targeted therapy, or immunotherapy?

  • How often will I need imaging to monitor for recurrence or metastasis?

  • What symptoms should I watch for?

  • Are there clinical trials available for this type of tumor?

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