Alveolar Rhabdomyosarcoma: Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC and Bibianna Purgina MD FRCPC
April 10, 2026

Alveolar rhabdomyosarcoma is an aggressive type of cancer that develops from immature skeletal muscle cells. It is classified as a sarcoma — a cancer arising from the body’s connective tissues. Although skeletal muscle is found throughout the body, this tumor can arise in locations with little or no actual muscle tissue. Alveolar rhabdomyosarcoma is most commonly found in the arms or legs, but it may also occur in the head and neck, spine, pelvis, or perineum. It is one of four main subtypes of rhabdomyosarcoma, and compared to the other subtypes, it grows quickly and has a higher risk of spreading to lymph nodes, lungs, and bones.

This article will help you understand the findings in your pathology report — what each term means and why it matters for your care. If you have been diagnosed with a different subtype of rhabdomyosarcoma, our article on embryonal rhabdomyosarcoma may also be helpful.

What causes alveolar rhabdomyosarcoma?

Most cases are caused by a specific genetic change called a fusion, in which two normally separate genes break apart and rejoin incorrectly. In alveolar rhabdomyosarcoma, the FOXO1 gene fuses with either PAX3 or PAX7, creating an abnormal fusion gene called PAX3–FOXO1 or PAX7–FOXO1. This fusion gene produces an abnormal protein that activates growth signals and blocks normal cell death, allowing tumor cells to multiply rapidly and without control.

A small number of alveolar rhabdomyosarcomas do not contain a detectable FOXO1 fusion — these are sometimes called fusion-negative cases and generally behave somewhat less aggressively. Doctors do not yet know what triggers these fusion events. They are not inherited and are not passed down through families.

What are the symptoms?

The symptoms of alveolar rhabdomyosarcoma depend on the tumor’s size and location. People usually notice symptoms only when the tumor becomes large enough to push on surrounding tissues or organs. Because this tumor can grow quickly, symptoms may worsen over weeks to months.

Common symptoms by location include:

• Arms or legs — A rapidly growing mass, swelling, pain, or difficulty moving the affected limb.
• Head and neck — Facial pain or swelling, nasal obstruction, nosebleeds, ear fullness, changes in vision, or headaches.
• Near the spine — Numbness, weakness, or pain in the legs, or difficulty walking.
• Pelvis or perineum — Problems with urination or bowel movements, pelvic pain, or a mass felt in the lower abdomen or groin.

How is the diagnosis made?

Imaging — typically MRI, CT scan, or both — is usually the first step. These tests identify the tumor’s size and location, determine whether it involves muscles, nerves, or bones, and assess whether the cancer has already spread to lymph nodes or distant organs such as the lungs or bone. MRI is particularly valuable for soft tissue detail and helps guide biopsy planning.

The diagnosis is then confirmed after a biopsy — a small sample of the tumor removed with a needle or through a small surgical incision — is examined under the microscope by a pathologist. After a biopsy confirms the diagnosis, treatment typically begins with chemotherapy and sometimes radiation before any attempt at surgical removal. The entire tumor is then removed surgically and sent for detailed pathologic examination.

Under the microscope, alveolar rhabdomyosarcoma is made up of small, round cells with dark blue nuclei — a pattern pathologists describe as a small round blue cell tumor. The tumor cells are arranged in nests separated by thin fibrous bands, and cells in the center of these nests often appear loosely attached or floating free from one another. This architecture resembles the small air spaces (alveoli) in the lung, which is how the tumor got its name. The tumor cells divide frequently, reflecting rapid growth, and they do not stick together well — a property described as discohesive. These features help distinguish alveolar rhabdomyosarcoma from other small round blue cell tumors that can look similar, such as Ewing sarcoma, neuroblastoma, and lymphoma.

To confirm the diagnosis, the pathologist uses immunohistochemistry (IHC) — a test that uses antibodies linked to colored dyes to detect specific proteins inside the tumor cells. Alveolar rhabdomyosarcoma characteristically shows strong positivity for muscle markers: desmin, myogenin, and MyoD1. These proteins confirm that the tumor cells have a skeletal muscle identity, supporting the diagnosis of rhabdomyosarcoma. Myogenin and MyoD1 in particular tend to show strong, diffuse (widespread) staining in alveolar rhabdomyosarcoma, which also helps distinguish it from embryonal rhabdomyosarcoma, where the staining is typically patchier.

Molecular testing is then performed to detect the defining FOXO1 gene rearrangement. This can be done using fluorescence in situ hybridization (FISH), which uses fluorescent probes to detect breaks or fusions involving the FOXO1, PAX3, or PAX7 genes, or next-generation sequencing (NGS), which reads the tumor’s DNA to identify the exact fusion gene present. Your pathology report will state which test was performed and whether a PAX3–FOXO1 fusion, a PAX7–FOXO1 fusion, or no fusion was detected. Identifying the specific fusion type matters for prognosis and treatment planning (see Biomarker and molecular testing below).

Histologic grade

Pathologists do not assign an FNCLCC grade to alveolar rhabdomyosarcoma. The FNCLCC system — the standard grading system for soft tissue sarcomas — is not applied here because alveolar rhabdomyosarcoma is already considered a high-grade cancer by definition. Its rapid cell division, high rate of spread, and characteristic molecular features all reflect high-grade behavior regardless of the individual scoring components used in the FNCLCC system. Your pathology report will therefore not include a numeric grade, and this is expected and appropriate for this diagnosis.

Tumor size

Tumor size is measured at the greatest dimension in centimeters. Smaller tumors — particularly those under 5 cm — are associated with a better prognosis and are less likely to have spread to other parts of the body at the time of diagnosis. Tumor size is also used to determine the pathologic tumor stage (pT). The size recorded in the report is measured from the surgically removed specimen, not from a biopsy.

Tumor extension

Alveolar rhabdomyosarcoma typically arises within a muscle but can invade surrounding structures as it enlarges. This spread beyond the original site is called tumor extension. The pathologist carefully examines all tissue submitted with the resection specimen to determine whether tumor cells have grown into adjacent muscles, bones, nerves, blood vessels, or organs. Extension into surrounding structures increases the pathologic tumor stage (pT) and may require additional surgery, radiation therapy, or both to improve local control.

Treatment effect

Because most patients with alveolar rhabdomyosarcoma receive chemotherapy and/or radiation therapy before surgery, the pathologist evaluates the surgical specimen to determine how well the tumor responded to that pre-operative treatment. This assessment is called the treatment effect.

The pathologist estimates the percentage of the tumor that is non-viable (dead) versus viable (still alive). A tumor that is 90% or more non-viable indicates an excellent response to pre-operative therapy and is associated with a better outcome. When a large proportion of viable tumor remains, additional treatment after surgery may be discussed. Your report will describe the estimated percentage of viable and non-viable tumors.

Lymphovascular invasion

Lymphovascular invasion means that tumor cells are present inside blood vessels or lymphatic channels within or around the tumor. Lymphatic channels connect to nearby lymph nodes, and blood vessels circulate throughout the body, so tumor cells that enter either pathway can travel to distant sites. Alveolar rhabdomyosarcoma frequently spreads through these pathways, and the presence of lymphovascular invasion in the pathology report is an adverse feature that increases the concern for regional or distant spread. Your report will state whether lymphovascular invasion was identified.

Perineural invasion

Perineural invasion means that tumor cells are growing along or around a nerve. Nerves run throughout the soft tissues, and tumor cells that reach them can use the nerve as a pathway to extend into surrounding tissues beyond the main tumor mass. This increases the risk that the tumor will grow back at the same site after treatment. Your pathology report will state whether perineural invasion was identified.

Surgical margins

A margin is the edge of the tissue removed during surgery. Margins are evaluated only after a surgery that removes the entire tumor — not after a biopsy, which takes only a small sample. The pathologist examines all cut surfaces of the specimen to determine whether tumor cells are present at the edges.

• Negative margin — No tumor cells are present at the cut edge. This suggests the tumor was completely removed in that area. The distance from the nearest tumor cells to the margin may also be recorded, as a wider clear margin is associated with a lower risk of local tumor recurrence.
• Positive margin — Tumor cells are present at the cut edge, raising concern that some cancer remains in the body. A positive margin increases the risk of local recurrence and may lead to a recommendation for additional surgery or radiation therapy.

Lymph nodes

Lymph nodes are small immune organs found throughout the body. Cancer cells can spread from the tumor to nearby lymph nodes through lymphatic channels — a process called metastasis. Alveolar rhabdomyosarcoma has a higher tendency to spread to regional lymph nodes than most other soft tissue sarcomas, which is why sentinel lymph node biopsy or regional node dissection is often performed as part of surgical staging.

Your pathology report will state the total number of lymph nodes examined and how many, if any, contain tumor cells. The report may also describe the size of tumor deposits within nodes and whether cancer cells have broken through the outer wall of a lymph node into the surrounding tissue — a finding called extranodal extension. Lymph node involvement increases the pathologic nodal stage (pN1) and is an important factor in determining the intensity of treatment required.

Biomarker and molecular testing

In alveolar rhabdomyosarcoma, the molecular test results — specifically the type of FOXO1 fusion gene detected — provide prognostically important information beyond simply confirming the diagnosis. This makes the fusion type a true biomarker that influences treatment planning.

FOXO1 fusion type (PAX3–FOXO1 vs. PAX7–FOXO1)

Almost all alveolar rhabdomyosarcomas contain a translocation involving the FOXO1 gene, fusing it with either PAX3 (on chromosome 2) or PAX7 (on chromosome 1). The specific fusion type has meaningful prognostic implications:

• PAX3–FOXO1 — The more common fusion, found in approximately 55–70% of cases. Tumors with this fusion tend to be more aggressive, with a higher rate of distant spread and a worse overall prognosis.
• PAX7–FOXO1 — Found in approximately 20% of cases. Tumors with this fusion generally have a somewhat better prognosis than PAX3–FOXO1 tumors, though they are still high-grade cancers.
• Fusion-negative — A small proportion of tumors that look like alveolar rhabdomyosarcoma under the microscope do not contain a detectable FOXO1 fusion. These tumors tend to behave more like embryonal rhabdomyosarcoma and generally carry a better prognosis. Your oncologist will take the fusion status into account when planning treatment and estimating the risk of recurrence.

Testing is performed using FISH or next-generation sequencing (NGS). Your report will state the test used and the specific result. For more information about molecular testing in cancer, visit our Biomarkers and Molecular Testing section.

Pathologic stage (pTNM)

The pathologic stage describes how far the cancer has spread, based on examination of the surgical specimen. It uses the internationally recognized TNM staging system, which considers the primary tumor (T), lymph node involvement (N), and distant metastasis (M). Metastasis to distant organs is typically determined by imaging rather than by pathology. In general, higher numbers indicate more advanced disease.

Note that children with rhabdomyosarcoma are also assigned a clinical grouping using the Intergroup Rhabdomyosarcoma Study Group (IRSG) system, which factors in tumor location, the extent of surgical removal, and whether lymph nodes or distant sites are involved. This clinical grouping guides the intensity of chemotherapy and radiation treatment. Your oncologist will explain which system applies to your situation and what the result means.

Tumor stage (pT)

The pT stage is based on tumor size and varies by body site. The most common locations are shown below.

Trunk and extremities (arms, legs, chest wall, abdomen):

• pT1 — Tumor is 5 cm or smaller.
• pT2 — Tumor is more than 5 cm but 10 cm or smaller.
• pT3 — Tumor is more than 10 cm but 15 cm or smaller.
• pT4 — Tumor is more than 15 cm.

Retroperitoneum (deep abdominal cavity behind the organs):

• pT1 — Tumor is 5 cm or smaller.
• pT2 — Tumor is more than 5 cm but 10 cm or smaller.
• pT3 — Tumor is more than 10 cm but 15 cm or smaller.
• pT4 — Tumor is more than 15 cm.

Thoracic visceral organs (organs within the chest):

• pT1 — Tumor is confined to one organ.
• pT2 — Tumor has grown into the connective tissue surrounding that organ.
• pT3 — Tumor has grown into at least one adjacent organ.
• pT4 — Multiple tumors are present.

If no viable tumor remains in the resection specimen after pre-operative treatment, the stage is recorded as pT0. If the specimen cannot be reliably assessed (for example, because it arrived as multiple fragments), it may be listed as pTX.

Nodal stage (pN)

• pN0 — No cancer found in any lymph nodes examined.
• pN1 — Cancer found in one or more regional lymph nodes.
• pNX — Lymph nodes were not assessed.

What is the prognosis for alveolar rhabdomyosarcoma?

Alveolar rhabdomyosarcoma is among the more aggressive soft tissue sarcomas. Prognosis depends on several factors that work together, and your oncologist will consider all of them when discussing your or your child’s situation.

The most important prognostic factors include:

• Stage at diagnosis — Localized disease (confined to the primary site with negative margins and no lymph node or distant spread) is associated with the best outcomes. Metastatic disease, particularly spread to bone marrow or multiple sites, carries a significantly worse prognosis. Overall five-year survival rates are approximately 50–70% for localized disease and under 30% for metastatic disease.
• Fusion type — PAX3–FOXO1-positive tumors carry a worse prognosis than PAX7–FOXO1-positive or fusion-negative tumors. Fusion-negative alveolar rhabdomyosarcoma behaves more like embryonal rhabdomyosarcoma and generally has a better outlook.
• Surgical margins — Complete removal with negative margins substantially reduces the risk of local recurrence and is associated with better survival.
• Response to neoadjuvant therapy — A strong pathologic response (90% or more tumor necrosis after pre-operative chemotherapy) is a favorable prognostic sign.
• Lymph node involvement — Spread to regional lymph nodes indicates more advanced disease and worsens the prognosis.
• Age — Outcomes in children, particularly those aged 1 to 9 years, tend to be more favorable than in adolescents or adults with the same diagnosis.
• Tumor size and location — Smaller tumors at favorable sites (such as the orbit or genitourinary tract) carry a better prognosis than large tumors at unfavorable sites (such as the parameningeal region or extremities).

What happens after the diagnosis?

Treatment for alveolar rhabdomyosarcoma requires a multidisciplinary team that includes a pediatric or adult oncologist, a radiation oncologist, and a surgeon experienced in sarcoma. The approach is guided by the patient’s age, tumor stage, tumor location, and FOXO1 fusion type.

For nearly all patients, treatment begins with multi-agent chemotherapy — this is essential regardless of stage and typically uses combinations of vincristine, actinomycin D, and cyclophosphamide (VAC), sometimes with additional agents for higher-risk disease. Chemotherapy reduces the size of the tumor before surgery, treats any microscopic spread, and is continued after local treatment.

Radiation therapy is added to control the tumor locally, particularly when complete surgical removal is not possible or when margins are positive or close. For some tumor locations — such as the orbit or parameningeal sites — radiation is a cornerstone of local therapy even when chemotherapy has produced a good response.

Surgery is performed to remove the tumor with negative margins whenever technically feasible. The timing of surgery (before or after chemotherapy) depends on the tumor’s location and resectability. For extremity tumors, limb-sparing resection is the goal whenever possible.

Because alveolar rhabdomyosarcoma can spread to lymph nodes, sentinel lymph node biopsy or regional lymph node dissection is often performed — particularly for extremity and trunk primaries — to determine the nodal stage and accurately guide treatment intensity.

After treatment is complete, regular follow-up imaging of the primary site, lungs, lymph nodes, and bones is required at defined intervals for several years. Given the risk of late recurrence, surveillance is maintained for at least five years and sometimes longer. Enrollment in a clinical trial or cooperative group study should be considered when available, as ongoing research continues to refine treatment for this disease.

Questions to ask your doctor

Your pathology report contains important information that will guide your care. The following questions may help you prepare for your next appointment.

• Which FOXO1 fusion gene was found — PAX3–FOXO1, PAX7–FOXO1, or was no fusion detected — and what does this mean for prognosis and treatment?
• What is the pathologic stage, and has the cancer spread to lymph nodes or other organs?
• How large is the tumor, and did it extend into any surrounding structures?
• Were the surgical margins negative? If positive, what treatment is recommended to address this?
• Were lymph nodes removed and examined, and were any positive for cancer?
• Was lymphovascular or perineural invasion present in the specimen?
• If pre-operative treatment was given, what percentage of the tumor was dead, and what does this mean for further treatment?
• What chemotherapy regimen is recommended, and for how long?
• Is radiation therapy part of the treatment plan, and which site will be treated?
• What follow-up imaging schedule is recommended, and which parts of the body will be monitored?
• Are there clinical trials available for this diagnosis?
• Are there any long-term side effects of treatment I should be aware of?