Spindle Cell/Sclerosing Rhabdomyosarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 17, 2026

Spindle cell/sclerosing rhabdomyosarcoma is an uncommon type of cancer that develops from cells that would normally become skeletal muscle. Skeletal muscle is the muscle that moves the arms, legs, and other parts of the body. It is a type of sarcoma. It is one of four main types of rhabdomyosarcoma, along with embryonal, alveolar, and pleomorphic rhabdomyosarcoma.

The name describes how the tumor looks under the microscope. “Spindle cell” means the tumor is made of long, thin cells arranged in bundles. “Sclerosing” means the tumor cells sit within dense, scar-like tissue. Many tumors show both patterns, which is why the World Health Organization (WHO) Classification of Tumors, 5th edition (2020), groups them. Your pathology report may use a shorter or more specific name, such as spindle cell rhabdomyosarcoma or MYOD1-mutant spindle cell/sclerosing rhabdomyosarcoma. Other reports name the group directly, as in congenital/infantile spindle cell rhabdomyosarcoma or intraosseous spindle cell rhabdomyosarcoma.

Spindle cell/sclerosing rhabdomyosarcoma can occur at any age, from newborns to older adults. It is not a single disease. It includes several groups defined by the genetic change inside the tumor cells, and these groups behave very differently. For this reason, the molecular test result is one of the most important findings in the report.

This article explains how spindle cell/sclerosing rhabdomyosarcoma is diagnosed and what the molecular result, margins, lymph nodes, and stage in your pathology report mean.

What causes spindle cell/sclerosing rhabdomyosarcoma?

Spindle cell/sclerosing rhabdomyosarcoma develops when immature muscle cells acquire a genetic change that keeps them dividing instead of maturing. The type of change usually falls into one of these groups:

A gene fusion in infants. Tumors in babies, including some present at birth, usually have a fusion involving genes called VGLL2, NCOA2, or CITED2. A fusion is an abnormal joining of two genes.
A MYOD1 mutation. Tumors in older children, adolescents, and adults often have a specific change in the MYOD1 gene, written as p.L122R. MYOD1 normally helps immature muscle cells mature and stop dividing. Some of these tumors also have a change in a gene called PIK3CA.
A fusion in bone. A rare group starts inside a bone, most often in the skull or face. The tumor cells in this group are often round rather than long and thin, a pattern pathologists call epithelioid. Most of these tumors have a fusion involving a gene called TFCP2, and a few have a fusion between genes called MEIS1 and NCOA2.
No identified change. Some tumors have none of these changes—many of these start in the abdomen or beside the testicle.

These genetic changes develop in the tumor cells during a person’s lifetime. They are not inherited and cannot be passed on to children.

What are the symptoms of spindle cell/sclerosing rhabdomyosarcoma?

The symptoms of spindle cell/sclerosing rhabdomyosarcoma depend on where the tumor starts:

Head and neck. A lump, facial swelling, a blocked nose, trouble swallowing, or changes in the voice.
Arms, legs, or trunk. A growing lump, swelling, pain, or difficulty moving the affected area.
Beside the testicle. Painless swelling in the scrotum.
Bones of the skull or face. Swelling, pain, or changes in the shape of the face.

How is the diagnosis made?

The diagnosis of spindle cell/sclerosing rhabdomyosarcoma is made after a pathologist examines a sample of the tumor under the microscope. The sample is usually obtained by a biopsy, using a needle or a small incision. In some locations, the whole tumor is removed at the first operation.

Under the microscope, the spindle cell pattern shows long, thin cells arranged in bundles that cross each other. The sclerosing pattern shows smaller, rounder cells arranged in cords, nests, or small clusters within dense, scar-like tissue. Tumors that start inside a bone may instead be made of round, epithelioid cells. Because these patterns can resemble alveolar rhabdomyosarcoma and other tumors, special tests are needed to confirm the diagnosis.

Molecular testing identifies the genetic change in the tumor. Next-generation sequencing (NGS) is often used because it can find both the MYOD1 mutation and the gene fusions in a single test. Every rhabdomyosarcoma is also tested for a FOXO1 fusion, and spindle cell/sclerosing rhabdomyosarcoma is expected to be negative. A FOXO1 fusion would point to alveolar rhabdomyosarcoma instead.

Once spindle cell/sclerosing rhabdomyosarcoma is confirmed, imaging tests show how far the tumor has spread. These usually include MRI or CT of the tumor site, a CT scan of the chest, and often a PET scan. The next section describes the immunohistochemistry tests used to confirm the diagnosis.

Immunohistochemistry

Immunohistochemistry uses antibodies to show which proteins tumor cells make. For spindle cell/sclerosing rhabdomyosarcoma, it confirms that the tumor cells are developing skeletal muscle cells. It also helps separate this tumor from other rhabdomyosarcomas and other spindle cell tumors. Your report may include some of the following:

Desmin. This muscle protein is positive in spindle cell/sclerosing rhabdomyosarcoma.
MyoD1. This protein is found only in developing skeletal muscle cells. It is usually positive, and tumors with a MYOD1 mutation typically show strong staining in nearly all of the tumor cells.
Myogenin. This protein is also found in developing skeletal muscle. It is usually positive in only scattered tumor cells. Strong staining in nearly every cell would point toward alveolar rhabdomyosarcoma instead.
ALK. ALK may be positive in tumors that start inside a bone and have a TFCP2 fusion.
S100 and SOX10. These proteins are negative. A positive result would suggest a nerve sheath tumor or melanoma instead.

Not every case of spindle cell/sclerosing rhabdomyosarcoma needs every stain. The pathologist chooses tests based on how the tumor looks, where it started, and the patient’s age.

Histologic grade

Pathologists do not assign an FNCLCC grade to spindle cell/sclerosing rhabdomyosarcoma. All types of rhabdomyosarcoma are treated as high-grade cancers, so your report will usually not include a grade. A report without a grade is not incomplete.

For this tumor, the molecular result gives far more information about expected behavior than a grade would. For example, tumors with a fusion in infants usually behave favorably, while tumors with a MYOD1 mutation do not.

Tumor size

Tumor size is the greatest dimension of the spindle cell/sclerosing rhabdomyosarcoma, measured in centimeters (cm). The size may come from imaging or from the tumor removed at surgery. Tumors larger than 5 cm are associated with a less favorable outcome than smaller tumors, and size is used to assign stage.

Tumor extension

Tumor extension describes whether spindle cell/sclerosing rhabdomyosarcoma has grown into nearby structures such as muscle, bone, nerves, blood vessels, or organs. After removing the tumor, the pathologist examines the surrounding tissue and reports which structures contain tumor cells. This information helps the team plan surgery and radiation therapy.

Treatment effect

Many people with spindle cell/sclerosing rhabdomyosarcoma receive chemotherapy, and sometimes radiation therapy, before surgery. When the tumor is removed after this treatment, the pathologist examines it to see whether living tumor cells remain.

For rhabdomyosarcoma, no specific percentage of dead tumor is used to guide treatment. Your team judges the response to treatment mainly with imaging and interprets the pathology findings alongside those results.

Lymphovascular invasion

Lymphovascular invasion means that cells from the spindle cell/sclerosing rhabdomyosarcoma are seen inside a small blood vessel or lymphatic channel. These vessels give cancer cells a route to lymph nodes and other parts of the body.

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. A pathologist may mention it when it is seen in spindle cell/sclerosing rhabdomyosarcoma. 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

Margins are the edges of tissue cut by the surgeon to remove a spindle cell/sclerosing rhabdomyosarcoma. The pathologist inks these edges and examines them under the microscope.

Negative margin. No tumor cells are seen at the inked edge. The report may also give the distance between the tumor and the closest margin.
Positive margin. Tumor cells reach the inked edge, and some tumor may have been left behind. Further surgery or radiation therapy may be considered.

Margins are only reported when the tumor was removed rather than sampled. For many rhabdomyosarcomas, complete removal at the first operation is not possible without harming an important structure. In those situations, a biopsy alone is the correct first step, and chemotherapy and radiation are used to control the tumor.

Lymph nodes

Lymph nodes are small immune organs found throughout the body, and rhabdomyosarcoma can spread to the nodes near where it started. When doctors remove nodes, the pathology report lists how many were examined and how many contain tumor cells.

For rhabdomyosarcoma of the arms and legs, and for tumors beside the testicle in boys aged 10 and older, nodes may be checked even when imaging looks normal. A sentinel lymph node biopsy, which removes only the first node or nodes the tumor would reach, is sometimes used. Finding a tumor in a lymph node raises both the stage and the risk group.

Biomarker and molecular testing

For spindle cell/sclerosing rhabdomyosarcoma, the molecular test result does more than confirm the diagnosis. It identifies the tumor group, one of the strongest predictors of how the tumor will behave. Your report may describe one of the following results:

VGLL2, NCOA2, or CITED2 fusion. These fusions are found mainly in infants and are a favorable finding. In published series, infants with these fusions who had long-term follow-up were alive, and none developed distant spread.
MYOD1 p.L122R mutation. This mutation is an unfavorable finding at any age, including in children. In one study of 30 people, only 18% were alive four years after diagnosis. Tumors that also have a PIK3CA mutation appear to behave even less favorably.
TFCP2 or MEIS1::NCOA2 fusion. These fusions are found in the rare group of tumors that start inside a bone, often called intraosseous spindle cell rhabdomyosarcoma. Reported cases have often behaved unfavorably.
No FOXO1 fusion. This result is expected for spindle cell/sclerosing rhabdomyosarcoma. The treating team uses it to assign the risk group.
No recurrent change identified. Some tumors have none of the changes above. The expected behavior of this group is less well defined.

No drug is currently approved to target the MYOD1 mutation or these fusions. The results may help identify clinical trials, especially for people with a MYOD1 mutation. You can learn more about tumor testing in our Biomarkers and Genetic Testing section.

Stage, clinical group, and risk group

Rhabdomyosarcoma in children, including spindle cell/sclerosing rhabdomyosarcoma, is described by three separate systems. All three may appear in the medical record.

Stage ranges from 1 to 4. This is based on where the tumor started, its size, whether lymph nodes are involved, and whether it has spread to distant sites. Sites are divided into favorable and unfavorable. Favorable sites include the area around the eye, the head and neck away from the lining of the brain, and the biliary tract. They also include the urinary and reproductive organs, except the bladder and prostate.
Clinical group, from I to IV. This is based on how much tumor was removed at the first operation. Group I means the tumor was completely removed. Group II means small amounts remained or a lymph node was involved. Group III means visible tumor remained, which includes tumors that were only biopsied. Group IV means the tumor had already spread to distant sites.
Risk group: low, intermediate, or high. This combines stage, clinical group, FOXO1 fusion status, and age and site. The risk group determines treatment intensity.

The pathology report provides several of these ingredients but usually does not state the risk group itself. The treating team assigns it once the imaging, surgery, and molecular results are available.

In adults, the pathology report may instead include a pathologic stage (pTNM) from the American Joint Committee on Cancer (AJCC), 8th edition. For tumors of the arms, legs, and trunk, the tumor stage (pT) is based on size. Other body sites use different rules.

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.

The nodal stage for adults is pN0 if no tumor cells were found in the lymph nodes examined and pN1 if tumor cells were found. If no lymph nodes were removed, the report will usually say that pN was not assigned.

What is the prognosis?

The outlook for a person with spindle cell/sclerosing rhabdomyosarcoma depends mainly on the molecular group of the tumor and on whether it has spread. The groups differ so much that a single survival figure for this diagnosis would be misleading.

Infants whose tumors have a VGLL2, NCOA2, or CITED2 fusion usually do very well. Tumors with a MYOD1 mutation have a much less favorable outlook, with only about 18 in 100 people alive four years after diagnosis in the largest study. For comparison, across all types of rhabdomyosarcoma in children, roughly 84% of those whose tumor had not spread were alive at five years.

Other features associated with a less favorable outcome of spindle cell/sclerosing rhabdomyosarcoma include:

Spread to distant sites at diagnosis. This is a strong predictor of a less favorable outcome in every molecular group.
An unfavorable primary site. This includes the arms and legs, the trunk, and sites next to the lining of the brain.
Larger tumor size. Tumors larger than 5 cm do less well than smaller tumors.
Lymph node involvement. Tumor found in nearby nodes is associated with a less favorable outcome.
Tumor remaining after surgery. A higher clinical group is associated with a less favorable outcome.
Age. Adults with rhabdomyosarcoma generally do less well than children.

What happens after the diagnosis?

After a team confirms spindle cell/sclerosing rhabdomyosarcoma, they usually plan care with an oncologist, surgeon, radiation oncologist, and pathologist. The findings in the pathology report, especially the molecular result, the margins, and the lymph nodes, are combined with imaging to assign the risk group and plan treatment.

Chemotherapy. Most people with rhabdomyosarcoma receive chemotherapy because tumor cells may be present elsewhere in the body without being visible on scans. Children are usually treated with a protocol developed by a cooperative research group, and current protocols include spindle cell/sclerosing rhabdomyosarcoma and fusion-negative rhabdomyosarcomas.
Surgery and radiation therapy. The original tumor is controlled with surgery, radiation, or both. The choice depends on the site and on how much tumor remains after surgery.
Treatment for adults. Adults are often treated with chemotherapy regimens developed for children, but fewer studies are available to guide treatment in adults.
Clinical trials. Clinical trials should be discussed, especially for tumors with a MYOD1 mutation, because standard treatment often works less well for this group.

After treatment ends, follow-up with regular imaging continues for many years. For children, long-term follow-up also monitors chemotherapy and radiation effects on growth, fertility, and organ function.

Questions to ask your doctor

Was molecular testing done, and which genetic change was found in the tumor?
Does the tumor have a MYOD1 mutation or a gene fusion?
Was the tumor tested for a FOXO1 fusion, and was the result negative?
Is the primary site considered favorable or unfavorable?
What stage and clinical group have been assigned, and what do they mean?
Which risk group does this fall into?
Was the whole tumor removed, and were the margins negative?
Were any lymph nodes examined, and did any contain tumor?
Has a pathologist with experience in sarcoma or children’s tumors reviewed the slides?
Will treatment be given on a cooperative group protocol or clinical trial?
If the tumor has a MYOD1 mutation, which clinical trials are available?
What follow-up will be needed after treatment, and for how long?