Spindle Cell/Sclerosing Rhabdomyosarcoma: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 11, 2026

Spindle cell/sclerosing rhabdomyosarcoma is a rare type of cancer that starts in skeletal muscle cells — the muscle responsible for voluntary movements such as walking and lifting. It is classified as a sarcoma and is one of four main subtypes of rhabdomyosarcoma. It can affect people of all ages, but its behavior, underlying genetic changes, and prognosis differ substantially between infants and young children on the one hand and adolescents and adults on the other. The name comes from the tumor’s appearance under the microscope: the spindle cell variant contains long, narrow cells growing in bundles, while the sclerosing variant features tumor cells embedded in abundant dense, fibrous (sclerotic) tissue.

This article will help you understand the findings in your pathology report — what each term means and why it matters for your care. Our articles on embryonal rhabdomyosarcoma and alveolar rhabdomyosarcoma describe the other subtypes of this disease.

What causes spindle cell/sclerosing rhabdomyosarcoma?

The genetic changes underlying spindle cell/sclerosing rhabdomyosarcoma vary with age, which is why this tumor is considered two biologically distinct diseases that share a similar microscopic appearance.

In infants and young children, the tumor is typically driven by a gene fusion — an abnormal joining of two genes that creates a new protein capable of driving uncontrolled cell growth. Known fusions in this group include SRF–NCOA2, TEAD1–NCOA2, VGLL2–NCOA2, and VGLL2–CITED2. These tumors most often arise in the head and neck or genitourinary tract and generally have a favorable prognosis when detected early.

In adolescents and adults, most spindle cell/sclerosing rhabdomyosarcomas carry a mutation in the MYOD1 gene — specifically the change called p.Leu122Arg. MYOD1 is a protein that normally helps immature muscle cells mature and stop dividing. When it is mutated, this control is lost, and cancer cells proliferate. MYOD1-mutant tumors are more aggressive and carry a substantially worse prognosis than the fusion-driven tumors of infancy. A small number of cases across all age groups do not show any identifiable recurrent genetic change.

None of these genetic changes are inherited. They arise during a person’s lifetime and are not passed down through families. In most cases, no specific environmental trigger is identified.

What are the symptoms?

Symptoms depend on where the tumor is located, and because spindle cell/sclerosing rhabdomyosarcoma can arise almost anywhere in the body, the presentation varies widely. The tumor tends to grow steadily, and symptoms may worsen over weeks to months.

• Arms, legs, or trunk — A growing lump or mass, swelling, pain, or reduced range of motion in the affected area.
• Head and neck — A visible or palpable mass, nasal obstruction, facial swelling, swallowing difficulties, or voice changes.
• Genitourinary tract (in young children) — Difficulty urinating, blood in the urine, or a palpable abdominal mass.
• Chest or abdomen — Pressure symptoms such as difficulty breathing, early fullness, or abdominal discomfort as the tumor enlarges.

How is the diagnosis made?

The diagnosis is made after a tissue sample is examined under the microscope by a pathologist. The first sample is usually obtained through a biopsy — a small piece of the tumor removed with a needle or through a small incision. After the biopsy confirms rhabdomyosarcoma, treatment typically begins with chemotherapy and sometimes radiation therapy before surgery. The entire tumor is then removed surgically and sent to pathology for complete evaluation.

Under the microscope, two main patterns are recognized. The spindle cell variant is made up of long, narrow spindle cells arranged in intersecting bundles (fascicles). The cells have pale pink cytoplasm and oval or elongated nuclei. This growth pattern can resemble that of other spindle cell tumors, such as leiomyosarcoma, making additional testing essential. Mitotic figures (cells in the act of dividing) and nuclear atypia (abnormal nuclear size and shape) are common. More primitive-looking areas containing rhabdomyoblasts — immature cells with a muscle identity — may also be seen. The sclerosing variant shows tumor cells arranged in cords, nests, or microalveolar clusters set within abundant dense fibrous (sclerotic) tissue, creating a hardened, scar-like background.

To confirm the muscle origin of the tumor cells, the pathologist uses immunohistochemistry (IHC) — a test that uses antibodies to detect specific proteins inside cells. Spindle cell/sclerosing rhabdomyosarcoma characteristically shows positivity for muscle markers, including desmin, MyoD1, and myogenin. Importantly, in the MYOD1-mutant form, the MyoD1 staining is often very strong and diffuse, while myogenin expression tends to be limited to only a few cells. These IHC patterns, together with the microscopic appearance, support the diagnosis and help distinguish this tumor from other spindle cell sarcomas.

Molecular testing is then performed to identify the specific genetic change present. Next-generation sequencing (NGS) is the preferred test as it can detect both point mutations (such as the MYOD1 p.Leu122Arg mutation) and gene fusions in a single assay. Fluorescence in situ hybridization (FISH) may also be used to assess for specific gene fusions. Your pathology report will state which test was performed and whether a MYOD1 mutation, a gene fusion, or no recurrent alteration was detected. This result is critical both for confirming the diagnosis and for establishing prognosis (see Biomarker and molecular testing below). Once the diagnosis is confirmed, imaging — typically MRI of the primary site plus CT of the chest, abdomen, and pelvis — determines the full extent of disease.

Histologic grade

Pathologists do not assign an FNCLCC grade to spindle cell/sclerosing rhabdomyosarcoma. Like other rhabdomyosarcoma subtypes, this tumor is already considered a high-grade cancer by definition, and the FNCLCC scoring system is not applied. 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 its greatest dimension in centimeters. Tumors smaller than 5 cm are less likely to have spread to other parts of the body and are associated with a better prognosis. Tumor size is also used to assign the pathologic tumor stage (pT). The size is recorded from the surgically removed specimen, not from a biopsy.

Tumor extension

Spindle cell/sclerosing rhabdomyosarcoma typically arises within a muscle or soft-tissue compartment but can invade adjacent 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 surrounding 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 achieve local control.

Treatment effect

Many patients with spindle cell/sclerosing rhabdomyosarcoma receive chemotherapy and/or radiation therapy before surgery. After the tumor is removed, the pathologist evaluates the specimen to determine how well the tumor responded to the pre-operative treatment.

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 significant proportion of viable tumor remains, additional treatment after surgery may be considered. Your report will describe the estimated percentage of viable and non-viable tumor.

Lymphovascular invasion

Lymphovascular invasion means that tumor cells are present inside blood vessels or lymphatic channels within or around the tumor. These vessels provide a route for cancer cells to travel to nearby lymph nodes or to distant organs such as the lungs. Lymphovascular invasion is an adverse feature that increases the concern for spread beyond the primary site. Your pathology 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 tissue 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. 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 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. If lymph nodes were removed during surgery, the pathologist examines them under the microscope and records whether tumor cells are present.

Your report will state the total number of lymph nodes examined and whether any contain tumor cells. It may also note whether cancer cells have broken through the outer wall of a lymph node into the surrounding tissue — a finding called extranodal extension, which is associated with a higher risk of recurrence. Lymph node involvement determines the pathologic nodal stage (pN1) and influences treatment planning.

Biomarker and molecular testing

In spindle cell/sclerosing rhabdomyosarcoma, the molecular test result — specifically whether a MYOD1 mutation or a gene fusion is detected — provides prognostically important information that goes beyond simply confirming the diagnosis.

MYOD1 mutation

The MYOD1 p.Leu122Arg mutation is found in most spindle cell/sclerosing rhabdomyosarcomas in adolescents and adults. This mutation is identified by next-generation sequencing (NGS) and has important prognostic implications. MYOD1-mutant tumors are significantly more aggressive than the fusion-driven infantile form of the disease. Response to standard rhabdomyosarcoma chemotherapy regimens is often poor, and the risk of recurrence and disease-related death is substantially higher. The detection of a MYOD1 mutation may prompt your oncologist to consider enrollment in clinical trials investigating alternative treatment strategies.

Gene fusions (infantile/congenital form)

In infants and young children, gene fusions involving VGLL2, SRF, TEAD1, NCOA2, or CITED2 are detected by NGS or FISH. The presence of one of these fusions, rather than a MYOD1 mutation, is a favorable prognostic finding. These tumors generally respond well to multimodal therapy and are associated with better long-term outcomes, particularly when detected early and treated with surgery and chemotherapy.

Your pathology report will state the specific molecular 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.

Children with rhabdomyosarcoma are also assigned a clinical grouping using the Intergroup Rhabdomyosarcoma Study Group (IRSG) system, which factors in tumor location, extent of surgical removal, and whether lymph nodes or distant sites are involved. Your oncologist will explain which system applies to your situation.

Tumor stage (pT)

The pT stage depends on tumor size and the body site where the tumor started. The most common locations are listed below. Head and neck tumors in the adult TNM system are assessed separately by your clinical team.

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

• 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 or abdomen):

• 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.

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.

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, 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 spindle cell/sclerosing rhabdomyosarcoma?

Prognosis in spindle cell/sclerosing rhabdomyosarcoma is strongly influenced by age at diagnosis and the underlying molecular change, making this one of the most prognostically heterogeneous rhabdomyosarcoma subtypes.

Infantile/congenital cases (those driven by gene fusions involving VGLL2, SRF, TEAD1, NCOA2, or CITED2) generally have a favorable prognosis. When diagnosed early and treated with surgery and chemotherapy, long-term survival rates are high and are comparable to or better than embryonal rhabdomyosarcoma.

MYOD1-mutant cases in adolescents and adults carry a significantly worse prognosis. These tumors are more likely to be refractory to standard chemotherapy, have higher rates of distant metastasis, and are associated with substantially lower five-year survival compared to other rhabdomyosarcoma subtypes. The outcome for adults with MYOD1-mutant disease remains poor with currently available therapies, and enrollment in clinical trials is strongly encouraged when available.

Additional factors affecting prognosis across all cases include:

• Stage at diagnosis — Localized disease with complete surgical resection offers the best chance of cure. Metastatic disease is associated with poor outcomes across all molecular subtypes.
• Surgical margin status — Negative margins are the strongest predictor of local control. Positive margins significantly increase the risk of local recurrence.
• Tumor size and location — Smaller tumors at surgically accessible sites carry a better prognosis.
• Response to neoadjuvant therapy — A good pathologic response (90% or more tumor necrosis) after pre-operative treatment is associated with better outcomes.

What happens after the diagnosis?

Treatment is planned by a multidisciplinary team comprising an oncologist, surgeon, and radiation oncologist with sarcoma experience. The approach depends on the patient’s age, tumor site, stage, and the specific molecular finding.

For infantile cases with favorable gene fusions, treatment typically combines surgery to remove the tumor with negative margins, chemotherapy using a multi-agent regimen (commonly including vincristine, actinomycin D, and cyclophosphamide — VAC), and radiation therapy when complete surgical removal is not possible or margins are positive. Outcomes in this group are generally favorable with standard multimodal therapy.

For MYOD1-mutant cases in adolescents and adults, the same combination of surgery, chemotherapy, and radiation is the starting framework, but these tumors frequently respond poorly to standard chemotherapy regimens. Clinical trial enrollment should be actively discussed, as ongoing studies are exploring novel agents targeting the mechanisms disrupted by MYOD1 mutation and other molecular pathways. Intensified chemotherapy regimens and new targeted approaches are under investigation.

After treatment is complete, long-term follow-up with regular imaging of the primary site and distant organs is required. Surveillance continues for at least five years, with closer monitoring in the early years after treatment when the risk of recurrence is highest. Long-term effects of treatment — particularly radiation in young children — should also be monitored over time.

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.

• Was molecular testing performed, and what was the result — was a MYOD1 mutation or a gene fusion detected?
• What does the molecular result mean for prognosis and treatment planning?
• What is the pathologic stage, and has the cancer spread to lymph nodes or other organs?
• How large is the tumor, and where exactly did it start?
• Did the tumor extend into any surrounding structures such as bone, nerves, or blood vessels?
• Were the surgical margins negative? If positive, what treatment is recommended?
• Were lymph nodes removed and examined, and were any positive for cancer?
• Was lymphovascular or perineural invasion present?
• If pre-operative treatment was given, what percentage of the tumor was dead, and what does the response mean for further management?
• What chemotherapy regimen is planned, and is my tumor likely to respond to it?
• Is radiation therapy part of the treatment plan?
• Are there clinical trials available for this diagnosis, particularly for MYOD1-mutant disease?
• What follow-up imaging schedule is recommended, and how long will surveillance continue?