Rhabdomyosarcoma

Slide 1 · Rhabdomyosarcoma

Skeletal muscle differentiation in a malignant mesenchymal tumor

Histopathology • Molecular pathogenesis • Clinicopathologic correlation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Identify the four rhabdomyosarcoma subtypes and their typical age distributions.

Recognize subtype-defining architecture, cytology, and skeletal-muscle differentiation.

Connect PAX::FOXO1 fusions, RAS-pathway activation, and MYOD1 alterations to phenotype.

Explain why subtype, site, and fusion status matter for clinical interpretation.

Slide 3 · Definition: Malignant skeletal muscle differentiation

Rhabdomyosarcoma is a malignant mesenchymal neoplasm that exhibits skeletal-muscle differentiation.

“Rhabdo-” refers to striated / skeletal muscle lineage.

Tumor location need not contain mature skeletal muscle.

Lineage assignment depends on morphology and, when needed, myogenic immunophenotype.

Slide 4 · Skeletal muscle tumors: Malignant predominance

Rhabdomyosarcoma

Rhabdomyoma

Malignant mesenchymal tumor

Multiple histologic and molecular classes

Important pediatric soft-tissue sarcoma

Benign skeletal-muscle tumor

May arise in heart or soft tissues

Frequently associated with tuberous sclerosis

Slide 5 · Lineage does not imply mature tissue of origin

Mesenchymal precursor

Cell of origin concept

Abnormal differentiation program

Oncogenic perturbation

Rhabdomyoblastic phenotype

Tumor resembles skeletal muscle

Slide 6 · Characteristic pediatric sites

Sinuses

Head and neck

Genitourinary tract

Rhabdomyosarcoma can arise where normal mature skeletal muscle is sparse.

Slide 7 · The pediatric–adult subtype divide

Common in younger patients

Older-age pattern

Embryonal rhabdomyosarcoma

Alveolar rhabdomyosarcoma

Usually present before age 20

Pleomorphic rhabdomyosarcoma: Adults

Spindle cell/sclerosing: Across age groups

Subtype changes the clinical context

Slide 8 · Four major histologic categories

Subtype

Distinctive finding / teaching clue

Embryonal

Primitive round/spindle cells in myxoid stroma

Alveolar

Fibrous septa; Discohesive central cells

Pleomorphic

Bizarre, sometimes multinucleated cells

Spindle cell/sclerosing

Fascicles / storiform cells ± dense collagen

Slide 9 · Subtype proportions

Percent of cases in the Robbins four-subtype overview

Embryonal

50%

Alveolar

20%

Pleomorphic

20%

Spindle/sclerosing

10%

These are the approximate proportions stated for the four-category textbook overview; They are not pediatric-only proportions.

Slide 10 · Do not transfer proportions between cohorts

Textbook overview

Pediatric-specific NCI framing

Embryonal 50%; Alveolar 20%

Pleomorphic 20%; Spindle/sclerosing 10%

General subtype summary

Childhood subtype mix differs

Embryonal forms a larger pediatric share

Cohort, age range, and classification affect proportions

Slide 11 · Subtype-specific molecular mechanisms

Alveolar

PAX3/7::FOXO1

Embryonal

RAS pathway

Spindle/sclerosing

MYOD1 in subset

Pleomorphic

Heterogeneous aberrations

Slide 12 · Why molecular differentiation matters

Myogenic regulator

Normal transcriptional control

Oncogenic perturbation

Fusion or mutation

Arrested / altered program

Persistent malignant phenotype

Slide 13 · Alveolar rhabdomyosarcoma: FOXO1 fusions

Recurrent rearrangement

PAX3 or PAX7

FOXO1

PAX3::FOXO1 / PAX7::FOXO1

Chimeric transcriptional regulator; Interference with myogenic differentiation

Slide 14 · PAX3::FOXO1: Chromosomal origin

T(2;13)

Characteristic translocation

PAX3 + FOXO1

Gene juxtaposition

PAX3::FOXO1

Chimeric product

Slide 15 · PAX7::FOXO1: Chromosomal origin

T(1;13)

Alternative rearrangement

PAX7 + FOXO1

Gene juxtaposition

PAX7::FOXO1

Chimeric product

Slide 16 · PAX3 and normal skeletal muscle programming

PAX3 is a transcription factor involved in initiating skeletal muscle differentiation.

A transcription factor regulates gene-expression programs.

Fusion to FOXO1 changes the regulatory context.

The textbook links the chimeric product to impaired differentiation.

Slide 17 · Fusion-driven differentiation arrest

PAX::FOXO1 fusion

Genomic rearrangement

Abnormal transcriptional control

Chimeric regulator

Impaired myogenic maturation

Persistence of immature tumor phenotype

Slide 18 · Embryonal: RAS-pathway activation

Somatic driver mutation

Recurrent molecular theme

RAS pathway activated

Abnormal signaling

Embryonal RMS biology

Subtype association

Slide 19 · MYOD1 and spindle/sclerosing tumors

Mutation in a subset

MYOD1

Myogenic program

Altered myogenic regulation

The association is subtype-specific but not universal: Not every spindle/sclerosing RMS has a MYOD1 mutation.

Slide 20 · Pleomorphic RMS lacks a single defining fusion

Pleomorphic rhabdomyosarcoma is genetically heterogeneous.

Multiple genomic abnormalities may coexist.

The textbook distinguishes its molecular landscape from the other three types.

Do not infer a recurrent PAX::FOXO1 fusion merely from pleomorphic cytology.

Slide 21 · Molecular associations are not absolutes

Specific association

Diagnostic caution

Alveolar: FOXO1 fusion often present

Spindle/sclerosing: MYOD1 in a subset

Embryonal: Frequent RAS activation

“Frequently” does not mean invariably

Histology and molecular status may diverge

Integrate morphology, phenotype, and molecular findings

Slide 22 · Embryonal RMS: Gross appearance

Soft, gray, infiltrative mass.

The macroscopic phenotype is nonspecific.

Infiltration illustrates the malignant behavior of the lesion.

Microscopy is required to establish subtype and differentiation.

Slide 23 · Embryonal RMS: Primitive cell population

Round primitive cells

Spindled primitive cells

Myxoid background

Schematic representation of the mixed round/spindle pattern.

Slide 24 · Why “embryonal” is morphologically appropriate

Tumor cells recapitulate multiple stages of skeletal muscle differentiation.

Primitive immature elements coexist with more differentiated cells.

Round and spindle morphologies occur in the same tumor.

Recognizable rhabdomyoblasts may appear within the population.

Slide 25 · The myxoid stroma in embryonal RMS

Loose myxoid matrix

Scattered tumor cells

Variable cellular maturity

Observe tissue context, not just isolated cell shapes.

Slide 26 · Rhabdomyoblasts: A differentiation clue

Eosinophilic cytoplasm

Developing skeletal muscle phenotype

Cross-striations may be visible

Cross-striations are supportive when identified; They are not required in every section.

Slide 27 · Cross-striations are not always demonstrable

When seen

When absent

Direct morphologic evidence of skeletal-muscle differentiation

Can be decisive in the correct context

Does not by itself exclude RMS

Seek architecture and myogenic immunophenotype

Slide 28 · Embryonal RMS: Integrated morphology

Soft, gray, infiltrative gross mass.

Variable primitive round and spindle cells.

Myxoid stromal background.

Variable rhabdomyoblastic maturation; Occasional visible cross-striations.

Slide 29 · Sarcoma botryoides is an embryonal variant

Embryonal RMS

Shared differentiation

Hollow viscus wall

Bladder or vagina

Botryoid growth pattern

Grapelike polypoid masses

Slide 30 · Botryoid RMS: Classic anatomical settings

Genitourinary locations

Why location helps

Vagina in infants / young children

Urinary bladder wall

Other hollow-viscus settings

Subepithelial or wall-based tumor

Lumen permits a polypoid mass

Gross pattern suggests the variant

Slide 31 · Botryoid RMS: Polypoid architecture

Surface epithelium

Grapelike polypoid lobules

Underlying tumor-bearing wall

Schematic anatomy; Not a gross specimen.

Slide 32 · Vaginal botryoid RMS: Age and gross pattern

The Chapter 22 vaginal example most often occurs in infants and children younger than 5 years.

Often a bulky polypoid / rounded mass.

Grapelike clustering explains the historical name “botryoides.”

Appearance is not sufficient to establish the diagnosis.

Slide 33 · Botryoid RMS: The cambium layer

Surface epithelium

Subepithelial cellular condensation

Deeper edematous stroma

The cambium layer is a concentrated subepithelial tumor-cell zone.

Slide 34 · Botryoid RMS: Cells in a loose deep stroma

Deeper tumor cells may lie in an edematous fibromyxoid matrix.

The subepithelial zone is more cellular (cambium layer).

An inflammatory infiltrate may accompany the loose stroma.

This variation creates a potential benign-polyp mimic.

Slide 35 · Botryoid RMS can resemble an inflammatory polyp

Potential pitfall

Features that demand attention

Polypoid vaginal lesion

Edematous matrix

Inflammatory cells

Subepithelial primitive-cell condensation

Malignant rhabdomyoblasts

Integrated tissue sampling and interpretation

Slide 36 · Alveolar RMS: Architecture explains its name

Fibrous septum

Cell aggregates

Central loss of cohesion

“Alveolar” describes the architecture, not pulmonary origin.

Slide 37 · Fibrous septa compartmentalize tumor cells

Intervening septa

Peripheral attached cells

Central loose tumor cells

At low power, scan for septa surrounding tumor-cell nests.

Slide 38 · Central discohesion is diagnostically useful

Central zone

Peripheral zone

Tumor cells become minimally cohesive

Spaces form within aggregates

Cells may appear suspended

Cells retain attachment to fibrous septa

Creates the characteristic alveolar arrangement

Slide 39 · Alveolar RMS: Cytologic profile

A relatively uniform round-cell population with scant cytoplasm.

The architecture is as important as the cytology.

Round, small-appearing malignant cells can resemble other neoplasms.

Integrate myogenic immunophenotype and fusion information when available.

Slide 40 · Embryonal versus alveolar RMS: Architecture

Embryonal

Alveolar

Primitive round and spindle elements

Myxoid background

Variable stages of maturation

Fibrous septa partition nests

Central discohesion

Uniform round cells with little cytoplasm

Slide 41 · Embryonal versus alveolar RMS: Schematics

Embryonal: Mixed maturation

Alveolar: Septa and loose centers

Original architectural schematics; Neither panel is a specimen.

Slide 42 · Pleomorphic RMS: Adult tumor morphology

Large, sometimes multinucleated, bizarre eosinophilic tumor cells.

Marked pleomorphism dominates the histology.

A malignant spindle/pleomorphic morphology alone is not lineage-specific.

Skeletal-muscle differentiation may require immunohistochemical confirmation.

Slide 43 · Multinucleation and bizarre forms

Large anaplastic tumor cell

Multiple nuclei

Eosinophilic cytoplasm

Stylized cytology; Do not interpret as an actual photomicrograph.

Slide 44 · Myogenin helps confirm rhabdomyoblastic lineage

Pleomorphic malignant cells

Morphologic overlap

Lineage uncertain on H&E

Differential remains broad

Myogenin IHC supports RMS

Skeletal-muscle-associated marker

Slide 45 · Pleomorphic RMS: A differential diagnosis problem

Why difficult

Resolution

Highly atypical pleomorphic cells

Histologic overlap with other pleomorphic sarcomas

No single specific architectural pattern

Identify skeletal-muscle differentiation

Use myogenin or appropriate myogenic immunophenotype

Avoid diagnosis from pleomorphism alone

Slide 46 · Spindle cell/sclerosing RMS: Fascicular pattern

Fusiform tumor cells

Long cellular fascicles

Vesicular chromatin

Fascicular arrangement is one recognized histologic pattern.

Slide 47 · A storiform pattern may also occur

Intersecting cell bundles

Storiform orientation

Spindle-cell population

Illustrative arrangement only; Not diagnostic in isolation.

Slide 48 · Sclerosing subtype: Collagen-rich background

Dense collagenous, sclerotic stroma may be especially prominent in adults.

Fibrous matrix alters the appearance of the spindle-cell population.

Rhabdomyoblasts may be encountered, but are not consistently conspicuous.

Histology must be interpreted with the subtype-specific molecular context.

Slide 49 · Occasional rhabdomyoblasts in spindle RMS

What to look for

Interpretive limitation

Fusiform malignant cells

Long fascicles or storiform arrays

Occasional cells with myogenic morphology

A spindle-cell pattern is nonspecific

Absence of a visible rhabdomyoblast is not exclusionary

Integrate myogenic lineage confirmation

Slide 50 · Four subtypes: Morphologic discriminators

Subtype

Distinctive finding / teaching clue

Embryonal

Heterogeneous immature cells; Myxoid matrix

Alveolar

Fibrous septa; Central discohesion

Pleomorphic

Bizarre eosinophilic / multinucleated cells

Spindle/sclerosing

Fusiform fascicles / storiform; Collagen

Slide 51 · From morphology to lineage confirmation

Recognize a malignant soft-tissue tumor

Identify dominant architectural / cytologic pattern

Seek skeletal-muscle differentiation

Assign a subtype using integrated evidence

Slide 52 · Morphology and molecular data play different roles

Microscopy

Ancillary testing

Identifies architecture and cell shape

Defines degree/pattern of maturation

Raises the subtype hypothesis

Myogenin supports skeletal muscle lineage

FOXO1 fusion status refines alveolar biology

MYOD1 alteration in spindle/sclerosing subset

Slide 53 · Fusion-negative alveolar morphology: An important caveat

Not every tumor showing alveolar histology has a FOXO1 fusion.

NCI PDQ reports that fusion-negative alveolar-pattern tumors resemble embryonal RMS molecularly and clinically.

Therefore, alveolar morphology and fusion-positive disease should not be treated as interchangeable.

Avoid assigning risk from architecture alone.

Slide 54 · FOXO1 fusion status carries prognostic information

Tumor morphology

Architectural phenotype

FOXO1 fusion result

Important molecular stratifier

Integrated risk assessment

Combined with disease extent and clinical factors

Slide 55 · A diagnostic biomarker is not automatically predictive

Myogenin

FOXO1 fusion status

Supports myogenic differentiation

Primarily a lineage / diagnostic marker

Does not alone quantify prognosis

Molecular association with alveolar RMS

Contributes to risk stratification

Not itself a treatment-response guarantee

Slide 56 · Molecular findings require context

FOXO1 fusion is strongly associated with alveolar rhabdomyosarcoma.

RAS-pathway changes commonly occur in embryonal disease.

MYOD1 alterations occur in only a subset of spindle/sclerosing lesions.

No marker should override an internally inconsistent morphologic and clinical picture.

Slide 57 · Clinical behavior: Aggressive malignant sarcoma

Rhabdomyosarcomas are aggressive neoplasms.

Both local extent and disseminated disease matter clinically.

Anatomic site influences feasibility of local treatment.

Histologic subtype and site correlate with survival.

Slide 58 · Risk assessment has multiple dimensions

Biology

Subtype and FOXO1

Extent

Metastatic status

Primary site

Anatomic risk

Patient factors

Age and resectability

Slide 59 · Clinical factors beyond histologic subtype

Age, tumor site, size, and resectability influence clinical risk.

Nodal involvement and metastasis at presentation matter.

FOXO1 fusion status is incorporated into contemporary pediatric risk assessment.

A pathologic subtype is only one component of management decisions.

Slide 60 · Primary location changes the therapeutic problem

Anatomical constraints

Clinical consequence

Sinuses and head/neck structures

Genitourinary organ preservation

Potential difficulty achieving complete resection

Local-control approach varies by site

Multidisciplinary decisions are required

Prognosis cannot be inferred from cell type alone

Slide 61 · Core multimodal treatment principles

Surgical/local assessment

Resection when feasible

Systemic chemotherapy

Addresses systemic risk

Radiation when indicated

Selected for local control needs

Slide 62 · Favorable botryoid prognosis is relative

Textbook observation

Do not overgeneralize

Botryoid variant has the most favorable prognosis

Its typical location and recognizability aid clinical interpretation

Still an invasive malignancy

Stage and treatment remain important

Not all embryonal lesions are botryoid

Slide 63 · Pleomorphic RMS: An adverse adult pattern

Pleomorphic rhabdomyosarcoma often carries a grave prognosis.

It is predominantly an adult subtype.

Tumor cells may overlap morphologically with other high-grade pleomorphic sarcomas.

Robbins characterizes the outcome as often fatal.

Slide 64 · Pediatric outlook depends on fusion and extent

FOXO1 status is more informative than alveolar histology alone in modern pediatric risk assessment.

Fusion-positive disease tends to have less favorable outcomes than fusion-negative RMS.

Metastatic disease at diagnosis substantially worsens prognosis.

Risk assignment also incorporates primary site, patient age, and surgical/pathologic group.

Slide 65 · Case: Polypoid vaginal mass in a young child

A 3-year-old has a bulky, grapelike vaginal mass. The biopsy reveals small malignant cells beneath epithelium with a dense subepithelial zone and loose deeper stroma.

Diagnostic question

Which rhabdomyosarcoma variant and microscopic feature are most characteristic?

Slide 66 · Case analysis: Botryoid embryonal RMS

Embryonal rhabdomyosarcoma, botryoid variant.

Grapelike polypoid appearance in a hollow viscus.

Characteristic subepithelial cambium layer.

Must not be mistaken for a benign inflammatory polyp.

Slide 67 · Case: Septated round-cell sarcoma

An adolescent has a soft-tissue malignancy composed of relatively uniform round cells. Fibrous septa divide tumor into aggregates with conspicuous central cellular discohesion.

Diagnostic question

What histologic subtype is suggested, and which fusion should be considered?

Slide 68 · Case analysis: Alveolar RMS

Alveolar rhabdomyosarcoma.

Fibrous septa and central discohesion define the classic architecture.

Evaluate for PAX3::FOXO1 or PAX7::FOXO1 fusion.

Fusion testing matters because not all alveolar-pattern tumors are fusion-positive.

Slide 69 · Case: Pleomorphic adult sarcoma

An older adult has a high-grade soft-tissue tumor. The neoplasm consists of large bizarre eosinophilic cells, including multinucleated forms.

Diagnostic question

What lineage-confirming stain helps evaluate pleomorphic RMS?

Slide 70 · Case analysis: Myogenin-supported lineage

Myogenin immunohistochemistry supports rhabdomyoblastic differentiation.

Marked pleomorphism is not specific for RMS.

The clinical and morphological pattern raises pleomorphic RMS.

Lineage confirmation is important before final classification.

Slide 71 · Case: Adult spindle-cell sarcoma

A soft-tissue tumor contains spindle-shaped malignant cells in long fascicles with areas of dense collagenous sclerosis.

Diagnostic question

Which RMS subtype should be considered, and what molecular alteration is associated with a subset?

Slide 72 · Case analysis: Spindle cell/sclerosing RMS

Spindle cell/sclerosing rhabdomyosarcoma.

Fascicular and sclerosing growth support the pattern.

A subset has MYOD1 mutations.

The spindle-cell phenotype alone is not sufficient for diagnosis.

Slide 73 · Integrated comparison: Histology, gene, age

Subtype

Distinctive finding / teaching clue

Embryonal

Young; Primitive round/spindle; RAS activation

Alveolar

Young; Septated nests; PAX::FOXO1 often

Pleomorphic

Adults; Bizarre cells; Heterogeneous genetics

Spindle/sclerosing

All ages; Fusiform + collagen; MYOD1 subset

Slide 74 · Five interpretive rules

RMS is malignant and shows skeletal-muscle differentiation.

Normal skeletal muscle is not required at the primary site.

Subtype-specific morphology is a starting point, not the whole diagnosis.

Myogenin supports myogenic lineage when morphology is ambiguous.

FOXO1 fusion status adds substantial molecular and prognostic information.

Slide 75 · Primary references and source boundaries

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026). Chapter 26, pp. 1102–1103.

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026). Chapter 22, p. 904 (botryoid vaginal RMS).

NCI. Childhood Rhabdomyosarcoma Treatment (PDQ®), Health Professional Version (supplementary pediatric interpretation).

NCI. Childhood Cancer Genomics (PDQ®), Health Professional Version (supplementary molecular interpretation).

Textbook subtype proportions apply to the textbook overview; pediatric-focused epidemiology may differ.