Skeletal Muscle Tumors

Slide 1 · Skeletal Muscle Tumors

Rhabdomyoma · Rhabdomyosarcoma

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish the rare benign rhabdomyoma from rhabdomyosarcoma.

Recognize four rhabdomyosarcoma subtypes by age, morphology, and molecular drivers.

Connect differentiation, anatomic site, and histologic pattern with diagnosis and behavior.

Slide 3 · Skeletal muscle neoplasia is predominantly malignant

A contrast with most other mesenchymal tissue lineages.

Most skeletal muscle neoplasms are malignant, represented by rhabdomyosarcoma.

Rhabdomyoma is the uncommon benign counterpart.

Uncommon benign

Rhabdomyoma

Benign designation

Predominant malignant

Rhabdomyosarcoma

Malignant skeletal-muscle differentiation

Slide 4 · Place these tumors within soft-tissue pathology

Soft-tissue tumors are mesenchymal neoplasms, not necessarily derived from a fully mature tissue cell.

Skeletal muscle differentiation is a phenotype recognized in the tumor.

Site of origin and line of differentiation must be considered separately.

Mesenchymal neoplasm

Skeletal-muscle phenotype

Rhabdomyoma or rhabdomyosarcoma

Slide 5 · Rhabdomyoma is the benign entity

Rhabdomyoma is the benign skeletal-muscle-type neoplasm.

The Robbins overview locates rhabdomyomas in the heart and in soft tissues.

Its existence does not negate the predominance of malignancy in this category.

Skeletal-muscle differentiation

Simplified myogenic silhouette

Schematic — not a pathologic specimen

Slide 6 · Rhabdomyoma and tuberous sclerosis

A frequent association between rhabdomyoma and tuberous sclerosis.

The association is especially relevant when interpreting cardiac rhabdomyomas.

Do not assume that every anatomically distinct rhabdomyoma has the same syndromic association.

Rhabdomyoma

Recognize anatomic site

Consider tuberous sclerosis association

Slide 7 · Rhabdomyoma occurs at distinct anatomic sites

Cardiac location illustrates the association with a genetic syndrome.

Soft-tissue lesions establish that benign rhabdomyomatous differentiation is not heart-restricted.

Anatomic setting is essential when communicating a precise diagnosis.

Cardiac

Benign rhabdomyoma

Syndromic setting may be relevant

Soft tissue

Benign rhabdomyoma

Anatomic context remains essential

Slide 8 · Benign versus malignant skeletal-muscle tumors

Feature

Rhabdomyoma

Rhabdomyosarcoma

Behavior

Benign

Malignant, aggressive

Frequency

Uncommon

Predominant category

Sites

Heart or soft tissues

Head and neck; genitourinary; other soft tissues

Slide 9 · Rhabdomyosarcoma: Defining concept

Malignant mesenchymal tumor exhibiting skeletal-muscle differentiation.

It is not synonymous with a malignancy arising inside pre-existing skeletal muscle.

Diagnosis integrates pathologic morphology with evidence of rhabdomyoblastic differentiation.

Skeletal-muscle differentiation

Simplified myogenic silhouette

Schematic — not a pathologic specimen

Slide 10 · Differentiation describes a phenotype, not a postcode

Pediatric tumors often occur at sites containing little normal skeletal muscle.

Examples include sinuses, head and neck, and the genitourinary tract.

This supports the model of an undifferentiated mesenchymal precursor acquiring myogenic identity.

Mesenchymal precursor

Myogenic differentiation program

Tumor at a nonmuscular site

Slide 11 · Four recognized rhabdomyosarcoma subtypes

Embryonal, alveolar, pleomorphic, and spindle cell/sclerosing types are described.

Each subtype has characteristic clinical and morphologic associations.

Molecular pathogenesis is heterogeneous across these groups.

Embryonal

Primitive cells; Myxoid stroma

Alveolar

Septa; Central dyscohesion

Pleomorphic

Bizarre eosinophilic cells

Spindle/sclerosing

Fascicles; Storiform architecture

Slide 12 · Subtype proportions

Embryonal rhabdomyosarcoma accounts for 50% in the stated overview.

Alveolar and pleomorphic types each account for 20%.

Spindle cell/sclerosing type accounts for 10%.

Reported subtype distribution

Embryonal: 50%

Alveolar: 20%

Pleomorphic: 20%

Spindle/sclerosing: 10%

Slide 13 · Age of onset helps contextualize the subtype

Subtype

Age pattern

Interpretive implication

Embryonal

Usually under 20 years

Pediatric differential

Alveolar

Usually under 20 years

Pediatric differential

Pleomorphic

Adults

Adult sarcoma work-up

Spindle/sclerosing

All ages

Age alone is insufficient

Slide 14 · Childhood rhabdomyosarcoma is a major soft-tissue malignancy

Embryonal and alveolar rhabdomyosarcoma as common pediatric soft-tissue sarcomas.

Presentation is usually before 20 years of age.

Subtype recognition is more informative than using a single undifferentiated label.

Pediatric emphasis

Embryonal / alveolar

Classically under 20 years

Adult emphasis

Pleomorphic

Most characteristic in adults

Slide 15 · Frequent pediatric anatomic presentations

Sinuses are a described site of pediatric rhabdomyosarcoma.

Other head-and-neck sites are frequent.

The genitourinary tract is another characteristic location.

Sinuses

Head and neck

Genitourinary tract

Slide 16 · Tumors need not arise in mature skeletal muscle

The sinonasal tract has relatively little native skeletal muscle.

Nevertheless, rhabdomyosarcoma can arise there.

Interpret lineage as differentiation acquired by a malignant mesenchymal cell, not simply tissue of origin.

Low native skeletal-muscle content

A myogenic sarcoma appears

Cell-of-origin ≠ mature myofiber

Slide 17 · Subtype classification reflects distinct biology

Clinical age and location differ by subtype.

Architecture and cytology differ by subtype.

Underlying recurrent fusions or other driver alterations also differ.

Clinical setting

Age and location

Morphology

Growth pattern and cytology

Genetics

Fusion or other driver alterations

Slide 18 · Molecular pathways vary across the four groups

Subtype

Reported alteration

Source-qualified frequency

Alveolar

PAX3/7::FOXO1 fusion

Frequent

Embryonal

RAS-pathway activation

Frequent

Spindle/sclerosing

MYOD1 mutation

Subset

Pleomorphic

Multiple diverse aberrations

Heterogeneous

Slide 19 · Alveolar RMS: PAX3::FOXO1 fusion

The t(2;13) translocation can juxtapose PAX3 and FOXO1.

PAX3 is a transcription factor involved in initiation of skeletal-muscle differentiation.

The chimeric fusion product dysregulates the differentiation program.

PAX3

FOXO1

T(2;13)

PAX3::FOXO1

Schematic fusion relationship

Slide 20 · Alveolar RMS: PAX7::FOXO1 fusion

A second recurrent fusion pairs PAX7 with FOXO1.

This to t(1;13).

These fusion types are alternatives within the alveolar molecular spectrum.

PAX7

FOXO1

T(1;13)

PAX7::FOXO1

Schematic fusion relationship

Slide 21 · Translocations and fusion partners must match

Rearrangement

PAX partner

Fusion gene

t(2;13)

PAX3

PAX3::FOXO1

t(1;13)

PAX7

PAX7::FOXO1

Slide 22 · Fusion proteins disrupt differentiation

Normal myogenic transcription factors help establish a differentiation program.

A PAX::FOXO1 chimera is associated with interference in that program.

The conceptual result is malignant proliferation without normal terminal maturation.

Myogenic transcriptional control

PAX::FOXO1 fusion protein

Disrupted differentiation

Slide 23 · Embryonal RMS: Frequent RAS-pathway activation

Embryonal tumors frequently contain acquired driver mutations.

Those alterations activate the RAS signaling pathway.

This differs mechanistically from the recurrent PAX::FOXO1 fusions of many alveolar tumors.

Somatic driver mutation

Activated RAS signaling

Embryonal rhabdomyosarcoma

Slide 24 · Embryonal and alveolar groups are molecularly distinct

For embryonal RMS, the source emphasizes signaling-pathway activation.

For alveolar RMS, the source emphasizes fusion transcription factors.

Genetic mechanism and histologic appearance should be correlated—not treated as interchangeable.

Embryonal

RAS-pathway drivers

Frequent; Somatic

Alveolar

PAX3/7::FOXO1 fusions

Frequent; Rearrangement-associated

Slide 25 · Spindle cell/sclerosing RMS: MYOD1 subset

A subset contains somatic alterations involving MYOD1.

MYOD1 is a major regulator of skeletal muscle differentiation.

The mutation is not described as universal for the entire spindle/sclerosing group.

MYOD1 regulator

Mutation in a subset

Altered myogenic program

Slide 26 · The MYOD1 qualifier is diagnostically important

“Subset” means a negative MYOD1 result would not logically exclude the category.

The histologic subtype also rests on spindle/sclerotic architecture.

Avoid equating all spindle-cell neoplasms with MYOD1-mutant rhabdomyosarcoma.

Supported inference

Some spindle/sclerosing RMS have MYOD1 alterations

Not every case must be MYOD1-mutant

Not supported

Architecture is characteristic

Spindling alone is not lineage proof

Slide 27 · Pleomorphic RMS is genetically heterogeneous

Multiple distinct genetic aberrations.

No single recurrent defining fusion is stated for this group.

Genetic diversity fits the importance of morphology plus myogenic differentiation evidence.

Adult predominance

Typical clinical setting

Marked pleomorphism

Characteristic histology

Diverse changes

Heterogeneous genomic background

Slide 28 · Fusion-driven versus heterogeneous biology

Pattern

Representative subtype

Source example

Fusion transcription factor

Alveolar

PAX3::FOXO1; PAX7::FOXO1

Signaling activation

Embryonal

RAS pathway

Differentiation regulator

Spindle/sclerosing

MYOD1 subset

Heterogeneous aberrations

Pleomorphic

No uniform source-specified fusion

Slide 29 · Microscopy: Recognize architecture and cytology

Embryonal RMS: Primitive round/spindle cells in myxoid stroma.

Alveolar RMS: Septated aggregates with poor central cohesion.

Pleomorphic RMS: Bizarre eosinophilic cells; Spindle/sclerosing RMS: Fascicles or storiform pattern.

Embryonal

Cellular diversity + myxoid

Alveolar

Septa + dyscohesive centers

Pleomorphic

Large bizarre cells

Spindle/sclerosing

Fascicles + sclerosis

Slide 30 · Embryonal RMS may be soft and infiltrative

The gross tumor is typically described as soft and gray.

Its margins may infiltrate surrounding tissues.

Gross appearance gives context, but histology is needed for subtype recognition.

Soft, gray tumor mass

Schematic — not a pathologic specimen

Slide 31 · Embryonal RMS has heterogeneous immature cells

Primitive round cells form part of the neoplasm.

Spindled immature cells coexist with the round population.

This admixture reflects cells at multiple apparent stages of myogenic differentiation.

Mixed primitive round and spindle cells

Architecture schematic — not a histology image

Slide 32 · Embryonal RMS often shows a myxoid background

Primitive tumor cells may lie in loose myxoid stroma.

A myxoid background is part of the characteristic tissue pattern.

Interpret stroma together with cellular features—not in isolation.

Myxoid matrix

Immature cells in myxoid stroma

Architecture schematic — not a histology image

Slide 33 · Rhabdomyoblasts signal muscle differentiation

Some tumor cells develop recognizable rhabdomyoblastic features.

Cross-striations can occasionally be visible in rhabdomyoblasts.

Their presence provides morphologic support for skeletal-muscle differentiation.

Variable rhabdomyoblast differentiation

Architecture schematic — not a histology image

Slide 34 · Absent cross-striations do not erase the diagnosis

Visible striations may be present, not that they must be present.

Immature neoplastic cells need not recapitulate a fully differentiated myofiber.

Use the complete morphologic and differentiation evidence.

Primitive neoplastic cells

Variable maturation

Striations may be absent

Slide 35 · Embryonal RMS: Integrate three morphologic clues

Primitive round and spindle cells.

Myxoid stroma.

Occasional differentiated rhabdomyoblasts.

Cell type

Round + spindled

Stroma

Myxoid

Differentiation

Rhabdomyoblasts; Possible striations

Slide 36 · Sarcoma botryoides is an embryonal variant

Botryoid RMS as a variant of embryonal RMS.

Its site association is with walls of hollow viscera.

The subtype and anatomic distribution must be documented together.

Embryonal RMS

Botryoid variant

Wall of hollow viscus

Slide 37 · Botryoid RMS: Characteristic organ sites

Urinary bladder is an explicitly cited location.

Vagina is another explicitly cited site.

The clinical scenario can bring a pediatric mass in a hollow viscus to attention.

Urinary bladder

Vagina

Hollow visceral wall

Slide 38 · Alveolar RMS is organized by fibrous septa

Fibrous partitions divide the tumor into cellular nests or aggregates.

The architecture resembles pulmonary alveoli, explaining the designation.

This is a pattern of growth, not actual lung epithelium.

Nests separated by fibrous septa

Architecture schematic — not a histology image

Slide 39 · Alveolar RMS: Loss of central cohesion

Cells near the centers of aggregates are minimally cohesive.

The reduced cohesion produces open-looking spaces within nests.

This structural relationship is more specific than the word “alveolar” alone.

Less cohesive central tumor cells

Architecture schematic — not a histology image

Slide 40 · Alveolar RMS: Peripheral attachment

Peripheral tumor cells retain an association with fibrous septa.

Central cells show less cohesion than septal-adjacent cells.

The center-versus-periphery contrast defines the source-described pattern.

Peripheral cells contact septa

Architecture schematic — not a histology image

Slide 41 · Alveolar RMS: Relatively uniform cytology

Tumor cells are often round and comparatively uniform.

Cytoplasm is scant.

Combine cytology with septated, dyscohesive architecture for pattern recognition.

Uniform round cells; Scant cytoplasm

Architecture schematic — not a histology image

Slide 42 · Embryonal versus alveolar architecture

Embryonal tumors contain primitive cell mixtures amid myxoid stroma.

Alveolar tumors form nests separated by fibrous septa.

Alveolar centers are less cohesive, while peripheral cells relate to the septa.

Embryonal morphology

Mixed round and spindle cells

Myxoid tissue matrix

Alveolar morphology

Round uniform cells

Fibrous septa and dyscohesion

Slide 43 · Pleomorphic RMS: Severe atypia

Large eosinophilic tumor cells are conspicuous.

Some cells are multinucleated and bizarre.

This pattern can overlap with other pleomorphic sarcomas.

Bizarre, variably sized eosinophilic cells

Architecture schematic — not a histology image

Slide 44 · Pleomorphic morphology is not lineage-specific

Bizarre sarcoma cells alone do not establish rhabdomyosarcoma.

The source explicitly notes overlap with other pleomorphic sarcomas.

Confirm skeletal-muscle differentiation rather than relying only on pleomorphism.

Pleomorphic malignant cells

Broad morphologic overlap

Establish myogenic phenotype

Slide 45 · Myogenin immunohistochemistry supports lineage

Myogenin as a skeletal-muscle-specific protein useful in diagnosis.

Myogenin immunohistochemistry is usually needed to confirm rhabdomyoblastic differentiation in pleomorphic lesions.

Immunostaining is interpreted in conjunction with architecture and cytology.

Morphologic suspicion

Myogenin immunohistochemistry

Support rhabdomyoblastic differentiation

Slide 46 · Spindle/sclerosing RMS: Fascicular architecture

The tumor contains fusiform malignant cells.

Nuclear chromatin can be vesicular.

Cells may form extended, parallel fascicles.

Spindle cells in long fascicles

Architecture schematic — not a histology image

Slide 47 · Spindle/sclerosing RMS: Storiform pattern

Spindle-cell fascicles may show storiform organization.

A storiform pattern is a radiating, interlacing fascicular architecture.

The pattern should be integrated with cytology and myogenic features.

Interlacing storiform fascicles

Architecture schematic — not a histology image

Slide 48 · Spindle/sclerosing RMS: Collagenous stroma

Dense sclerotic collagen can accompany the neoplastic spindled cells.

The source notes that sclerosis may be more common in adults.

The amount of sclerosis is variable; It is not the only diagnostic feature.

Spindle cells with collagenous sclerosis

Architecture schematic — not a histology image

Slide 49 · Spindle/sclerosing RMS: Rhabdomyoblasts may be seen

Recognizable rhabdomyoblasts are occasionally present.

The dominant spindle-cell architecture may obscure the myogenic identity.

A single favorable morphologic clue should be weighed with the whole lesion.

Occasional rhabdomyoblasts may occur

Architecture schematic — not a histology image

Slide 50 · Histology comparison: Four diagnostic patterns

Subtype

Cells

Architecture / stroma

Embryonal

Primitive round + spindle

Myxoid; occasional rhabdomyoblasts

Alveolar

Uniform round; scant cytoplasm

Fibrous septa; central dyscohesion

Pleomorphic

Large, bizarre eosinophilic

Pleomorphic; possible giant cells

Spindle/sclerosing

Fusiform; vesicular chromatin

Fascicular or storiform; collagenous

Slide 51 · Correlating morphology with molecular class

Histologic pattern

Molecular association

Alveolar

PAX3::FOXO1 or PAX7::FOXO1

Embryonal

RAS-pathway activation

Spindle/sclerosing

MYOD1 mutation in subset

Pleomorphic

Genetically heterogeneous

Slide 52 · A pathology-oriented integration sequence

Establish the clinical context: Age and anatomical site.

Review growth pattern, cells, and background stroma.

Use skeletal-muscle differentiation evidence and, where appropriate, the subtype-associated genetic mechanism.

Clinical context

Histologic pattern

Myogenic evidence

Molecular correlation

Slide 53 · Clinicopathologic patterns are informative—not rules

Age and location can suggest a subtype.

Microscopic architecture provides more direct phenotypic evidence.

The Robbins associations do not make anatomic site or age independently diagnostic.

Age

Raises suspicion

Site

Adds context

Histology + phenotype

Supports classification

Slide 54 · Rhabdomyosarcoma behaves aggressively

The textbook characterizes rhabdomyosarcomas as aggressive tumors.

Locoregional extent and other clinical factors matter in management planning.

Histologic subtype and location correlate with survival.

Malignant myogenic tumor

Aggressive behavior

Need for combined clinical care

Slide 55 · Treatment in the Robbins overview is multimodal

Surgical resection is one common component.

Chemotherapy is generally included.

Radiation therapy may be used, depending on the clinical situation.

Surgery

Local tumor control

Chemotherapy

Systemic treatment

Radiation

May complement local treatment

Slide 56 · Outcome differs by histologic type and location

The source links patient survival with histologic subtype.

Anatomic location is another prognostic correlate.

These qualitative statements do not supply a universal survival percentage or staging algorithm.

Histologic subtype

Anatomic location

Correlated with outcome

Slide 57 · Botryoid variant has a favorable relative prognosis

Among the named morphologic types, botryoid RMS as most favorable.

Botryoid tumors belong to the embryonal family.

Prognosis should still be interpreted in the actual clinical context.

Variant

Botryoid RMS

Most favorable among described groups

Source prognostic statement

Pleomorphic RMS

Often fatal in the cited overview

Slide 58 · Pleomorphic RMS has an adverse clinical course

Pleomorphic rhabdomyosarcoma principally occurs in adults.

It has marked cytologic atypia and genetic heterogeneity.

The Robbins clinical summary describes pleomorphic RMS as often fatal.

Setting

Adulthood

Morphology

Bizarre eosinophilic cells

Clinical behavior

Often fatal (source description)

Slide 59 · Teaching vignette: Pediatric hollow-viscus mass

An illustrative pediatric mass arises in the wall of the urinary bladder.

Biopsy shows primitive round/spindled cells in a myxoid background.

Which subtype/variant should enter the differential first?

Interpretive answer

Embryonal rhabdomyosarcoma, with the botryoid variant considered in this anatomic context.

Illustrative teaching vignette—not patient data

Slide 60 · Teaching vignette: Septated round-cell tumor

An illustrative lesion contains round cells in nests divided by fibrous septa.

Cells are less cohesive centrally but contact septa at the periphery.

What is the characteristic histologic category and fusion class?

Interpretive answer

Alveolar RMS; PAX3::FOXO1 or PAX7::FOXO1 is a frequent molecular association.

Illustrative teaching vignette—not patient data

Slide 61 · Teaching vignette: Adult pleomorphic sarcoma

An illustrative adult lesion shows bizarre eosinophilic malignant cells.

Histology overlaps with other pleomorphic sarcomas.

What additional differentiation evidence is specifically highlighted by Robbins?

Interpretive answer

Myogenin immunohistochemistry to support rhabdomyoblastic differentiation.

Illustrative teaching vignette—not patient data

Slide 62 · Avoid common interpretive errors

Do not require visible striations in every rhabdomyosarcoma.

Do not equate every spindle-cell tumor with MYOD1-mutant RMS.

Do not reverse PAX3/PAX7 translocation mapping or mistake apparent “alveoli” for epithelial differentiation.

Morphologic overreach

Visible striations are optional

Molecular overreach

MYOD1 is a subset association

Extraction vigilance

PAX3 t(2;13), PAX7 t(1;13)

Slide 63 · Integrated summary: Biologic classification

Rhabdomyoma is benign; Skeletal-muscle neoplasms are otherwise predominantly malignant.

Rhabdomyosarcoma is divided into embryonal, alveolar, pleomorphic, and spindle/sclerosing groups.

The groups are distinguished by morphology, typical patient setting, and differing genetic events.

Embryonal

RAS frequently

Alveolar

PAX::FOXO1 frequently

Pleomorphic

Diverse aberrations

Spindle/sclerosing

MYOD1 in subset

Slide 64 · Integrated summary: Microscopic recognition

Myxoid primitive cell mixtures favor the embryonal pattern.

Septate nests with dyscohesive centers favor the alveolar pattern.

Bizarre eosinophilic cells or spindled/sclerotic patterns require evidence of myogenic differentiation.

Embryonal

Myxoid + immature

Alveolar

Septa + dyscohesion

Pleomorphic

Bizarre eosinophilic

Spindle/sclerosing

Fascicular; Collagenous

Slide 65 · Sources and scope

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. Chapter 26: Bones, Joints, and Soft Tissue Tumors.

Primary passage: “Skeletal Muscle Tumors” (printed p. 1102); adjacent rhabdomyosarcoma morphology and clinical continuation (pp. 1102–1103).

All mechanisms and numeric subtype proportions follow the supplied Robbins text. No external staging criteria or quantitative outcomes are added.