Soft Tissue Tumors

Slide 1 · Soft Tissue Tumors

Pathogenesis • Classification • Adipocytic and Fibroblastic Tumors

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Define the soft tissue compartment and distinguish tumor behavior categories.

Explain predisposition, putative origin, and the two broad cytogenetic patterns.

Interpret representative sarcoma fusion events without confusing molecular alterations with malignancy.

Compare lipoma, liposarcoma subtypes, and nodular fasciitis using clinicopathologic features.

Slide 3 · Soft tissue: The pathology convention

Included: Mesenchymal soft tissues

Excluded: Bone / joints

Excluded: CNS / blood / lymphoid

Non-epithelial tissues outside skeleton and joints.

Central nervous system and hematopoietic/lymphoid tissues are excluded.

In this chapter, soft tissue pathology chiefly concerns neoplasms.

Slide 4 · Why the soft tissue category is anatomically broad

Differentiation

Fat and fibroblastic lineages

Anatomic setting

Superficial, deep, extremity, trunk, retroperitoneum

Behavior

Benign through malignant

The term designates a diagnostic compartment—not one unique cell type.

Slide 5 · Most soft tissue neoplasms are benign

Approximately 100-fold more frequent

Benign soft tissue tumors versus sarcomas — skeletal muscle tumors excepted

Benign tumors approximately 100-fold more frequently than sarcomas, excepting skeletal muscle neoplasms.

A soft tissue mass is not automatically a sarcoma.

Clinical context and tissue classification remain essential.

Slide 6 · Soft tissue sarcoma: Uncommon, clinically consequential

Frequency

Clinical significance

Rare relative to all cancers

Can cause substantial morbidity and mortality

Most masses are benign

Aggressive sarcomas require precise diagnosis

Slide 7 · Anatomic distribution has diagnostic value

Extremities predominate

Thigh: Frequent site

Subtype location also matters

Most soft tissue tumors arise in extremities.

The thigh is a particularly frequent site.

Location also helps narrow a specific subtype (e.g., deep thigh vs superficial trunk).

Slide 8 · Sarcomas across the age spectrum

Childhood

Hereditary predisposition deserves attention

Young adult

Some simple-karyotype tumors predominate

Older adult

Many complex-karyotype tumors occur

Most soft tissue sarcomas occur in adults, but childhood disease is important.

Slide 9 · A typical sarcoma has no established precursor lesion

Carcinomas

Sometimes recognizable precursor lesions

Sarcomas

Precursors generally undefined

Many carcinomas evolve through recognizable preinvasive lesions.

Slide 10 · Most sarcomas are sporadic

Sporadic context

Acquired oncogenic alterations

Sarcoma phenotype

In most cases, no specific predisposing cause is identified.

Somatic oncogenic changes drive neoplastic growth.

A smaller subset occurs with germline predisposition or identified exposures.

Slide 11 · Inherited susceptibility syndromes

NF1

Neurofibromatosis type 1

Li–Fraumeni

Inherited tumor predisposition

Gardner

APC-associated context

NF1-associated predisposition, Li–Fraumeni syndrome, and Gardner syndrome among examples.

Slide 12 · Environmental and acquired associations

Recognized associations include radiation exposure.

Burns, toxins, and chronic inflammation are also noted.

These exposures account for a minority of sarcomas; Association is not a universal cause.

Slide 13 · Mesenchymal stem cell: A proposed origin

Mesenchymal progenitor

Acquired driver alterations

Tumor with lineage features

Robbins frames sarcomas as arising from mesenchymal stem cells acquiring driver events.

Resulting neoplasms may express different differentiation programs.

This is a pathogenic model, not proof that each tumor derives from a fully mature tissue cell.

Slide 14 · Differentiation does not prove the cell of origin

Where tumor arises

Anatomic soft tissue site

What tumor resembles

Mature or primitive mesenchymal lineage

Rhabdomyosarcoma can occur where mature skeletal muscle is sparse.

Slide 15 · Molecular heterogeneity is a defining feature

Genomic alteration

Biologic pathway

Distinct tumor histotype

Sarcoma genotypes range from recurrent specific rearrangements to highly complex copy-number changes.

Different mechanisms may converge on malignant growth.

The genetic pattern can be diagnostically more informative than gross tumor appearance.

Slide 16 · Two broad cytogenetic patterns

Simple karyotype

Few recurrent changes

Complex karyotype

Numerous gains/losses

One recurrent event

Multiple altered chromosomes

Schematic only (not patient cytogenetics)

Slide 17 · Simple-karyotype sarcomas

Recurrent rearrangement

Specific fusion

Recognizable sarcoma subtype

Often relatively limited chromosomal abnormalities, sometimes a defining rearrangement.

Many arise in younger individuals and have limited pleomorphism.

Diagnostic molecular markers can reflect early driving events.

Slide 18 · Rearrangement-driven tumors and diagnosis

Pattern recognition

Targeted molecular assay

Integrated diagnosis

A recurrent fusion may be sufficiently specific to support entity assignment.

Detection can refine a differential suggested by age, site, and histology.

The genetic result must still be interpreted in clinicopathologic context.

Slide 19 · Complex-karyotype sarcomas

Schematic genomic instability — not a measured karyogram

Schematic only (not patient cytogenetics)

Slide 20 · Simple and complex karyotypes compared

Feature

Simple karyotype

Complex karyotype

Genomic pattern

Few recurrent changes

Multiple gains and losses

Usual morphology

Often limited pleomorphism

Often pleomorphic

Typical setting

Many younger patients

Often adults

Examples

Synovial sarcoma

Leiomyosarcoma; undifferentiated sarcoma

Slide 21 · Gene fusions create more than one oncogenic mechanism

Transcription

EWSR1 family; PAX::FOXO1

Ligand signaling

COL1A1::PDGFB

Kinase signaling

ETV6::NTRK3

Some fusion proteins dysregulate transcription or chromatin remodeling.

Slide 22 · Selected EWSR1-associated fusions

Tumor

Characteristic fusion

Interpretive role

Ewing sarcoma

EWSR1::FLI1 / EWSR1::ERG

Gene-regulatory fusion

Extraskeletal myxoid chondrosarcoma

EWSR1::NR4A3

Entity-associated rearrangement

Desmoplastic small round cell tumor

EWSR1::WT1

Entity-associated rearrangement

Clear-cell sarcoma

EWSR1::ATF1

Entity-associated rearrangement

Slide 23 · FUS::DDIT3 blocks adipocytic differentiation

FUS::DDIT3

Adipocytic differentiation impaired

Myxoid liposarcoma

Myxoid liposarcoma commonly carries FUS::DDIT3 from t(12;16).

The chimeric protein interferes with normal adipocytic maturation.

Primitive adipocytic cells accumulate within a myxoid vascular matrix.

Slide 24 · SS18::SSX affects chromatin-remodeling complexes

SS18 + SSX family

Chromatin regulation altered

Synovial sarcoma

Synovial sarcoma is linked to t(X;18).

Reported fusion partners include SSX1, SSX2, and SSX4.

The resulting proteins dysregulate chromatin-remodeling complexes.

Slide 25 · PAX::FOXO1 affects muscle differentiation

PAX3 / PAX7 + FOXO1

Myogenic transcription altered

Alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma may contain PAX3::FOXO1 or PAX7::FOXO1.

These act as chimeric transcription factors.

They interfere with skeletal muscle differentiation programs.

Slide 26 · COL1A1::PDGFB: Promoter-driven growth signaling

COL1A1::PDGFB

PDGFB overexpression

Autocrine growth signal

Dermatofibrosarcoma protuberans is associated with COL1A1::PDGFB.

The rearrangement promotes PDGFB overexpression.

PDGF signaling then supports autocrine proliferative stimulation.

Slide 27 · ETV6::NTRK3: Constitutive kinase activation

ETV6::NTRK3

Constitutive kinase

RAS/MAPK signaling

Infantile fibrosarcoma is associated with ETV6::NTRK3.

The chimeric kinase is constitutively active.

Downstream RAS/MAPK pathway activation supports proliferation.

Slide 28 · Fusion-positive does not equal malignant

Molecular feature

Clonal driver rearrangement

Biologic outcome

May be benign, intermediate, or malignant

Nodular fasciitis can harbor MYH9::USP6 but usually behaves in a self-limited fashion.

Slide 29 · The behavioral continuum in soft tissue tumors

Benign

Intermediate / locally aggressive

Malignant

Benign lesions may require minimal treatment.

Intermediate/local-aggressive lesions can be infiltrative or recurrent without meaningful metastatic risk.

High-grade sarcomas may metastasize and cause death.

Slide 30 · Benign is not synonymous with small

Lipomas are generally soft, mobile, and painless.

Occasional benign lipomas may be large or intramuscular.

Size, depth, and margin are contextual findings rather than isolated proof of malignancy.

Slide 31 · Locally aggressive tumors demand a separate category

Growth

Infiltrative locally

Clinical risk

Recurrence; Functional injury

Metastasis

Not characteristic for desmoid tumor

Infiltrative growth creates morbidity through local tissue involvement.

Slide 32 · A sarcoma diagnosis requires biologic specificity

Aggressive soft tissue malignancies are classified as sarcomas.

The term is not applied uniformly to every locally aggressive nonmetastasizing tumor.

A precise tumor entity is more informative than the generic label “sarcoma.”

Slide 33 · Integrated classification has three evidence domains

Morphology

Immunohistochemistry

Molecular genetics

Integrated tumor classification

Morphology: Architecture, matrix, cytology, and differentiation.

Immunohistochemistry: Protein-expression evidence of lineage or state.

Molecular testing: Defining rearrangements or other genomic alterations.

Slide 34 · Morphologic pattern guides the first differential

Adipocytic

Mature fat / atypia / lipoblasts

Spindle-cell

Fascicles, collagen, vascular pattern

Round / pleomorphic

Cell shape, atypia, necrosis

Mature adipocytes suggest an adipocytic tumor.

Slide 35 · Immunohistochemistry refines lineage assignment

Histologic hypothesis

IHC result

Integrated histotype

Marker patterns should answer a specific morphologic question.

A single stain rarely constitutes an entire diagnosis.

Combining immunophenotypic and molecular information with morphology.

Slide 36 · Molecular testing serves the diagnostic differential

Context + morphology

Selected genetic test

Behavioral assessment

Recurrent fusions support recognition of selected entities.

Results are strongest when predicted by clinical and morphologic features.

Genomic findings do not alone convey grade or stage.

Slide 37 · Grade and stage answer different questions

Grade

Stage

Histologic tumor characteristics

Anatomic extent of disease

Biologic aggressiveness

Local and metastatic spread

Pathologic prognostic dimension

Clinical-anatomic prognostic dimension

Slide 38 · A practical integrated diagnostic sequence

Age / site / depth

Histology / lineage

Ancillary testing

Entity + behavior

Establish clinical setting: Age, site, and superficial versus deep location.

Determine differentiation pattern and cytologic atypia in tissue.

Select ancillary tests only when they resolve a defined differential.

Slide 39 · Adipocytic lesions illustrate the full spectrum

Lipoma

Well-differentiated liposarcoma

Myxoid / pleomorphic liposarcoma

Lipoma is the most common benign adult soft tissue tumor.

Liposarcoma is as the commonest sarcoma in adulthood.

Subtype-level distinction matters because molecular pathways and outcomes differ.

Slide 40 · Lipoma: Characteristic presentation

Patient

Middle adulthood

Site

Superficial trunk or proximal limb

Clinical pattern

Soft, painless, mobile

Typically arises in middle adulthood.

Slide 41 · Conventional lipoma: Defining morphology

Original simplified schematic — not a histologic specimen

Usually well encapsulated and composed of mature adipocytes.

Adipocytic cells resemble normal fat at low and high power.

Rare examples are intramuscular, large, or poorly circumscribed.

Slide 42 · HMGA2 dysregulation in solitary lipoma

12q rearrangement

HMGA2 dysregulation

Benign lipoma

Chromosome 12q rearrangements are common in solitary lipomas.

They disrupt regulation of HMGA2, a chromatin-structure regulator.

A genomic alteration can occur in a lesion with benign clinical behavior.

Slide 43 · Lipomatosis is distinct from a solitary lipoma

Solitary lipoma

Single localized fatty mass

Lipomatosis

Multiple lipomas in a limb

Lipomatosis denotes multifocal lipomas involving a limb.

Slide 44 · Atypical adipocytic findings change the differential

Scattered atypical hyperchromatic stromal cells are described in well-differentiated liposarcoma.

Deep location can increase the clinical implications of an adipocytic lesion.

Benign mature adipose tissue alone should not determine a deep tumor’s final classification.

Slide 45 · Liposarcoma: Adult sarcoma of adipocytic differentiation

Deep proximal extremity

Retroperitoneum

Genetic subtype changes prognosis

Typically presents in deep soft tissue of proximal extremities or retroperitoneum.

The textbook places many patients in the fifth to sixth decades.

Different histologic subtypes have different genetics and behavior.

Slide 46 · Three principal liposarcoma subtypes

Well differentiated

MDM2 amplification

Myxoid

FUS::DDIT3 fusion

Pleomorphic

Complex genotype

Well-differentiated: Mature adipocytes with scattered atypical spindle cells.

Slide 47 · Well-differentiated liposarcoma: MDM2 amplification

MDM2 amplification

P53 degradation

Growth-control escape

MDM2 at 12q13–q15 is amplified.

MDM2 promotes proteasomal degradation of p53.

The pathway weakens an important tumor-suppressor response.

Slide 48 · MDM2 and p53: Pathogenetic direction

Increased MDM2

Reduced p53 activity

Tumor-cell survival / growth

The MDM2 protein negatively regulates p53 stability.

MDM2 overexpression can reduce p53-mediated suppressive responses.

The oncogenic driver here is amplification, not a characteristic fusion.

Slide 49 · Well-differentiated liposarcoma: Morphology

Original simplified schematic — not a histologic specimen

Mature adipocytes remain a dominant component.

Scattered stromal spindle cells show atypical, hyperchromatic nuclei.

These atypical stromal cells distinguish the lesion from a straightforward lipoma.

Slide 50 · ALT versus well-differentiated liposarcoma

Resectable extremity

Atypical lipomatous tumor

Retroperitoneum

Well-differentiated liposarcoma

“atypical lipomatous tumor” for resectable extremity lesions.

Slide 51 · Why site matters in well-differentiated adipocytic tumors

Anatomic compartment

Resectability / recurrence

Clinical outcome

Extremity lesions may be completely removed and behave indolently.

Retroperitoneal lesions are more difficult to eradicate completely.

Repeated local recurrence can produce major morbidity even without high-grade morphology.

Slide 52 · Myxoid liposarcoma: Fusion-driven disease

FUS::DDIT3

Differentiation arrest

Myxoid tumor phenotype

The t(12;16) rearrangement yields FUS::DDIT3.

A differentiation-arrest mechanism is proposed.

The characteristic tumor contains primitive adipocytic cells in a myxoid background.

Slide 53 · Myxoid liposarcoma: Pattern recognition

Original simplified schematic — not a histologic specimen

Abundant basophilic myxoid extracellular matrix.

Delicate arborizing capillary network.

Primitive round-to-stellate cells and immature adipocytes/lipoblasts.

Slide 54 · Myxoid liposarcoma: An unusual metastatic site

Bone is a recognized metastatic destination in myxoid liposarcoma.

This differs from the predominant metastatic patterns of many other soft tissue sarcomas.

Histotype-specific patterns can influence clinical interpretation.

Slide 55 · Pleomorphic liposarcoma: Genomically complex

Schematic genomic instability — not a measured karyogram

Schematic only (not patient cytogenetics)

Slide 56 · Pleomorphic liposarcoma: Morphology and behavior

Original simplified schematic — not a histologic specimen

Sheets of anaplastic cells with bizarre nuclei.

Variable numbers of immature adipocytes/lipoblasts.

Aggressive behavior with frequent metastasis, unlike many well-differentiated lesions.

Slide 57 · Major liposarcoma forms compared

Subtype

Molecular profile

Microscopic anchor

Well differentiated

MDM2 amplification

Mature fat + atypical spindle cells

Myxoid

FUS::DDIT3

Myxoid matrix + delicate vessels

Pleomorphic

Complex abnormalities

Anaplastic cells + lipoblasts

Slide 58 · Liposarcoma classification is clinically consequential

Morphology and molecular events classify the tumor; Site modifies consequences.

A well-differentiated retroperitoneal tumor may be morbid through repeated local recurrence.

Myxoid and pleomorphic subtypes carry metastatic potential, with the latter particularly aggressive.

Slide 59 · Lipoma versus well-differentiated liposarcoma

Feature

Lipoma

WDLPS / ALT

Typical level

Often superficial

Often deep

Microscopic finding

Mature adipocytes

Mature fat + atypical spindle cells

Molecular association

HMGA2 rearrangement

MDM2 amplification

Behavior

Benign

Site-dependent local morbidity

Slide 60 · Fibroblastic tumors: Different behavior with similar cells

Nodular fasciitis

Deep fibromatosis

Adult fibrosarcoma

The fibrous lineage spans self-limited proliferations, infiltrative fibromatoses, and malignant spindle tumors.

Spindle morphology does not uniquely imply malignancy.

Growth rate, nuclear atypia, mitotic character, and molecular data refine classification.

Slide 61 · Nodular fasciitis: Rapid growth can still be benign

Site

Forearm, chest, back

Tempo

Rapid, weeks to months

Behavior

Usually self-limited

A self-limited fibroblastic/myofibroblastic proliferation.

Slide 62 · Nodular fasciitis: Why reactive was reconsidered

Rapid nodule

MYH9::USP6 fusion

Clonal self-limited lesion

Regression and occasional preceding trauma led to a historical reactive interpretation.

Detection of a recurrent USP6 fusion demonstrated clonal proliferation.

Clonality and transient growth are biologically compatible.

Slide 63 · MYH9::USP6: Molecular signature of nodular fasciitis

MYH9::USP6

Possible Wnt/β-catenin activity

Self-limited proliferation

Robbins records a rearrangement involving MYH9 and USP6.

The fusion may augment Wnt/β-catenin signaling.

It provides molecular evidence against a purely nonspecific reactive process.

Slide 64 · Nodular fasciitis: Tissue-culture-like histology

Original simplified schematic — not a histologic specimen

Plump, immature-appearing fibroblasts and myofibroblasts.

Loose, randomly oriented growth resembling cells in tissue culture.

May involve subcutis, fascia, deep dermis, or muscle.

Slide 65 · Nodular fasciitis: Maturation across the lesion

Original simplified schematic — not a histologic specimen

Hypercellular myxoid and hypocellular fibrous zones may coexist.

Extravasated erythrocytes, giant cells, and lymphocytes can occur.

A zonation pattern helps recognize the entity.

Slide 66 · Frequent mitoses do not always mean sarcoma

Nodular fasciitis

Mitoses may be frequent; Atypical forms absent

Malignancy concern

Assess atypia, architecture, necrosis and context

Nodular fasciitis can show conspicuous mitotic activity.

Slide 67 · Nodular fasciitis versus malignant spindle-cell tumors

Criterion

Nodular fasciitis

Concern for sarcoma

Growth

Rapid but self-limited possible

May also grow rapidly

Cytology

Plump but relatively bland

May show marked atypia

Mitoses

Frequent; atypical absent

Atypical mitoses may occur

Molecular clue

MYH9::USP6

Depends on tumor subtype

Slide 68 · Representative soft tissue tumor families

Lineage

Benign example

Malignant example

Adipocytic

Lipoma

Liposarcoma

Smooth muscle

Leiomyoma

Leiomyosarcoma

Vascular

Hemangioma

Angiosarcoma

Peripheral nerve sheath

Schwannoma / neurofibroma

Malignant peripheral nerve sheath tumor

Slide 69 · Other diagnostic archetypes among sarcomas

Synovial sarcoma

Spindle ± pseudoglandular

Undifferentiated sarcoma

High-grade pleomorphic / anaplastic

Fibrosarcoma

Monomorphic spindle fascicles

Synovial sarcoma: Uniform spindle fascicles and sometimes pseudoglandular structures.

Slide 70 · Site and age can sharpen the differential

Superficial trunk

Lipoma context

Retroperitoneum

WDLPS context

Young adult forearm

Nodular fasciitis context

A superficial middle-aged patient with a mobile fatty mass fits common lipoma settings.

Slide 71 · What molecular results can—and cannot—establish

Identity

Biologic behavior

Extent

Integrated tumor classification

A recurrent molecular alteration may strengthen tumor-entity assignment.

The same genomic testing does not replace assessment of local behavior or metastatic extent.

A benign self-limited proliferation may also harbor an oncogenic fusion.

Slide 72 · Case 1: A mobile superficial fatty mass

Illustrative case — not a real patient

A hypothetical middle-aged patient has a slow-growing painless, mobile subcutaneous mass over the proximal arm.

Excision shows a well-circumscribed lesion of mature adipocytes.

Question: Which diagnosis best fits, and what molecular pathway can be involved?

Slide 73 · Case 1: Clinicopathologic resolution

Diagnostic reasoning

Conventional lipoma best matches this presentation and morphology.

A subset is associated with chromosome 12q rearrangements altering HMGA2.

Absence of atypical stromal cells helps distinguish this pattern from well-differentiated liposarcoma.

Slide 74 · Case 2: A deep adipocytic tumor

Illustrative case — not a real patient

A hypothetical patient in their 50s has a large retroperitoneal fatty mass.

Biopsy shows predominantly mature fat with scattered atypical hyperchromatic stromal cells.

Question: How would tumor site and MDM2 status affect the classification?

Slide 75 · Case 2: Diagnostic and behavioral resolution

Diagnostic reasoning

The morphology supports well-differentiated liposarcoma rather than ordinary lipoma.

MDM2 amplification fits the subtype.

Retroperitoneal location makes complete removal harder and local recurrence clinically significant.

Slide 76 · Case 3: A rapidly growing forearm nodule

Illustrative case — not a real patient

A hypothetical young adult develops a subcutaneous forearm nodule enlarging over several weeks.

Histology shows plump myofibroblasts in a loose matrix with numerous ordinary mitoses.

Question: Does rapid growth mandate a sarcoma diagnosis?

Slide 77 · Case 3: Recognizing a self-limited neoplasm

Diagnostic reasoning

Nodular fasciitis is a leading consideration in the described setting.

MYH9::USP6 can support a clonal lesion with self-limited behavior.

Atypical mitotic figures and other malignant features must still be evaluated.

Slide 78 · Integrated summary: Five governing principles

Most soft tissue tumors are benign; The sarcomas are heterogeneous and consequential.

Tumor entity is defined by integrating location, morphology, immunophenotype, and molecular findings.

Simple fusion-driven and complex genomically unstable sarcomas differ in biology.

A driver alteration does not, by itself, establish malignancy.

Liposarcoma subtype and anatomic site materially shape behavior.

Slide 79 · References and source notes

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026). Section: “Soft Tissue Tumors.”

WHO Classification of Tumours Editorial Board. Soft Tissue and Bone Tumours. WHO Classification of Tumours, 5th ed., vol. 3. IARC, 2020.

Mechanism and morphology figures in this deck are original editable teaching schematics—not diagnostic specimen photographs.

Molecular and behavioral examples emphasize the textbook account; WHO (2020) is included for classification context.