Benign Fibrous Histiocytoma Dermatofibroma

Slide 1 · Benign Fibrous Histiocytoma

(Dermatofibroma)

Clinical presentation · Pathogenesis · Morphology · Differential diagnosis

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives: Recognize and distinguish dermatofibroma

AT THE END OF THIS LECTURE

Recognize the usual clinical phenotype and indolent course.

Explain the uncertain lineage and evidence of true neoplasia.

Identify defining dermal, epidermal, and collagen-related microscopic findings.

Distinguish common and selected atypical variants from dermatofibrosarcoma protuberans (DFSP).

Choose a morphology-led diagnostic and management approach.

Slide 3 · The dermis is the anatomic compartment of origin

WHY LOCATION MATTERS

Dermis contains fibroblastic, vascular, neural, and other mesenchymal elements.

Dermatofibroma is primarily a dermal spindle-cell proliferation.

Epidermal alterations above the lesion are secondary diagnostic clues.

Subcutaneous extension is possible and must be interpreted in context.

Schematic — not a histologic specimen

Slide 4 · Dermatofibroma is a benign fibrohistiocytic neoplasm

CORE CONCEPT

Represents the commonest form within the benign fibrous histiocytoma family.

Morphologically related lesions have uncertain, probably heterogeneous lineage.

Typical lesions are localized, slowly evolving, and biologically indolent.

The term “benign” describes the usual course; It does not replace morphologic assessment of variants.

Slide 5 · Related names do not imply identical biology

DIAGNOSTIC COMPARISON

Dermatofibroma / BFH

Common benign dermal fibrohistiocytic lesion.

May show epidermal acanthosis and collagen entrapment.

Typically small and confined largely to dermis.

Dermatofibrosarcoma protuberans

Distinct infiltrative dermal neoplasm.

Often grows into fat in a honeycomb pattern.

Has characteristic COL1A1::PDGFB pathway in many cases.

Slide 6 · A firm tan-to-brown papule is the clinical prototype

CLINICAL RECOGNITION

Firm papule or small nodule with tan to brown coloration.

Most lesions measure less than 1 cm.

Common clinical sites include the lower extremities.

Clinical appearance must be interpreted alongside evolution and symptoms.

Clinical schematic

Slide 7 · Patient distribution: A lesion encountered in adults

TYPICAL PATIENT CONTEXT

Usually recognized in adulthood.

Particularly frequent on the legs of younger and middle-aged women.

Occurs across ethnic groups and can arise elsewhere on the skin.

Demography modifies clinical suspicion but is not itself diagnostic.

Slide 8 · The lower leg is a common but not exclusive site

ANATOMIC DISTRIBUTION

Lower extremity location supports—but does not establish—the clinical diagnosis.

The lesion may occur on the trunk or upper extremities.

A site unusual for the common phenotype should trigger closer assessment of morphology and history.

Clinical schematic

Slide 9 · Symptoms range from none to localized tenderness

CORE CONCEPT

Many dermatofibromas are asymptomatic.

Some are tender or irritated by repetitive friction.

Symptoms alone cannot separate benign dermatofibroma from its mimics.

A new symptom in a changing lesion warrants clinical reassessment.

Slide 10 · Size evolution often reflects a slow, fluctuating course

Initial papule

Small fluctuations

Long-term persistence

Possible flattening

MECHANISTIC REASONING

Slight increases or decreases in size over time.

A growing lesion may occasionally reach several centimeters; Most remain small.

Abrupt, disproportionate growth is a reason to reassess rather than assume a typical course.

Slide 11 · Most lesions are small; Size is context dependent

CORE CONCEPT

Most dermatofibromas are under 1 cm in diameter.

Actively growing lesions occasionally become several centimeters across.

Large size alone is not a histologic diagnosis.

Size, depth, growth pattern, and cytology should be considered together.

Slide 12 · The dimple sign reflects tethering of the overlying skin

CLINICAL EXAMINATION

Lateral compression may cause central depression of a typical dermatofibroma.

Dimpling is a supportive clinical observation, not a definitive diagnostic test.

Absence of the sign does not exclude a variant dermatofibroma.

Clinical schematic

Slide 13 · Dermoscopic clues can support clinical recognition

SUPPLEMENTARY CLINICAL CORRELATION

A central white scar-like area with a peripheral pigment network is a common dermoscopic pattern.

Dermoscopic appearances are variable across lesions and skin tones.

Asymmetry, ulceration, or atypical evolution may justify biopsy.

Histopathology resolves uncertainty when clinical findings are discordant.

Slide 14 · Clinical assessment asks three distinct questions

Does it look typical?

Is it evolving atypically?

Is tissue diagnosis required?

MECHANISTIC REASONING

Combine distribution, firmness, size, tenderness, and history.

Consider dermoscopy as supportive rather than conclusive evidence.

Proceed to tissue assessment when clinical uncertainty could change care.

Slide 15 · Clinical-to-pathologic correlation is multidimensional

STRUCTURED APPRAISAL

Surface

Tan or brown color

Possible dimple sign

Epidermal thickening may be present

Dermal mass

Firm spindle-cell lesion

Collagen trapping

Mid-dermal centered

Biologic behavior

Usually indolent

Variable slight size changes

Clinical red flags require review

Slide 16 · The precise initiating cause remains unresolved

PATHOGENESIS

Some lesions follow an antecedent minor injury.

An abnormal repair or inflammatory response has been proposed.

Other lesions carry kinase-related fusion genes, favoring a neoplastic process.

Reactive and neoplastic observations should not be collapsed into a single proven pathway.

Slide 17 · The trauma-associated hypothesis is a model, not a proof

Antecedent injury

Inflammatory / repair response

Fibroblastic proliferation

Collagen remodeling

MECHANISTIC REASONING

Analogy with hypertrophic scar or keloid is proposed.

Association with trauma cannot by itself establish causation.

The molecular findings suggest that some lesions are true neoplasms.

Slide 18 · Kinase-fusion evidence supports true neoplasia in a subset

Somatic gene fusion

Altered kinase signaling

Clonal proliferation

MECHANISTIC REASONING

Rearrangements involving kinases including ALK and protein kinase C are reported in subsets.

No single driver is claimed to explain all conventional dermatofibromas.

Fusion-associated subsets help reconcile lesion heterogeneity.

Slide 19 · Epithelioid fibrous histiocytoma highlights molecular diversity

CORE CONCEPT

Epithelioid fibrohistiocytic lesions may carry ALK rearrangements.

Some authorities regard this phenotype as distinct from conventional dermatofibroma.

An ALK result must be interpreted alongside cell morphology and lesion architecture.

A molecular association is not evidence that every dermatofibroma is ALK driven.

Slide 20 · Dermal dendritic-cell differentiation is implicated

CORE CONCEPT

Dermal dendritic cells a likely partial component of lineage.

The historical term fibrous histiocytoma reflects mixed fibroblastic/histiocytic appearances.

Immunophenotype can support differentiation, but lineage is not uniquely established by a single stain.

Avoid overspecifying the cell of origin.

Slide 21 · Reactive and neoplastic models are not interchangeable

DIAGNOSTIC COMPARISON

Reactive hypothesis

Reported history of local trauma in some cases.

Morphology may resemble altered scar-like collagen deposition.

Does not explain all lesions or prove initiating mechanism.

Neoplastic evidence

Somatic kinase-related fusions found in some cases.

Supports autonomous proliferation in at least a subset.

Does not define one universal pathway for the entire family.

Slide 22 · The classic lesion is a nonencapsulated dermal nodule

Schematic — not a histologic specimen

LOW-POWER ARCHITECTURE

Bland spindle-cell proliferation is centered in the mid-dermis.

Typical mass is identifiable but lacks a true capsule.

Overlying epidermis often shows reactive thickening.

The relationship to collagen and fat aids recognition.

Slide 23 · Gross morphology: Firm, tan-to-brown, circumscribed papule

MACROSCOPIC PATTERN

Surface lesion commonly appears as a firm papule.

Pigmentation ranges from tan to brown.

The clinical border may be more apparent than the microscopic border.

Morphologic examination determines whether an atypical lesion remains consistent with BFH.

Clinical schematic

Slide 24 · Overlying epidermal hyperplasia is a recurring diagnostic clue

Epidermal hyperplasia

Schematic — not a histologic specimen

READ THE MORPHOLOGY

Many lesions exhibit thickened epidermis above the dermal mass.

Rete ridges often extend downward.

The surface reaction may be disproportionate to the cytologic blandness beneath.

Not every dermatofibroma shows identical epidermal change.

Slide 25 · Pseudoepitheliomatous hyperplasia has a precise meaning

Elongated rete ridges

Schematic — not a histologic specimen

EPIDERMAL RESPONSE

The rete architecture is elongated and can be pigmented.

This reactive-appearing epithelial proliferation overlies a mesenchymal lesion.

“Pseudoepitheliomatous” does not mean invasive squamous carcinoma.

Always inspect the underlying dermis before labeling a reactive epithelial change neoplastic.

Slide 26 · Basal epidermal hyperpigmentation alters lesion color

Basal pigmentation

Schematic — not a histologic specimen

MICROSCOPIC–CLINICAL LINK

Basal-layer pigmentation may be accentuated above the lesion.

This contributes to a clinically brown appearance in some cases.

Surface pigmentation alone cannot distinguish a dermal fibrohistiocytic lesion from a melanocytic mimic.

Slide 27 · Bland spindle cells are the principal lesional component

Bland spindle-cell pattern

Schematic — not a histologic specimen

READ THE MORPHOLOGY

Cells are elongated with relatively benign-appearing nuclei.

The proliferation usually occupies a collagen-rich dermal background.

Cytologic evaluation should be integrated with growth pattern and lesion depth.

Marked atypia demands variant assessment and exclusion of mimics.

Slide 28 · The lesion is centered in the mid-dermis

Mid-dermal location

Schematic — not a histologic specimen

MAPPING TUMOR DEPTH

Mid-dermal localization is characteristic.

Relatively superficial epidermal changes may accompany deeper spindle-cell proliferation.

A biopsy limited to one component can obscure the full diagnostic pattern.

Slide 29 · Circumscribed does not mean encapsulated

DIAGNOSTIC COMPARISON

Architectural circumscription

A recognizable tumor is demarcated from nearby dermis.

Margins can merge microscopically with native collagen.

This is the usual meaning of well-defined BFH.

True fibrous capsule

An external encasing layer completely surrounds a mass.

A typical dermatofibroma lacks this complete capsule.

Capsule absence alone is not evidence of malignancy.

Slide 30 · Peripheral collagen entrapment is especially informative

Entrapped collagen bundles

Schematic — not a histologic specimen

DERMAL INTERFACE

Spindle cells invest and surround individual collagen bundles.

Entrapped native collagen helps localize the advancing lesion boundary.

A sampled region without the edge may omit this classic clue.

Use the finding in combination with overall architecture.

Slide 31 · Why collagen trapping helps lesion recognition

Dermal collagen present

Spindle cells proliferate

Bundles become invested

Entrapment recognized

MECHANISTIC REASONING

The finding reflects interactions between lesional cells and pre-existing matrix.

It supports a dermatofibroma pattern but is not a standalone molecular criterion.

Slide 32 · Occasional subcutaneous extension requires contextual assessment

Possible focal fat extension

Schematic — not a histologic specimen

READ THE MORPHOLOGY

Lesional cells can extend into subcutaneous fat.

This finding does not automatically convert a benign lesion to DFSP.

Broad infiltration, architecture, and ancillary immunohistochemistry help resolve difficult cases.

Slide 33 · A combined architectural pattern is more diagnostic than a single sign

Integrated architecture

Schematic — not a histologic specimen

READ THE MORPHOLOGY

Mid-dermal spindle-cell proliferation with bland morphology.

Reactive acanthosis with elongated or pigmented rete ridges.

Collagen entrapment at the periphery.

Nonencapsulation with an otherwise predominantly circumscribed outline.

Slide 34 · Cytologic blandness does not eliminate all diagnostic pitfalls

CORE CONCEPT

Typical cells show no conspicuous malignant pleomorphism.

Cellularity, atypia, mitoses, depth, and vascular spaces vary among subtypes.

Atypical histology requires more than the label “dermatofibroma.”

Borderline cases should be evaluated for DFSP and other dermal spindle-cell entities.

Slide 35 · Conventional dermatofibroma: Integrated feature checklist

Domain

Expected finding

Diagnostic use

Clinical

Small, firm, tan–brown papule

Supports but does not prove diagnosis

Compartment

Predominantly dermal proliferation

Distinguishes superficial and deep pattern

Cytology

Bland spindle cells

Favors common BFH phenotype

Epidermis

Hyperplasia / pigmented rete

Associated surface response

Matrix

Collagen entrapment

Helpful peripheral pattern

Margin

Nonencapsulated but defined

Avoids equating no capsule with malignancy

Slide 36 · Histologic variants are a family of patterns

STRUCTURED APPRAISAL

Cell-rich

Cellular variant

More densely cellular spindle cells

May be diagnostically challenging

Blood-related

Aneurysmal variant

Blood-filled spaces

Hemosiderin and hemorrhagic change

Unusual cytology

Epithelioid or atypical patterns

May have distinctive molecular associations

Require appropriate differentials

Slide 37 · Cellular dermatofibroma shows increased tumor density

Increased spindle-cell density

Schematic — not a histologic specimen

VARIANT MORPHOLOGY

More crowded spindle cells than in the common form.

May show more extensive subcutaneous growth.

Some lesions have increased mitotic activity and more conspicuous storiform architecture.

Greater overlap with DFSP requires careful sampling.

Slide 38 · Cellular variants merit particular attention to margins

CORE CONCEPT

DermNet describes a greater chance of local recurrence than with conventional dermatofibroma.

Apparent infiltration and high cellularity invite a DFSP differential.

Immunostains may be useful but are not decisive in isolation.

Extent of excision and follow-up depend on the specific pathology and clinical context.

Slide 39 · Aneurysmal dermatofibroma contains extravascular blood

Schematic — not a histologic specimen

VARIANT MORPHOLOGY

Blood pools and hemorrhagic spaces may develop within the tumor.

Hemosiderin may accumulate around the spaces.

The lesion can look blue-brown and may enlarge after bleeding.

The apparent spaces are not equivalent to normal endothelial-lined vessels.

Slide 40 · Blood-filled spaces are not automatically a vascular neoplasm

DIAGNOSTIC COMPARISON

Aneurysmal dermatofibroma

Blood occupies cleft-like spaces in a fibrohistiocytic tumor.

Spaces lack a true endothelial lining.

Additional dermatofibroma architecture may be present.

True vascular structures

Vessels have endothelial lining.

Endothelial immunomarkers highlight actual vascular channels.

The dominant growth pattern must be assessed.

Slide 41 · Hemosiderotic dermatofibroma may mimic a pigmented lesion

CORE CONCEPT

Hemosiderin deposition can make the lesion appear dark.

Pigmentation should be localized morphologically: Epidermal melanin versus dermal iron.

A suspected melanocytic tumor requires an appropriate clinical and histologic differential.

Pigment alone cannot establish biologic lineage.

Slide 42 · Epithelioid fibrous histiocytoma is a distinct diagnostic challenge

CORE CONCEPT

Usually comprises epithelioid rather than ordinary slender spindle cells.

ALK rearrangements and ALK protein expression are described in many examples.

Some authorities separate this lesion from conventional BFH.

Morphology and immunophenotype are interpreted together.

Slide 43 · Atypical dermatofibroma can display alarming large cells

CORE CONCEPT

“Monster cells” or focal pleomorphic cells are recognized in atypical variants.

The presence of atypia mandates evaluation of the full lesion.

Rare aggressive outcomes have been reported, but historical case classification can be uncertain.

Do not extrapolate this behavior to all common dermatofibromas.

Slide 44 · Lipidized and atrophic variants broaden the morphology

CORE CONCEPT

Lipidized lesions can contain conspicuous foamy histiocytes and collagen sclerosis.

Atrophic lesions have reduced thickness and prominent altered collagen.

These patterns may be clinically or microscopically unfamiliar.

An integrated pattern-based diagnosis remains preferable to a single isolated finding.

Slide 45 · Variants: Practical discriminating clues

Variant

Feature to seek

Potential pitfall

Cellular

Crowded spindle cells; deeper growth

DFSP

Aneurysmal

Extravascular blood spaces

Vascular lesion

Hemosiderotic

Iron deposition / brown color

Melanocytic lesion

Epithelioid

Rounder eosinophilic cells; ALK association

Other epithelioid tumors

Atypical

Focal enlarged pleomorphic cells

Sarcoma

Atrophic/lipidized

Depression or lipid-rich histiocytes

Other fibroblastic processes

Slide 46 · DFSP is the critical spindle-cell malignant comparator

DIAGNOSTIC COMPARISON

Dermatofibroma

Usually a small firm papule.

Predominantly dermal with collagen entrapment.

Benign behavior in the conventional type.

Dermatofibrosarcoma protuberans

Slowly enlarging indurated plaque or protuberant nodule.

May infiltrate subcutaneous fat extensively.

Can recur locally; Metastasis is uncommon.

Slide 47 · Surface epidermis often points in opposite directions

DIAGNOSTIC COMPARISON

Dermatofibroma

Overlying epidermis may be thickened.

Downward elongated and sometimes hyperpigmented rete ridges.

Pseudoepitheliomatous pattern supports DF.

DFSP

Epidermis is often attenuated or thinned.

Surface may ulcerate in larger lesions.

The epidermal clue must be interpreted with depth and architecture.

Slide 48 · DFSP fat infiltration can produce a honeycomb pattern

Schematic — not a histologic specimen

COMPARATOR SCHEMATIC

DFSP commonly infiltrates adipose compartments from a dermal origin.

Isolated fat cells become surrounded by spindle-cell proliferation.

This broad infiltrative architecture differs from usual superficial BFH.

Minor fat extension in BFH means the feature is not absolute.

Slide 49 · Storiform growth is not uniquely diagnostic of DFSP

DIAGNOSTIC COMPARISON

In dermatofibroma

Variable swirled or short fascicular spindle-cell proliferation.

Collagen entrapment and reactive epidermis are useful companions.

Cellular BFH can be particularly storiform.

In DFSP

Tightly packed spindle cells often form a prominent radial “cartwheel.”

Usually accompanies an infiltrative, deeply extending growth pattern.

Do not diagnose DFSP from a storiform pattern alone.

Slide 50 · DFSP has a distinct COL1A1::PDGFB oncogenic mechanism

COL1A1–PDGFB rearrangement

PDGFB overexpression

Autocrine receptor signaling

Tumor-cell growth

MECHANISTIC REASONING

This pathway belongs to DFSP—not to conventional dermatofibroma.

The signaling mechanism to receptor-targeted therapy in selected advanced DFSP.

Slide 51 · Factor XIIIa and CD34 can support the distinction

DIAGNOSTIC COMPARISON

Dermatofibroma

Often factor XIIIa-positive in lesional cells.

Typically CD34-negative or only focally positive.

Staining patterns vary across BFH variants.

DFSP

Typically diffuse CD34 staining.

Factor XIIIa highlights mainly entrapped or peripheral dendritic cells.

A panel complements, rather than replaces, architecture.

Slide 52 · How to interpret a discordant immunostain panel

Observation

Appropriate interpretation

CD34 focal at periphery

Can occur in dermatofibroma, especially cellular cases.

CD34 diffuse in lesional spindle cells

Raises support for DFSP in the right architecture.

Factor XIIIa lesional labeling

Supports BFH, but not absolutely specific.

Overlapping or equivocal stains

Review full lesion, depth, and alternate markers as appropriate.

Scant superficial biopsy

May be insufficient to judge infiltrative architecture.

Slide 53 · Morphology-led diagnostic matrix: BFH versus DFSP

Feature

Dermatofibroma

DFSP

Typical skin site

Often leg

Often trunk

Surface epidermis

Hyperplasia frequent

Often thinned

Dermal interface

Collagen trapping

More infiltrative

Subcutis

Occasional extension

Honeycomb invasion common

Immunophenotype

FXIIIa+, often CD34–

Diffuse CD34 typical

Molecular pathway

Heterogeneous; kinase fusions in subset

COL1A1::PDGFB hallmark

Slide 54 · Other lesions can enter the clinical or histologic differential

CORE CONCEPT

Pigmented melanocytic lesions: Resolve with epidermal/dermal pattern and lineage assessment.

Hypertrophic scar or keloid: Trauma-associated collagen alteration, but no identical BFH architecture.

Vascular lesions: Genuine endothelial-lined channels rather than aneurysmal blood pools.

Other dermal spindle-cell neoplasms: Evaluated with depth, cytology, and targeted markers.

Slide 55 · Tissue diagnosis begins with architecture, not an antibody

Assess depth

Inspect epidermis

Assess lesional interface

Evaluate cells

Choose ancillary tests

MECHANISTIC REASONING

Look for collagen entrapment, subtype-specific features, and fat infiltration.

Order ancillary tests when the morphology and differential warrant them.

Reconcile discordant results rather than privileging one positive stain.

Slide 56 · Biopsy sampling determines which clues are accessible

STRUCTURED APPRAISAL

Surface

Can show acanthosis

May show hyperpigmented rete

Absent in deep-only sample

Lesion body

Shows spindle-cell density

Reveals hemorrhage or atypia

Helps subtype assignment

Deep interface

Can disclose fat invasion

Distinguishes penetration patterns

May be omitted by shallow sampling

Slide 57 · Clinically atypical lesions deserve histologic assessment

CORE CONCEPT

Consider biopsy when there is recent conspicuous enlargement, ulceration, or asymmetric pigmentation.

Histology should assess the whole growth pattern whenever feasible.

Tenderness or clinical pigmentation alone does not identify a malignant lesion.

Resolve the specific differential rather than using a nonspecific label.

Slide 58 · Usual dermatofibroma often needs no active treatment

CORE CONCEPT

A typical asymptomatic lesion can be clinically observed.

Reassurance is usually sufficient once the diagnosis is adequately established.

Removal may be appropriate for symptoms, cosmetic concerns, or unresolved diagnostic doubt.

Management is not interchangeable with that for DFSP.

Slide 59 · Excision strategy depends on the actual diagnosis

DIAGNOSTIC COMPARISON

Common dermatofibroma

Clinical observation can be reasonable when typical.

Symptomatic or uncertain lesions may be removed.

Routine oncologic margins are not a feature of usual DF.

DFSP / concerning variant

DFSP generally requires definitive complete surgical excision.

Cellular/atypical BFH may need closer margin assessment.

Specialist input is appropriate for disputed or higher-risk pathology.

Slide 60 · A useful pathology report communicates pattern and certainty

CORE CONCEPT

Identify the lesion and recognized variant when warranted.

Describe relevant epidermal, dermal, collagen, and deep-margin findings.

State atypia, cellularity, or hemorrhagic changes when diagnostically material.

Document ancillary results together with interpretive limitations.

State uncertainty and need for correlation when a differential remains unresolved.

Slide 61 · Case application: Stable firm nodule on the shin

CLINICAL VIGNETTE

A 32-year-old woman has a small, firm tan-brown papule on the lower leg that has changed little over several years. Histology shows bland spindle cells in the mid-dermis, epidermal hyperplasia, and collagen entrapment.

CLINICOPATHOLOGIC INTERPRETATION

Pattern is most consistent with conventional dermatofibroma.

The combination of location, evolution, and morphology supports a benign diagnosis.

Slide 62 · Case application: Hypercellular lesion extends into fat

CLINICAL VIGNETTE

A densely cellular cutaneous spindle-cell lesion involves the dermis and focally reaches subcutaneous tissue. Overlying epidermis is reactive; CD34 is focal rather than diffusely strong.

CLINICOPATHOLOGIC INTERPRETATION

Consider cellular dermatofibroma and DFSP in the differential.

Evaluate growth architecture, fat pattern, lesional factor XIIIa, and completeness of sampling.

Slide 63 · Case application: Suddenly darkening blue-brown nodule

CLINICAL VIGNETTE

A previously small dermal nodule enlarges and becomes blue-brown. Sections reveal bland fibrohistiocytic areas and irregular spaces filled with blood but lacking a vascular lining.

CLINICOPATHOLOGIC INTERPRETATION

Aneurysmal dermatofibroma is a coherent histologic explanation.

Exclude a true vascular tumor and consider appropriate excision assessment.

Slide 64 · Case application: Indurated plaque with fat honeycombing

CLINICAL VIGNETTE

A slow-growing indurated plaque on the trunk forms a protuberant nodule. Histology shows tightly packed storiform cells, attenuated epidermis, and honeycomb invasion of adipose tissue.

CLINICOPATHOLOGIC INTERPRETATION

The constellation supports dermatofibrosarcoma protuberans rather than ordinary DF.

Confirm the diagnosis with appropriate ancillary studies and plan definitive management.

Slide 65 · Knowledge check: Associate clue with interpretation

Clue

Most appropriate interpretation

Hyperpigmented elongated rete

Supports reactive epidermal response over BFH

Peripheral collagen entrapment

Useful classic BFH interface feature

Blood lakes without endothelial lining

Think of aneurysmal BFH

Diffuse lesional CD34 + honeycomb fat

Strongly consider DFSP

Kinase fusion involving ALK

Possible fibrohistiocytic molecular subset; not universal

Growing lesion with marked atypia

Reassess variant and malignant differential

Slide 66 · Integrated clinicopathologic synthesis

CORE CONCEPT

Dermatofibroma is usually a small, benign, firm cutaneous lesion.

The diagnostic core is a dermal spindle-cell proliferation with epidermal reaction and collagen entrapment.

Variant patterns explain deviations in cellularity, cytology, or hemorrhage.

Distinction from DFSP relies on architecture, fat infiltration, and supportive immunophenotype.

The reactive-versus-neoplastic debate is nuanced by kinase fusions in subsets.

Slide 67 · References and recommended diagnostic reading

PRIMARY AND SUPPLEMENTARY READING

Primary textbook

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed (2026). Ch 25, p 1044.

Dermatopathology

Cheng H, Emanuel P. Dermatofibroma pathology. DermNet NZ (2013).

Clinical correlation

Oakley A. Dermatofibroma. DermNet NZ (revised 2020).

Comparator

DermNet NZ. Dermatofibrosarcoma protuberans pathology.

Immunohistochemistry

CD34 and factor XIIIa in DF versus DFSP. Am J Dermatopathol (1993); PMID 7694515.