Actinic Keratosis

Slide 1 · Actinic Keratosis

Sun-damaged keratinocytes · dysplasia · progression risk

Clinicopathologic teaching lecture

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Identify the clinical phenotype and distribution of actinic keratosis.

Explain epidermal dysplasia, keratinization, and dermal solar elastosis.

Relate ultraviolet injury to precursor lesions and squamous carcinoma.

Distinguish a keratosis from in situ and invasive carcinoma.

Explain the rationale and mechanisms of local lesion eradication.

Slide 3 · Actinic keratosis in the epidermal tumor spectrum

A localized lesion is understood in relation to progressive squamous atypia.

Sun-injured epidermis

→

Actinic keratosis

→

SCC in situ

→

Invasive SCC

These are morphologic states, not an inevitable sequence for every lesion.

The critical transitions are extent of atypia and basement-membrane breach.

Slide 4 · Core definition: A dysplastic keratinocytic lesion

An actinic keratosis arises most often in chronically sun-damaged skin.

It is associated with surface hyperkeratosis and basal epidermal atypia.

Progressively greater epithelial dysplasia may culminate in cutaneous SCC.

Interpretive anchor

Premalignant potential is a property of the lesion category—not a prediction for each plaque.

Slide 5 · Why “actinic” and why “keratosis”?

Actinic — causal setting

Keratosis — surface change

Sun exposure is the predominant association.

UV injury links skin damage to keratinocyte mutation.

Typical distribution favors exposed skin.

Stratum corneum is abnormally thickened.

Roughness can be more evident than color.

Pronounced keratin may create a horn.

Slide 6 · High-risk host phenotype

Light pigmentation is associated with a particularly high incidence.

The association is consistent with reduced protection against UV injury.

Cumulative exposure and host susceptibility act together.

Do not overextend the textbook statement

The source does not provide an individual risk score or a precise lesion-transformation percentage.

Slide 7 · Etiologic exposures linked to similar lesions

Ultraviolet exposure

Principal association in sun-damaged skin.

Typical exposure pattern involves uncovered sites.

Ionizing radiation

May induce lesions resembling AK.

Non-solar radiation is a separate exposure category.

Chemical carcinogens

Industrial hydrocarbons may contribute.

Arsenical exposure may induce similar lesions.

Slide 8 · Anatomic distribution is an etiologic clue

OBSERVED FEATURES

Face is a common involved site.

Arms and dorsal hands are frequently affected.

The lip can show an analogous actinic lesion.

WHY DISTRIBUTION MATTERS

A lesion’s location supports interpretation when it matches the pattern of cumulative environmental injury.

Slide 9 · Typical lesion size and clinical scale

Most lesions are described as less than 1 cm across.

Small size does not exclude biologically important dysplasia.

The clinical surface may be more conspicuous than the lesion’s elevation.

Size is descriptive, not a diagnostic cutoff

The source gives typical dimensions, not an absolute threshold or staging rule.

Slide 10 · Clinical color does not have one fixed appearance

Tan–brown

A pigmented lesion can retain the characteristic rough surface.

Color alone is insufficient for identification.

Red

Erythematous lesions are within the usual spectrum.

Inspection must be combined with surface assessment.

Skin-colored

Some lesions are subtle on visual inspection.

The rough consistency is an important clue.

Slide 11 · The characteristic tactile clue

CLINICAL RECOGNITION

A rough, sandpaper-like surface is typical.

The sensation is explained by abnormal keratin accumulation.

“Scaly” is a structural observation, not a histologic diagnosis.

Slide 12 · How excessive keratin produces surface scale

Clinical texture is a consequence of altered epidermal differentiation.

Atypical keratinocytes

→

Disordered maturation

→

Thick stratum corneum

→

Rough palpable scale

This is a conceptual explanation of the source morphology, not a proven single molecular pathway.

Slide 13 · Hyperkeratosis can dominate the gross appearance

The cornified surface may become markedly thickened.

An exuberant keratin mass can obscure the underlying dysplastic focus.

Gross keratin quantity does not alone define epithelial invasion.

Diagnostic separation

A horn describes keratin architecture; Histology is needed to establish the underlying lesion.

Slide 14 · A cutaneous horn: Architecture versus etiology

“Cutaneous horn” names the keratin projection, not a unique disease.

Excessive scale

→

Compact keratin buildup

→

Hornlike projection

The horn phenotype as a possible manifestation of actinic keratosis.

One must not equate a dramatic horn with confirmed invasive carcinoma.

Slide 15 · Actinic cheilitis: The lip counterpart

Sun-exposed lip epithelium may develop comparable actinic change.

The name actinic cheilitis for this presentation.

The shared pathologic theme is sunlight-associated squamous epithelial injury.

Vocabulary distinction

Cheilitis specifies the involved lip; Keratosis denotes the analogous cutaneous setting.

Slide 16 · Reading a rough plaque clinically

OBSERVED FEATURES

Record color as tan–brown, red, or skin-colored.

Assess rough scale and the degree of keratin buildup.

Consider whether the site matches chronic sun exposure.

Interpret all features together—not any feature alone.

CLINICOPATHOLOGIC CORRELATION

A coherent clinical description guides the differential; The defining cellular changes are microscopic.

Slide 17 · Transition from gross to microscopic anatomy

EPIDERMAL MICROSCOPY

The defining abnormality begins in epithelial cells—not in the visible scale alone.

The basal portion of the epidermis is the key initial compartment.

The stratum corneum and superficial dermis show associated injury patterns.

Slide 18 · Epidermal reference: Surface, basal layer, dermis

Keratinizing epithelium lies above the dermal connective tissue.

The superficial stratum corneum reflects terminal differentiation.

The basal compartment adjoins the epidermal–dermal junction.

Stratum corneum

Epidermis

Dermis

Schematic · not a histologic section

The schematic encodes compartments only; It is not an H&E photomicrograph.

Slide 19 · The central microscopic feature is basal atypia

Atypical keratinocytes populate low epidermal layers.

Nuclear and cytoplasmic irregularity marks epithelial dysplasia.

The upper epidermis may remain comparatively mature.

Stratum corneum

Epidermis

Dermis

Schematic · not a histologic section

Slide 20 · Basal hyperplasia can accompany dysplasia

The basal-cell population may be expanded.

Increased epithelial cellularity coexists with basal atypia.

This is one recognized architectural response in AK.

Stratum corneum

Epidermis

Dermis

Schematic · not a histologic section

Slide 21 · Epidermal atrophy is an alternative response

Some actinic keratoses have a thinner epidermis rather than hyperplasia.

Atypia cannot be ruled out because the epithelial compartment is attenuated.

Architecture and cytology must be assessed separately.

Stratum corneum

Epidermis

Dermis

Schematic · not a histologic section

Slide 22 · Hyperplasia and atrophy are not contradictory

Hyperplastic pattern

Atrophic pattern

Basal-cell proliferation is increased.

Epidermal thickness can be accentuated.

Dysplasia remains the defining lesion.

Epidermal thickness is reduced.

Atypical cells remain in basal layers.

Thin epithelium does not imply a harmless lesion.

Slide 23 · Dyskeratosis and eosinophilic cytoplasm

Atypical basal keratinocytes often show pink or reddish cytoplasm.

This appearance with dyskeratosis.

The observation links aberrant cellular keratinization to atypical morphology.

Terminology

Dyskeratosis describes abnormal premature keratinization of individual keratinocytes.

Slide 24 · Intercellular bridges help establish squamous identity

Intercellular bridges may be recognized between atypical keratinocytes.

This with basal cell carcinoma, where bridges are not visible.

This is a morphologic clue, not a stand-alone immunohistochemical test.

Slide 25 · Parakeratosis: The terminal differentiation defect

Abnormal cornification helps explain the characteristic scale.

Cornified surface thickens

→

Nuclei remain in cornified cells

→

Parakeratotic scale

In normally cornified epidermis, mature surface corneocytes lose nuclei.

Robbins explicitly describes retained nuclei in the stratum corneum.

Slide 26 · Hyperkeratosis versus parakeratosis

Hyperkeratosis

Parakeratosis

Thickness of stratum corneum is increased.

Clinically contributes to scale and horns.

Answers: “How much keratin?”

Nuclei persist in cornified cells.

Signals altered keratinocyte maturation.

Answers: “How is cornification occurring?”

Slide 27 · Solar elastosis: The dermal injury companion

Superficial dermis contains thickened, blue-gray elastic fibers.

The term elastosis for this appearance.

It likely reflects abnormal elastic-fiber production by sun-damaged fibroblasts.

Compartment logic

Epidermal dysplasia and dermal elastosis are related signatures of sun injury, not the same lesion.

Slide 28 · Three histologic compartments to examine

Cornified surface

Hyperkeratosis may be conspicuous.

Parakeratosis is commonly present.

Living epidermis

Basal keratinocytic atypia is central.

Hyperplasia or atrophy may occur.

Superficial dermis

Solar elastosis may be visible.

Elastosis points to chronic photodamage.

Slide 29 · Integrating surface scale with lower-layer atypia

The surface keratin is conspicuously expanded.

Keratinocyte atypia is concentrated toward the base.

Dermal solar injury supplies etiologic context.

Stratum corneum

Epidermis

Dermis

Schematic · not a histologic section

Schematic relationships only: Not a specimen Fig. 25.12.

Slide 30 · Morphologic observations and what they establish

Observation

What it supports

What it does not prove

Rough scale

Surface keratin abnormality

Degree of invasion

Basal atypia

Epidermal dysplasia

Full-thickness in situ disease

Dermal elastosis

Background sun injury

Specific lesion subtype

Cutaneous horn

Marked keratin production

A unique histologic diagnosis

Slide 31 · Progression is defined by atypia and invasion

INCREASING EXTENT OF ATYPIA

Orderly epidermis

Basal atypia (AK)

Full-thickness atypia

Breach of junction

Schematic illustration · conceptual epithelial compartments, not specimens

Slide 32 · Full-thickness atypia indicates SCC in situ

Atypical enlarged, hyperchromatic nuclei extend through the epidermis.

The basement membrane has not been breached.

This as squamous cell carcinoma in situ.

Stratum corneum

Epidermis

Dermis

Schematic · not a histologic section

Slide 33 · Invasive SCC crosses the basement membrane

Atypical squamous epithelium extends into dermal tissue.

Dermal invasion—not merely thick surface scale—establishes invasive disease.

The basement membrane is the decisive anatomic boundary.

Stratum corneum

Epidermis

Dermis

Schematic · not a histologic section

Slide 34 · Actinic keratosis versus SCC in situ

Actinic keratosis

Squamous carcinoma in situ

Basal or lower-epidermal atypia predominates.

Marked hyperkeratosis/parakeratosis possible.

Upper epidermal differentiation may persist.

Atypical nuclei involve all epidermal levels.

No invasion beyond basement membrane.

Often presents as a sharply defined red scaling plaque.

Slide 35 · Carcinoma in situ versus invasive SCC

Carcinoma in situ

Invasive SCC

Atypia extends through epidermal thickness.

Atypical cells remain above the junction.

Potential precursor to deeper invasion.

Epithelial nests or tongues enter dermis.

Nodular growth or ulceration may occur.

Keratinization varies with differentiation.

Slide 36 · From phenotype to mechanism

MOLECULAR PATHOGENESIS

Cumulative ultraviolet injury supplies a mutagenic background for keratinocytic neoplasia.

Specific molecular findings are described in the adjacent SCC section.

Mechanistic associations must not be treated as a universal deterministic pathway.

Slide 37 · UV injury and the precursor–carcinoma pathway

UV-mediated DNA damage as a dominant causal influence.

UV radiation

→

Keratinocyte DNA injury

→

Driver mutations

→

Clonal dysplasia

The adjacent squamous carcinoma section discusses mutations in precursors, including AK.

Slide 38 · The mutagenic signature: Pyrimidine dimers

Ultraviolet injury can create adjacent pyrimidine-base lesions.

DNA absorbs UV

→

Pyrimidine dimers form

→

Repair may fail

→

Mutation is retained

TP53 mutations in these tumors often map to UV-associated pyrimidine dimer sites.

Slide 39 · TP53 dysfunction appears early

TP53 mutations are frequent in AKs in lightly pigmented individuals.

The pattern supports early p53 pathway dysfunction during UV-driven tumorigenesis.

A precursor-associated mutation does not imply that invasion has already occurred.

Level of evidence in source

A high frequency is described, but no exact percentage is needed for this teaching point.

Slide 40 · P53 pathway: Lesion state versus molecular event

Histologic lesion

Molecular context

Basal atypia defines the morphological diagnosis.

Invasion remains a separately assessed feature.

Surface keratin and dermal elastosis add context.

TP53 changes may already be present in AK.

UV-type sequence damage supplies an etiologic link.

Molecular alteration is not synonymous with invasive SCC.

Slide 41 · DNA repair determines persistence of UV injury

Nucleotide excision repair removes UV-induced pyrimidine lesions.

UV-induced dimer

→

Repair recognition

→

Damaged segment excision

→

DNA restoration

The scheme is conceptual; Xeroderma pigmentosum to illustrate repair failure.

Slide 42 · Xeroderma pigmentosum is a causal illustration

Inherited defects in nucleotide excision repair impair dimer correction.

Error-prone handling permits rapid mutation accumulation.

The syndrome underscores why DNA maintenance limits skin carcinogenesis.

Slide 43 · Sunlight also alters local immune defense

A transient effect on cutaneous innate immunity.

UV exposure

→

Transient innate immune defect

→

Reduced elimination of injured cells

The precise mechanism is described as uncertain.

Do not portray this as a fully mapped pathway.

Slide 44 · Three distinct influences on keratinocyte evolution

Damage input

UV radiation introduces DNA lesions.

Other physical or chemical insults may contribute.

Genome maintenance

Repair competency modifies mutation persistence.

TP53 dysfunction may emerge early.

Host response

Immune surveillance can help remove damaged cells.

UV-associated impairment is mechanistically uncertain.

Slide 45 · Later SCC drivers are not AK diagnostic criteria

RAS pathway activation and reduced Notch signaling are described in cutaneous SCC.

These are mechanisms contributing to multistep malignant transformation.

Robbins does not define an AK diagnosis by routine RAS/NOTCH testing.

Slide 46 · Natural history has several possible paths

Regression

Some lesions diminish spontaneously.

Current lesion behavior is not fixed.

Persistence

Some lesions remain stable for years.

Stability does not eliminate precursor significance.

Progression

Some evolve toward squamous carcinoma.

Potential for malignancy motivates treatment.

Slide 47 · Natural history must not be oversimplified

RISK INTERPRETATION

A lesion may regress or persist, yet the group has meaningful malignant potential.

The source supports eradication, not a numerical patient-specific prediction.

A deterministic “every AK becomes SCC” claim is incorrect.

Slide 48 · What can—and cannot—be concluded from AK

Finding

Supported inference

Not supported

Sun-exposed site

Etiologic plausibility

Specific mutation status

Basal epidermal atypia

Precancerous morphology

Already invasive SCC

Persistent lesion

One possible disease course

Guaranteed future carcinoma

Hyperkeratotic horn

Marked surface keratin

Invasion by appearance alone

Slide 49 · Actinic keratosis and basal cell carcinoma differ

Actinic keratosis

Basal cell carcinoma

Atypical squamous cells near epidermal base.

Intercellular bridges may be apparent.

May progress toward squamous carcinoma.

Separate neoplasm associated with Hedgehog activation.

Intercellular bridges are not visible.

Not the endpoint of an AK-to-SCC sequence.

Slide 50 · A focused differential based on architecture

Entity

Epidermal atypia

Defining boundary

Actinic keratosis

Mainly basal/lower layers

No invasive growth established

SCC in situ

Full epidermal thickness

Basement membrane intact

Invasive SCC

Variable epithelial differentiation

Dermal invasion present

Basal cell carcinoma

Basaloid tumor; Distinct phenotype

Separate Hedgehog-linked entity

Slide 51 · Diagnostic reasoning: Clinical to microscopic

Start with clinical morphology; Establish the tissue-level distinction.

Rough sun-exposed lesion

→

Evaluate lesion morphology

→

Assess depth of atypia

→

Determine presence of invasion

The source describes distinguishing morphology but does not issue a modern biopsy triage guideline.

Slide 52 · Why treatment is justified despite possible regression

The course of an individual lesion cannot be predicted from appearance alone.

Enough lesions transform to warrant local eradication.

Intervention addresses a lesion with established malignant potential.

Core principle

A preventive rationale does not mean every individual actinic keratosis is destined to become cancer.

Slide 53 · Local eradication methods

Gentle curettage

Mechanical removal of the local lesion.

A lesion-directed procedural approach.

Freezing

Local destruction through cryotherapy.

A lesion-directed physical approach.

Topical agents

Drug-based treatment of affected epithelium.

The text mentions topical chemotherapeutic agents.

Slide 54 · Imiquimod illustrates immunologic therapy

A topical approach may recruit host innate immune recognition.

Imiquimod application

→

TLR activation

→

Innate immune signaling

→

Response to dysplastic cells

Efficacy presumably to stimulation of host immunity; Causal wording remains qualified.

Slide 55 · Direct destruction and immune activation

Physical / cytotoxic strategies

Immune-oriented strategy

Curettage removes altered epithelium.

Freezing locally destroys affected tissue.

Topical chemotherapy targets lesional cells.

Imiquimod activates Toll-like receptors.

Innate immune response is engaged.

Immune activity may contribute to clearance.

Slide 56 · Interpreting the imiquimod efficacy statement

Eradication of up to half of lesions in the described context.

That statement is not a universal clearance rate for all regimens or populations.

The excerpt supplies no comparator arm or modern treatment ranking.

Evidence calibration

Avoid turning a textbook teaching figure into a personalized treatment probability.

Slide 57 · Applied case: Subtle facial plaque

Hypothetical teaching vignette

A lightly pigmented adult has a small facial lesion that is difficult to see but feels coarse and rough.

Which clinical observation most strongly supports actinic keratosis?

A. Rough sandpaper-like surface

B. A painless lesion

C. Uniform color

Slide 58 · Case interpretation: The surface is informative

The source specifically emphasizes sandpaper-like consistency.

Typical sun-exposed distribution increases etiologic plausibility.

Appearance supports a working diagnosis; Cellular dysplasia remains microscopic.

Slide 59 · Applied case: Markedly keratinized projection

Hypothetical teaching vignette

A small sun-exposed plaque produces a tall compact projection of surface keratin.

What is the most precise descriptive term?

A. Cutaneous horn

B. Invasive SCC confirmed clinically

C. Basal cell carcinoma

Slide 60 · Case interpretation: Horn does not equal carcinoma

A cutaneous horn may arise from excessive keratin in actinic keratosis.

Keratin accumulation is distinct from tumor-cell invasion.

The underlying epithelial architecture establishes the lesion category.

Slide 61 · Applied case: Basal keratinocyte atypia

Hypothetical teaching vignette

A conceptual epidermal section shows dysplastic cells mainly in lower epidermal layers, with parakeratosis and solar elastosis.

Which diagnosis best matches this pattern?

A. Actinic keratosis

B. SCC in situ

C. Invasive SCC

Slide 62 · Case interpretation: Lower-layer dysplasia

Basal-predominant atypia is the characteristic AK distribution.

Parakeratosis and elastosis reinforce the morphologic context.

Full-thickness atypia or basement-membrane breach would alter the classification.

Slide 63 · Applied case: Full-thickness epidermal atypia

Hypothetical teaching vignette

A conceptual section shows atypical nuclei in the entire epidermis, with no cells extending into dermis.

Which tissue-level conclusion follows?

A. Squamous cell carcinoma in situ

B. Actinic keratosis limited to basal cells

C. Invasive squamous carcinoma

Slide 64 · Case interpretation: In situ is not invasive

The distinguishing change is full-epidermal-thickness atypia.

Carcinoma remains in situ while the basement membrane is intact.

Invasive growth requires atypical tumor epithelium in underlying dermis.

Slide 65 · A final morphology-based classification exercise

READ FROM LEFT TO RIGHT, THEN NAME THE TISSUE BOUNDARY

Normal

Basal dysplasia

Carcinoma in situ

Invasive SCC

Schematic illustration · conceptual epithelial compartments, not specimens

Slide 66 · Three evidence layers in a good pathology diagnosis

Clinical morphology

Rough scale, size, color, distribution.

Cutaneous horn and lip involvement are variants.

Histologic morphology

Basal atypia, hyperkeratosis, parakeratosis.

Dyskeratosis and dermal elastosis may support.

Biologic context

UV-linked mutations, including TP53.

Uncertain progression and potential SCC evolution.

Slide 67 · Consolidation: Five high-yield propositions

AK is a sun-associated dysplastic keratinocytic lesion.

The surface is often rough and hyperkeratotic.

Atypia is concentrated in the lower epidermis.

Full-thickness atypia means SCC in situ; Dermal invasion means invasive SCC.

Lesion eradication is justified by nonzero malignant potential.

Slide 68 · References and source scope

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th ed. (2026).

Chapter 25, The Skin: Actinic Keratosis, pp. 1039–1040.

Adjacent context: Squamous Cell Carcinoma, pp. 1040–1041.

The lecture diagrams are original editable conceptual schematics—not histologic specimens.