Actinic keratosis and field change

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Examine by sight and touch across the whole field. Record thin versus thick keratosis, tenderness, induration, ulceration and lip or ear involvement and palpate regional nodes when invasion is suspected. Erythema may be subtle in darker skin; scale, surface friction and focal thickening remain detectable. Actinic keratosis is less common in deeply pigmented skin, but a suspicious lesion still requires equivalent urgency.

Choose between lesion-directed and field-directed care. Cryotherapy suits a small number of discrete lesions. Field therapy treats visible and subclinical dysplasia but produces a larger inflammatory burden. Shared decisions include treatment duration, work and caring commitments, ability to apply to a mapped area, expected pain, cosmetic outcomes, immune status and capacity for review.

Before prescribing 5-fluorouracil, mark the exact field, exclude invasive foci and explain that inflammation is expected but uncontrolled pain, infection, severe ulceration or systemic symptoms require contact. Review after healing, not at peak reaction alone. Persistent focal thickening after a technically adequate field course needs biopsy rather than repeating treatment indefinitely.

Key points

Actinic keratosis is a rough, sandpaper-like keratotic macule or papule on chronically ultraviolet-exposed skin and may be easier to feel than see.
Field change means multiple lesions plus background mottling, atrophy and telangiectasia across a sun-damaged anatomical area.
Biopsy a tender, indurated, rapidly enlarging, bleeding, ulcerated, thick or treatment-resistant lesion to exclude invasive SCC.
Observation with protection is reasonable for selected thin asymptomatic lesions in lower-risk patients when follow-up and safety-netting are reliable.
Cryotherapy is lesion directed; topical 5-fluorouracil, imiquimod, diclofenac, tirbanibulin or photodynamic therapy can address a field according to site and licence.
For 5-fluorouracil 5% cream, a common licensed field regimen is once or twice daily for three to four weeks, with site-specific specialist directions.
Explain the expected sequence of redness, soreness, erosion and healing before field treatment and supply a stop and contact plan.
Use shade, clothing, hats and broad-spectrum sunscreen while avoiding messages that imply past ultraviolet damage can be reversed completely.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Cumulative ultraviolet exposure

Repeated ultraviolet injury creates clonal atypical keratinocytes on chronically exposed scalp, face, ears, forearms and dorsal hands.

Phenotypic and age risk

Lighter skin, bald scalp, older age and outdoor work increase cumulative damage, while any skin tone can develop actinic neoplasia.

Immune suppression

Transplantation and other impaired immune surveillance increase lesion number, progression risk and coexistence of invasive SCC.

Field cancerisation

Clinically normal-appearing sun-damaged skin can contain multiple mutated clones, explaining new lesions after successful spot treatment.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Partial-thickness dysplasia
Atypical keratinocytes begin in basal epidermis and produce disordered maturation, hyperkeratosis and a rough adherent surface.
2
Clonal field expansion
Multiple independent or related dysplastic clones occupy a photodamaged area, only some of which are clinically visible.
3
Inflammatory hyperkeratosis
Variable keratin and inflammation create thin gritty macules through thick plaques and horns, with grade not reliably predicting invasion alone.
4
Progression to SCC
A minority acquire additional changes, breach basement membrane and become invasive SCC; no individual lesion’s future can be predicted perfectly.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Gritty thin keratosis

A poorly defined rough macule feels like fine sandpaper on scalp, face, ear or dorsal hand.

Thick hyperkeratotic lesion

Adherent scale or a horn increases diagnostic uncertainty and requires careful assessment of the indurated base.

Actinic field change

Multiple keratoses sit within mottled, wrinkled, telangiectatic and atrophic chronically sun-damaged skin.

Actinic cheilitis

Persistent roughness, blurring and scale of the lower-lip vermilion has higher-consequence SCC potential and needs specialist assessment.

Invasive warningRed flag

New pain, induration, rapid growth, ulceration, bleeding or treatment resistance requires urgent biopsy for SCC.

Red flags requiring action

Induration, tenderness, rapid growth, ulceration, bleeding, a thick cutaneous horn, lip involvement, treatment resistance or immune suppression raises invasive SCC concern and requires urgent diagnostic review.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Full field examinationFirst step
Why
Map lesion burden, background damage and any focus suspicious for invasion.
Interpretation and limitations
Count is less important than identifying a tender indurated outlier and high-risk host or site.
02
Dermoscopy
Why
Support actinic keratosis diagnosis and identify atypical vascular, follicular or keratin features.
Interpretation and limitations
Dermoscopy cannot exclude invasion when palpation or evolution is concerning.
03
Punch, shave or excision biopsy
Why
Assess a thick, tender, indurated, ulcerated or resistant lesion for SCC.
Interpretation and limitations
Sample deeply enough to assess basement membrane and dermis; superficial keratin alone is inadequate.
04
Baseline treatment-field photograph
Why
Define application boundaries and compare clearance after healing.
Interpretation and limitations
Use consent, secure storage, scale and consistent lighting; the image does not replace palpation.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Invasive SCC

Induration, pain, rapid growth, ulceration, bleeding or a thick recurrent horn requires biopsy rather than further field treatment.

Bowen disease

A larger persistent well-demarcated scaly plaque represents full-thickness in-situ SCC and may need biopsy and different treatment planning.

Seborrhoeic keratosis

A waxy stuck-on lesion with comedo-like openings favours benign keratosis, though irritated lesions can remain uncertain.

Inflammatory dermatitis

Itchy confluent scale with a product or eczematous distribution lacks discrete sandpaper keratoses but can coexist on damaged skin.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Few discrete lesionsTreat or observe individuallyFirst stepA small number of typical thin lesions has no invasive warning and host risk is low.

1Discuss observation versus lesion-directed cryotherapy based on symptoms, site and preference.
2For cryotherapy, obtain consent for pain, blister, pigment change and scar and use anatomy-appropriate freeze technique.
3Review non-clearance or focal thickening and biopsy rather than repeatedly freezing an uncertain lesion.

02Field cancerisationTreat the mapped fieldMultiple keratoses and subclinical photodamage occupy a defined area without suspected invasive SCC.

1Mark the field, photograph with consent and choose 5-fluorouracil or another licensed option according to site, immune status and patient capacity.
2Give exact application, expected inflammation, skin-care, infection and stop instructions and plan activity around the reaction.
3Review after epithelial healing for field response and biopsy any persistent indurated or hyperkeratotic focus.

03Possible invasive diseaseBiopsy before field treatmentPain, induration, ulceration, rapid growth, horn, lip involvement or treatment resistance is present.

1Stop empirical destructive or field therapy and document size, depth impression, symptoms, immune state and nodes.
2Arrange urgent specialist biopsy with sufficient depth to assess invasion.
3DefinitiveRoute confirmed SCC through the skin-cancer multidisciplinary pathway and reassess the surrounding field after definitive treatment.

Key medicines and prescribing safety1 treatment · regimens, roles and cautions

A licensed field treatment for actinic keratoses that targets visible and subclinical atypical keratinocytes.

Fluorouracil 5% cream

Apply a thin layer once or twice daily to the precisely defined actinic field for three to four weeks, or as directed for the anatomical site.

Avoid pregnancy, eyes and mucosa; review severe inflammation, infection and dihydropyrimidine dehydrogenase deficiency risk, wash hands and follow site-specific specialist instructions.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Invasive squamous-cell carcinoma

Progression produces a firm destructive tumour with metastatic potential, particularly in immune suppression or high-risk sites.

Field recurrence

New visible keratoses arise from the altered field even after complete treatment of every current lesion.

Treatment inflammation

5-fluorouracil, imiquimod and photodynamic therapy deliberately inflame dysplastic skin and can interrupt sleep, work and social activity.

Pigment and scar change

Cryotherapy and severe inflammatory reactions can produce lasting hypo- or hyperpigmentation and occasional scarring, especially after overtreatment.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Review during field therapy if pain, erosion or uncertainty threatens adherence and distinguish expected inflammation from infection or excessive injury.
Assess clearance after the reaction heals, palpating for persistent induration and not relying on colour alone.
Biopsy a focal lesion that persists, thickens, bleeds or becomes tender after adequate treatment.
Continue whole-field and whole-skin surveillance according to immune state and previous keratinocyte cancer burden.
Reinforce ultraviolet protection and self-awareness without presenting sunscreen as a guarantee against future cancer.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Fingers can diagnose

A thin actinic keratosis may be more obvious as roughness under a fingertip than as colour change.

Field treatment reveals damage

Previously invisible dysplastic foci often inflame during 5-fluorouracil, illustrating subclinical field cancerisation.

A horn is not a grade

Thick keratin does not reliably distinguish in-situ from invasive disease; the base determines urgency.

Reaction and cure differ

Severe redness does not prove complete clearance, and a modest reaction does not automatically prove failure.

11Common pitfallsFrequent interpretation and management errors.

01
Starting field cream over an indurated tender focus without excluding invasive SCC.
02
Failing to mark the treatment boundary and allowing accidental eye or lip exposure.
03
Interpreting expected 5-fluorouracil inflammation as allergy without assessing severity.
04
Repeating cryotherapy on a resistant thick lesion instead of obtaining histology.
05
Treating visible spots only while ignoring a heavily damaged surrounding field and future surveillance.