Residency · Residency · Dermatology

Actinic Keratosis: Field Therapy and Chemoprevention

Epidemiology and Significance

Actinic keratosis is the most common precancerous skin lesion, with a prevalence of 11 to 26 percent in Northern Hemisphere populations. It is more common in males, fair skin types (Fitzpatrick I and II), older individuals, and those with high cumulative UV exposure. The presence of AKs serves as a marker of significant photodamage and elevated risk for both SCC and BCC. A patient with more than 10 AKs has a 10 to 15 percent cumulative risk of developing at least one SCC over 10 years.

Pathogenesis

Field Cancerization Concept

UV-induced mutations create genetically altered but clinically normal keratinocyte clones throughout sun-damaged skin. Clinically visible AKs represent only the "tip of the iceberg," as subclinical AKs outnumber visible lesions. This concept of field cancerization explains why new AKs continuously arise in previously treated areas and provides the rationale for field-directed therapy. Key early mutations include TP53 (occurring very early in the process), CDKN2A, NOTCH1, RAS, and KNSTRN.

AK-to-SCC Progression (The Controversy)

The traditional view holds that AKs progress through a linear continuum from AK I (mild) to AK III (severe) to invasive SCC. The estimated progression rate for an individual AK to SCC is widely debated, ranging from 0.025 to 0.6 percent per lesion per year. However, up to 60 percent of SCCs arise in or adjacent to clinically identified AKs. An alternative view holds that SCCs can arise de novo without passing through recognizable AK stages, and not all AKs progress. The clinical implication is that because we cannot predict which individual AKs will progress, treatment of all AKs and the affected field is justified.

Histopathology

AK is graded histologically by the extent of epidermal atypia. AK I (mild) shows basal atypia confined to the lower third of the epidermis. AK II (moderate) shows atypia extending to the lower two-thirds. AK III (severe) shows full-thickness atypia, equivalent to SCC in situ or Bowen disease. Variants include hypertrophic (acantholytic), atrophic, pigmented, lichenoid, and proliferative AK. Bowenoid AK, which shows full-thickness dysplasia involving adnexal structures, must be distinguished from Bowen disease.

Clinical Features

AKs present as rough, scaly, erythematous macules or papules on sun-exposed skin. They are classically "better felt than seen," with a gritty "sandpaper" texture detected on palpation. Common sites include the bald scalp, face, ears, dorsal hands, and forearms. A cutaneous horn is a conical keratinous projection; while AK is the most common underlying diagnosis, SCC must be excluded. Hypertrophic AK displays thickened, adherent scale and can be difficult to distinguish from SCC clinically, warranting biopsy. Field signs of chronic photodamage include diffuse erythema, telangiectasia, dyspigmentation, laxity, and a sallow color.

<image>Clinical photograph showing multiple actinic keratoses on sun-damaged bald scalp with background photodamage illustrating the concept of field cancerization</image>

Lesion-Directed Therapy

Cryotherapy (Liquid Nitrogen)

Cryotherapy is the most common treatment for individual AKs. A 5- to 10-second freeze is used for thin AKs, with single or double freeze-thaw cycles. Clearance rates range from 67 to 99 percent depending on technique and freeze time. It is rapid, requires no prescription, and is well-tolerated. However, it treats only visible lesions without addressing the subclinical field, and carries a risk of hypopigmentation, especially in darker skin types. It does not provide histologic diagnosis.

Shave Removal / Biopsy

Shave removal is indicated for hypertrophic or suspicious AKs to exclude SCC. It serves both therapeutic and diagnostic purposes.

Curettage

Curettage is effective for thicker, hyperkeratotic AKs and can be combined with electrodesiccation.

Field-Directed Therapy

Topical 5-Fluorouracil (5-FU)

5-FU is a thymidylate synthase inhibitor that selectively destroys rapidly proliferating atypical keratinocytes. The 5 percent cream (Efudex) is applied twice daily for 2 to 4 weeks on the face and 3 to 6 weeks on extremities and scalp. Lower-concentration formulations include 0.5 percent cream (Carac) applied daily for 4 weeks and 4 percent cream applied daily for 4 weeks. Clearance rates range from 50 to 90 percent at 12 months depending on duration and compliance. The expected treatment course progresses through erythema, erosion, ulceration, crusting, and re-epithelialization over 2 to 4 weeks post-treatment. Advantages include treatment of subclinical lesions and an extensive evidence base. Limitations include significant inflammation, discomfort, cosmetic downtime, and poor compliance. The landmark VAC trial demonstrated that 5-FU field therapy reduced SCC risk by 75 percent at 1 year compared to placebo.

Imiquimod

Imiquimod is a TLR7 agonist that induces local interferon-alpha, TNF-alpha, and IL-12, activating innate and adaptive immunity. The 5 percent cream (Aldara) is applied twice weekly for 16 weeks or three times weekly for 4 weeks. The 3.75 percent cream (Zyclara) is applied daily for two 2-week treatment cycles separated by a 2-week rest period for the face and scalp. Clearance rates range from 45 to 85 percent depending on regimen. The immunologic mechanism may provide longer-lasting field clearance, but side effects include flu-like symptoms (fatigue, myalgia), significant local inflammation, and a longer treatment course.

Ingenol Mebutate (Picato) -- WITHDRAWN

Ingenol mebutate was previously approved for short-course (2 to 3 day) field therapy but was withdrawn from the market (2020 in the EU, with an FDA warning in 2020) after a long-term safety study showed increased incidence of skin cancer in the treatment area. It is no longer recommended.

Photodynamic Therapy (PDT)

PDT uses a topical photosensitizer that is preferentially absorbed by dysplastic keratinocytes. Activation by light generates reactive oxygen species that destroy the abnormal cells. ALA-PDT uses aminolevulinic acid with blue light at 417 nm after a 1-hour incubation. MAL-PDT uses methyl aminolevulinate with red light at 630 nm after a 3-hour incubation. Daylight PDT applies MAL with 2 hours of natural daylight exposure and is less painful with similar efficacy for thin AKs. Clearance rates are 70 to 90 percent for thin AKs but lower for thick or hypertrophic lesions. Advantages include excellent cosmesis and single-visit field treatment, while limitations include pain during light exposure (especially ALA-PDT), the potential need for multiple sessions, and cost.

Diclofenac 3% Gel

Diclofenac gel is a COX-2 inhibitor that reduces prostaglandin-mediated proliferation. It is applied twice daily for 60 to 90 days with clearance rates of 30 to 50 percent. It produces minimal inflammation and is well-tolerated but requires a long treatment duration, has lower efficacy than other field therapies, and is best suited for mild or thin AKs.

Tirbanibulin 1% Ointment (Klisyri)

Tirbanibulin is a dual Src kinase inhibitor and tubulin polymerization inhibitor FDA-approved in 2020. It is applied daily for 5 days on the face and scalp (up to 25 cm2). Complete clearance rates are approximately 44 to 54 percent at day 57. It offers a short treatment course with minimal to moderate local reactions, though long-term data remain limited and it is approved only for the face and scalp.

<image>Side-by-side clinical photographs showing the expected inflammatory response during topical 5-fluorouracil field therapy at baseline, week 2, and week 4, with healing at week 8</image>

Chemoprevention

Nicotinamide (Vitamin B3)

The ONTRAC trial demonstrated that nicotinamide 500 mg twice daily reduced new AK counts by 11 percent and new non-melanoma skin cancers (BCC plus SCC) by 23 percent at 12 months in high-risk patients. It is well-tolerated, inexpensive, and does not cause photosensitivity. However, the effect is not maintained after discontinuation. It is important to specify nicotinamide (niacinamide), not niacin, which causes flushing.

Systemic Retinoids

Acitretin at 10 to 25 mg per day reduces new SCC incidence by 25 to 30 percent in high-risk patients, particularly transplant recipients. It is teratogenic and contraindicated in women of childbearing potential, requiring 3 years of contraception after discontinuation. Side effects include cheilitis, xerosis, hyperlipidemia, and hepatotoxicity. The protective effect is lost upon discontinuation.

Sunscreen

Daily sunscreen use reduces AK counts and new SCC development. The Nambour trial demonstrated that regular sunscreen use reduced new SCC by 40 percent over 4.5 years. Broad-spectrum SPF 30 or higher with regular reapplication is recommended, combined with protective clothing and behavior modification.

Difluoromethylornithine (DFMO)

DFMO is an ornithine decarboxylase inhibitor studied in combination with sulindac. While there is some evidence for non-melanoma skin cancer chemoprevention, it is not in clinical use.

<image>Algorithm for management of actinic keratoses showing decision pathways for isolated lesions (lesion-directed cryotherapy), multiple AKs with field cancerization (field-directed therapy), and high-risk patients (chemoprevention strategies)</image>

Monitoring and Follow-Up

Clinical surveillance every 6 to 12 months is recommended for patients with a history of multiple AKs. Any AK that is hypertrophic, tender, rapidly enlarging, or non-responsive to standard treatment should be biopsied to exclude SCC. Post-treatment assessment at 8 to 12 weeks after field therapy evaluates clearance and determines the need for retreatment. AK recurrence is the rule rather than the exception; ongoing sun protection and periodic retreatment are expected.

Clinical Pearls

The phrase "better felt than seen" captures the clinical reality that palpation of a rough, gritty texture on sun-exposed skin is often more sensitive than visual inspection alone for detecting AKs. Field therapy (5-FU, imiquimod, PDT) is superior to lesion-directed therapy (cryotherapy) for patients with numerous AKs because it treats subclinical disease. The VAC trial provided the strongest evidence that a single course of topical 5-FU significantly reduces subsequent SCC development, supporting field therapy as cancer prevention. Ingenol mebutate has been withdrawn due to safety concerns and should no longer be prescribed. Nicotinamide 500 mg twice daily is the best-tolerated chemoprevention strategy for high-risk patients with multiple non-melanoma skin cancers; its effect ceases when stopped, so ongoing use is required.

References

• Jansen MHE, et al. Five-year results of a randomized controlled trial comparing effectiveness of photodynamic therapy, topical imiquimod, and topical 5-fluorouracil in patients with superficial basal cell carcinoma. J Invest Dermatol. 2018;138(3):527-533
• Thompson SC, et al. Reduction of solar keratoses by regular sunscreen use. N Engl J Med. 1993;329(16):1147-1151
• Chen AC, et al. A phase 3 randomized trial of nicotinamide for skin-cancer chemoprevention (ONTRAC). N Engl J Med. 2015;373(17):1618-1626
• Cantisani C, et al. Actinic keratosis pathogenesis update and new patents. Recent Pat Inflamm Allergy Drug Discov. 2019;13(2):105-113
• Weinstock MA, et al. Chemoprevention of basal and squamous cell carcinoma with a single course of fluorouracil, 5%, cream (VAC trial). JAMA Dermatol. 2018;154(2):167-174

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