AK is the most common precancerous skin lesion, strongly associated with cumulative UVA + UVB exposure.Per-lesion annual SCC transformation rate ~0.025–16%, average ~1%/year, but actinic keratoses are often multiple, so the cumulative lifetime risk is significant.cSCC patients have a 30–50% 5-year risk of a second skin cancer, requiring long-term surveillance.
What is actinic keratosis (AK)?
AK is a UV-driven precancerous lesion confined to the epidermis. It presents as a rough, erythematous patch with overlying scale on chronically sun-exposed skin (face, scalp, ears, forearms, dorsal hands). Olsen grading divides AK into I (better felt than seen), II (visible and palpable), and III (thick, hyperkeratotic — must rule out SCC). Modern emphasis is on "field cancerization" — multiple AKs reflect widespread sub-clinical UV damage, requiring field-directed treatment rather than treating one lesion at a time (Eisen 2021, JAAD).
AK to SCC progression
Per-lesion annual transformation rate: 0.025–16% (average ~1%/year). 60–80% of cSCCs arise from or are adjacent to AK. Spontaneous regression occurs in ~20–30% within 1 year but recurs. High-risk groups: organ transplant recipients, immunosuppressed, ≥10 AKs, hypertrophic AK, lip/ear/scalp lesions.
Bowen disease (cutaneous SCC in situ)
Bowen disease is full-thickness epidermal SCC that has not yet breached the basement membrane. It presents as a sharply demarcated, erythematous, scaly plaque, commonly on the legs (women), scalp, trunk, perianal/genital. Lifetime invasive transformation: 3–5%. BAD 2022 R1: consider punch biopsy when diagnosis is uncertain. First-line: 5-FU 5% (R7 ⇈), cryotherapy (R14 ⇈), curettage with cautery (R18 ⇈), red-light PDT (R21 ⇈), surgical excision (R23 ↑↑) (Sharma 2022, BJD).
Invasive cSCC
cSCC is the second most common non-melanoma skin cancer. High-risk features (BAD 2020): tumour > 20 mm (or > 10 mm head/neck/hands/feet), depth > 4 mm, poor differentiation, perineural/lymphovascular invasion, ear/lip/scalp/genital sites, immunosuppression, recurrence. Treatment: standard surgical excision with ≥ 4–6 mm margin and ≥ 1 mm histological clearance (R5/R7 ↑↑); Mohs micrographic surgery (R16 ↑) for high-risk; SSMDT review for symptomatic/recurrent (R15/R27 ↑↑); radiotherapy when surgery not feasible (R18 ↑↑); regional lymphadenectomy + adjuvant RT for nodal disease (R32–R34 ↑↑); cemiplimab/immune checkpoint inhibitors for advanced/metastatic (R37 ↑) (Keohane 2020, BJD).
Prevention
Daily SPF ≥ 30 broad-spectrum sunscreen reduces AK and cSCC incidence by ~40% (Thompson 1993, Green 1999). Avoid 10 AM–4 PM peak UV; wide-brim hat, UPF 50+ clothing, UV400 sunglasses; no tanning beds (IARC group 1 carcinogen).
Follow-up
Low-risk cSCC: single 4–6-month post-treatment visit. High-risk: 5-year follow-up (every 3–6 months for years 1–2, then every 6–12 months). Educate patients on monthly self-examination; cSCC patients have a 30–50% risk of a second skin cancer within 5 years.
Bottom line
AK → Bowen → invasive cSCC is one continuous UV-driven spectrum. Early diagnosis, early treatment, lifelong sun protection, and structured follow-up are the cornerstones. For multiple AKs, switch from one-by-one cryotherapy to field-directed therapy (5-FU, imiquimod, PDT, tirbanibulin). Any AK that suddenly enlarges, hardens, ulcerates, or bleeds should be biopsied to rule out SCC.