What are actinic keratoses?

Actinic Keratoses: Clinical Description

Actinic keratoses (AKs), also known as solar keratoses, are keratotic lesions occurring on chronically sun-exposed adult skin that represent focal areas of abnormal keratinocyte proliferation and differentiation with a low risk of progression to invasive squamous cell carcinoma. 1

Definition and Pathological Features

The cardinal histological feature of AK is epithelial dysplasia, which may be restricted to the basal layer or extend to full-thickness atypia (at which point it becomes squamous cell carcinoma in situ, or Bowen disease). 1, 2

Key pathological characteristics include:

• Disorderly arrangement and maturation of epithelial cells 1
• Multiple buds of epithelial cells at the membrane zone, but critically no invasion is present 1, 2
• Several histological variants exist: hypertrophic, bowenoid, lichenoid, acantholytic, and pigmented types 1, 2

Clinical Presentation

AKs present as discrete, sometimes confluent, patches of erythema and scaling on predominantly sun-exposed skin. 2

Typical locations include:

• Face, scalp, and ears 2
• Dorsa of hands 1
• Other chronically sun-exposed areas 2

Clinical characteristics:

• Lesions are often asymptomatic but may occasionally be sore or itch 2
• Most commonly affect middle-aged and elderly individuals 2
• Particularly prevalent in fair-skinned individuals (Fitzpatrick skin types I and II) 2

Etiology and Risk Factors

AKs result from chronic exposure to ultraviolet (UV) radiation, with UVB-specific p53 mutations demonstrated in these lesions, providing molecular evidence for sunlight's causative role. 1, 2

Major risk factors include:

• Fair skin in individuals with chronic sun exposure 1
• Immunosuppression, particularly in organ transplant recipients 1, 2
• Long-term treatment for inflammatory bowel and rheumatological disease 1
• Chronic sunbed use 1, 2
• Arsenic exposure 1

Epidemiology

Prevalence increases significantly with age:

• 19-24% of individuals aged >60 years have at least one AK in UK studies 1, 2
• 36% of men aged 40-49 years have AKs 1
• By age 70, >70% of dermatology clinic attendees have AKs, with 70% on the head and neck 1
• In the Rotterdam study, 49% of men and 28% of women (mean age 72 years) had AKs 1, 2

The rate of new AKs is estimated at 149 per 1000 person-years, though the true incidence is likely underestimated due to difficulties in reliable measurement. 1, 2

Natural History and Field Cancerization

AKs represent a chronic disease with a dynamic natural history:

• 25-70% of lesions may spontaneously resolve over 1-4 years 1, 2
• Lesions regress and relapse over time, particularly grade 1 and 2 lesions 1
• The presence of a single lesion is a marker of excessive sun exposure and is associated with development of further lesions 1

Malignant transformation risk:

• Less than one in 1000 AKs develops into invasive squamous cell carcinoma per year 2
• However, the majority of SCCs arise from AKs 3
• Risk increases substantially in patients with more than 5 AKs 3

Field Cancerization Concept

Subclinical (invisible) AK lesions contain the same genetic changes as visible lesions and occur up to 10 times more frequently than visible AKs. 4, 5

• A field of change can extend up to 7 cm around primary lesions 4
• The entire affected area represents genetically similar lesions due to chronic UV exposure 4, 5
• Understanding field cancerization is critical because treating only visible lesions leads to high recurrence rates 5