Solar Keratosis (Actinic Keratosis)
Definition
Actinic keratosis (AK) is an intraepithelial neoplastic lesion that may progress into invasive squamous cell carcinoma (SCC) or may spontaneously regress.
ICD-O code 8070/0
Synonyms
Solar keratosis; keratinocytic intraepithelial neoplasia: senile keratosis
Epidemiology
AKS occur more commonly in more sun exposed regions, in people with fair skin, and in males; their incidence increases with advancing age.
Etiology
Risk factors include cumulative intermittent sun exposure, chronic immunosuppression, arsenic exposure, PUVA therapy, and chronic cutaneous inflammation. Exposure to ultraviolet (UV) radiation (UVB and to a lesser extent UVA radiation) plays the major role in carcinogenesis .
Localisation
The lesions occur on sun-exposed areas such as the face, lower lip (actinic cheilitis), bald scalp, lateral neck, forearms, and dorsal hands, as well as on the lower legs and trunk.
Clinical features
AK commonly presents as an asymptomatic scaly erythematous patch or plaque, usually 2—10 mm in diameter, surmounted with a rough adherent scale often resembling sandpaper. There are often multiple lesions, flesh-coloured and confluent, with a background of chronic sun damage.
Genetic profile
TP53 mutations are the most common genetic alteration in AK. Other mutations include mutations of CDKN2A on chromosome 9p21 — the gene encoding p16 (p161NK4a) and p14ARF — and mutations of CDKN2B — the gene encoding p15 (p151NK4b). Loss of heterozygosity has been documented in as many as four loci and several chromosomes.
Genetic susceptibility
The genetic factors associated with an increased risk for the development of AK are those that result in constitutional sensitivity to sunlight, such as childhood freckling, blue eyes, and blond or red hair.
Oculocutaneous albinism; Rothmund Thomson, Cockayne, and Bloom syndromes; and xeroderma pigmentosum are associated with the development of AKs in earlier life.
Prognosis and predictive factors
AK may regress spontaneously, remain stable, or progress to invasive SCC; cases in which there are >5 AKS have an increased risk of progression. The annual rate of malignant transformation is usually cited as < 1 per 1 000 cases, but transformation rates are higher among the higher-risk cases.
Reference
WHO classification of skin tumours, 4th edition (2018): 51-52.