Although it is assumed that the rate of conversion of actinic keratoses into squamous cell carcinoma is low, this has been derived from short-term studies.

Actinic keratoses represent areas of scaly skin that develop after years of chronic sun exposure. Such lesions are very common and age-related with one study indicating a prevalence of 49% in men and 28% in women. However, a more worrying concern is that these lesions can transform into a squamous cell carcinoma, for which a key risk factor is known to be cumulative sun exposure.

The rate of conversion of an actinic keratosis into a squamous cell carcinoma (sCC) is unclear and highly variable and thus whether the presence of actinic keratoses represent an adequate proxy measure for sun-damage remains to be seen. Given this uncertainty, a team from the Department of Dermatology and Mohs surgery, Kaiser Permanente, California, US, undertook a large, community-based, retrospective and longitudinal study, to assess the risk of sCC, 10 years after a diagnosis of actinic keratosis.

The team included patients who had a recorded consultation in 2009 and for which a diagnosis of an actinic keratosis was made and excluded those with a history of sCC, melanoma, organ transplant and HIV within two years of their diagnosis.

The patients with actinic keratoses were age and demographically matched with a group of patients without the lesion. All of the clinical data was obtained from the patient’s electronic health record starting two years before the date of diagnosis and until the end of the follow-up period, February 2020.

The primary outcome measure was the incidence of sCC which was obtained from pathologic data.

Actinic keratoses findings

A total of 220,236 patients with a diagnosis of an actinic keratosis and 220,236 matched control patients were included in the analysis. The sample had a mean age of 64.1 years (52.5% female) and were predominately (89.5%) of White ethnicity.

After 10 years of follow-up, the incidence of sCC in those originally diagnosed with an actinic keratosis was 17.1% compared with 5.7% in the matched control patients. Furthermore, the risk of developing a sCC was found to be seven times higher among those aged 49 years or younger with an the lesion, (hazard ratio, HR = 6.77 95% CI 5.50 – 8.32).

The authors calculated that the incidence rates of sCC were 1.92% per year after a diagnosis of 1 or more actinic keratoses and 0.83% per year among those without the lesion.

Interestingly, the number of actinic keratoses present (which represents more severe sun-damage) was only modestly associated with the risk of sCC. They concluded that the results should be used to educate patients on the signs of progression to a sCC and when it would be appropriate to contact a healthcare practitioner.

Citation
Madani S et al. Ten-Year Follow-up of Persons with Sun-Damaged skin associated with subsequent development of cutaneous squamous cell carcinoma. JAMA Dermatol 2021.