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 (AK) 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 AKs 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 AK into a squamous cell carcinoma (sCC) is unclear and highly variable and thus whether the presence of AKs 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 AK. The team included patients who had a recorded consultation in 2009 and for which a diagnosis of an AK was made and excluded those with a history of sCC, melanoma, organ transplant and HIV within 2 years of their AK diagnosis. The AK patients were age and demographically matched with a group of non-AK patients. 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.
A total of 220,236 patients with a diagnosis of an AK 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 AK was 17.1% compared with 5.7% in the matched control patients. Furthermore, the risk of developing a sCC was found to be 7 times higher among those aged 49 years or younger with an AK, (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 AKs and 0.83% per year among those without an AK. Interestingly, the number of AKs 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.
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.