Malignant Potential of Actinic Keratoses and the Controversy Over Treatment

Abstract

Dermatologists routinely treat actinic keratoses (AKs) based on the belief that these lesions are premalignant and that treatment reduces morbidity. Marks et al1 make a convincing case for conservative management by suggesting that malignant transformation of AKs is rare and that metastasis from such transformation is uncommon. They present their data as the relationship between AKs detected during an initial visit and squamous cell carcinoma (SCC) on examination 1 year later. Using this lesion-oriented approach, Marks and colleagues1,2 have shown that the average malignant transformation rate for an actinic keratosis is only a fraction of a percent per year. Previous clinical estimates of malignant potential ranged as high as 25% and were based on patients with several AKs followed up for many years.3,4 Historical estimates were thus more patient oriented. We will present a mathematical model resulting in patient-oriented statistics that can be more easily compared with traditional clinical data. Using Marks and colleagues'1,2 data, we will show that the theoretical risk of malignant transformation for an average patient with AKs followed up for 10 years would be 6.1% or 10.2%, depending on the study analyzed. METHODSMathematical extrapolation requires the assumption that each AK is an independent event. One must assume that transformation of an AK is independent of other lesions and that the transformation rate is constant over the life span of the AK. The actual transformation rate for a given AK is likely dependent on a number of genetic, environmental, and temporal factors. Marks et al1,2 presented an average trans¬ formation rate for one AK followed up for 1 year. One may extrapolate that rate to arrive at the risk of malignant trans¬ formation for a hypothetical patient with an average number of AKs followed up for a period of years. RESULTSIn 1986 Marks et al1 published results suggesting that the yearly incidence rate of SCC occurring in persons with AKs was 0.24%. The average number of AKs for patients with lesions was 7.7. One SCC was detected at the 1-year follow-up for every 429 AKs present at the initial examination, for an annual incidence rate of 0.24%. Methods were not designed to determine if the SCC arose in the site of a previous AK; therefore, this may not be a true determination oftransformation rate.To compare Marks and colleagues'1,2 data with previ¬ ous estimates of lifetime malignant potential, we ex¬ trapolated the data based on a transformation rate of 0.24% per year for a hypothetical patient with a con¬ stant 7.7 AKs. Again, we stress that the assumption of

Malignant Skin Neoplasms

“…6 It is estimated that up to 25% regress spontaneously but 0.1% to 10% may undergo malignant transformation to squamous cell carcinoma. [7][8][9] The frequency of actinic keratosis correlates with cumulative UV exposure. High-risk populations include the elderly and people receiving immunosuppressive therapy, psoralen plus ultraviolet A therapy, and arsenic exposure.…”

Utility of Step Sections

“…19 However, because patients typically have numerous AK and are at risk for several years, the actual rate of malignant transformation for an individual patient might be on the order of 10% over a 10-year period. 20 Lesions clinically diagnosed as hyperkeratotic AK might have an especially high rate of progression to SCC. 21 Whereas relatively few AKs overall progress to fully evolved SCC, the converse frequency of AK in proximity to SCC ranges from 82% 22 to 97%, 23 indicating their common pathogenesis.…”

The Majority of Cutaneous Squamous Cell Carcinomas Arise in Actinic Keratoses

“…One study estimated that the risk was 1 in 1000 per year, while another author used the same data to calculate the risk at 10% after 10 years. 2,3 In addition, retrospective studies of SCCs have given con¯icting results as to the percentage of SCCs that have histological evidence they arose in AKs. One found that 24% of SCCs were contiguous with an AK, while another gave this ®gure as 94%.…”