Histopathology of incipient intraepidermal squamous cell carcinoma ("actinic keratosis")

Actinic keratoses (AKs) are proliferations of transformed, neoplastic keratinocytes confined to the epidermis induced by exposure to ultraviolet (UV) radiation in sunlight. They are extremely prevalent and are seen in most middle-aged to elderly Caucasian persons with a history of significant sun exposure. These lesions are currently termed AKs while they remain confined to the epidermis, but when they extend more deeply to involve the papillary and/or reticular dermis, they are termed squamous cell carcinoma (SCC). The term AK was developed on the basis of clinical appearance and texture of these lesions without regard to histopathology or pathobiology.

This study was based on a current and historical literature review.

AKs and SCC represent the same disease process at different stages of evolution. Neoplastic transformation occurs in keratinocytes exposed to UV radiation manifest primarily as mutations in the p53 gene. In time, these cells proliferate in the epidermis and eventually extend into the dermis at which point metastatic spread can occur. Cytologic atypia is visible in early stages and is identical to that seen in SCC that is present in the dermis or in metastatic lesions. This sequence of events is directly analogous to that of evolving carcinoma of the uterine cervix that has been termed cervical intraepithelial neoplasia.

AKs are malignant neoplasms in evolution and demonstrate histologic and molecular genetic features of malignancy. Because the name currently used to describe these lesions does not reflect the pathobiology of the process, we propose that a new, more appropriate name be given to these lesions. We propose that a classification scheme analogous to that used by gynecologists for evolving carcinoma of the uterine cervix using the terminology "keratinocytic intraepidermal neoplasia" or another term such as "solar keratotic intraepidermal SCC" be used to define these lesions and that the term actinic (solar) keratosis be eliminated.