Actinic keratosis with atypical basal cells (AK I) is the most common lesion associated with invasive squamous cell carcinoma of the skin

Direct invasion from proliferating basaloid atypical keratinocytes limited to the epidermal basal layer (AK I), known as the differentiated pathway, was the most common form of progression to cutaneous iSCC in our series. On the other hand, stepwise progression from AK I to AK II and AK III (classic pathway) was seen to be operative in a substantial proportion of iSCC cases. All AK lesions, irrespective of intraepidermal neoplasia thickness, are therefore potentially invasive and tumour advance along adnexal structures might facilitate iSCC development from AK I lesions.