Cutaneous squamous cell carcinomas are associated with basal proliferating actinic keratoses.

Summary: Background: In addition to the extent of atypical keratinocytes throughout the epidermis, actinic keratoses (AKs) are histologically characterized by downward‐directed basal‐layer expansion. It is not known whether this growth pattern correlates with the risk of developing invasive squamous cell carcinoma (iSCC). Objectives: To characterize the prevalence of downward‐directed basal‐layer expansion of AKs adjacent to iSCC. Methods: The epidermis overlying and adjacent to iSCCs was assessed histologically. We determined the histological grade (AK I–III), basal growth pattern (PRO I–III) and accompanying parameters such as adnexal involvement. Results: Among 307 lesions, 52·4% of AKs were histologically classified as AK grade I, 38·1% as AK II and 6·8% as AK III (χ2‐test, P < 0·001). Only 2·6% of adjacent epidermal samples did not show any atypical keratinocytes. The epidermis adjacent to iSCCs was classified as having a PRO I basal growth pattern in 25·7%, PRO II in 31·9% and PROIII in 39·4% of cases. Only 2·9% of AKs showed no basal growth (χ2‐test, P < 0·001). In total 118 AKs (48·8%) showed extension into adnexal structures. These AKs were graded as PRO I in 18·6% of cases, PRO II in 30·5% and PRO III in 50·8%. The epidermis above iSCCs could be assessed only for upwards‐directed growth and showed no significant differences in the three AK grades (P = 0·42). Conclusions: Basal proliferative AKs, as well as atypical keratinocytes restricted to the lower third of the epidermis, are most commonly seen adjacent to iSCC, with less evidence for full‐thickness epidermal dysplasia. Our study supports the important role of dysplastic keratinocytes in the epidermal basal layer and their potential association with iSCC. What's already known about this topic? Actinic keratosis (AK) can progress into invasive squamous cell carcinoma (iSCC).The established histological classification of AKs lacks predictive properties to determine progression risk.A recently proposed classification scheme has reported on the histological changes in the epidermis, particularly the downward‐directed proliferation of atypical keratinocytes at the basal layer What does this study add? iSCCs are associated with basal proliferation of AKs.In our study, AK grade I is confirmed to be the dominant AK grade adjacent to iSCC.iSCCs are more commonly associated with marked basal proliferation and with adnexal involvement, along with atypical keratinocytes restricted to the lower third of the epidermis (AK grade I). Linked Editorial:Welzel. Br J Dermatol 2019; 180:699–700. Respond to this article [ABSTRACT FROM AUTHOR]