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 ( AK s) 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 AK s adjacent to iSCC . Methods The epidermis overlying and adjacent to iSCC s 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 AK s were histologically classified as AK grade I, 38·1% as AK II and 6·8% as AK III (χ 2 ‐test, P < 0·001). Only 2·6% of adjacent epidermal samples did not show any atypical keratinocytes. The epidermis adjacent to iSCC s 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 AK s showed no basal growth (χ 2 ‐test, P < 0·001). In total 118 AK s (48·8%) showed extension into adnexal structures. These AK s were graded as PRO I in 18·6% of cases, PRO II in 30·5% and PRO III in 50·8%. The epidermis above iSCC s could be assessed only for upwards‐directed growth and showed no significant differences in the three AK grades ( P = 0·42). Conclusions Basal proliferative AK s, 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 .