Cutaneous squamous cell carcinomas are associated with basal proliferating actinic keratoses

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).

To characterize the prevalence of downward-directed basal-layer expansion of AKs adjacent to iSCC.

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.

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).

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.