OBJECTIVE. This article describes a new subtype of actinic keratosis that exhibits proliferative characteristics both histologically and clinically. We describe three representative cases occuring in the presence of infiltrative squamous cell carcinoma (SCC) and/or basal cell carcinoma (BCC). METHODS. Histories of each lesion in the three cases discussed were obtained. The lesions were removed by Mohs micrographic surgery. Permanent sections, stained with hematoxylin and eosin, were examined and studied under light microscopy. RESULTS. All three lesions had failed conventional treatment with liquid nitrogen and/or 5-fluorouracil (5-FU). Histologic examination of the lesions revealed sheets of dysplastic cells growing along the basal layer of the epidermis and migrating down hair follicles and sweat ducts. An associated infiltrative SCC and/or BCC was found in each case. CONCLUSIONS. Proliferative actinic keratosis is resistant to standard therapies because of deep migration of abnormal cells along hair follicles and sweat ducts. It has a strong propensity to develop infiltrative SCC and may occur concomitantly with BCC.
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