[Actinic keratosis, Bowen's disease, keratoacanthoma and squamous cell carcinoma of the skin].

Actinic (solar) keratosis is an intraepidermal squamous neoplasm of sun-damaged skin and by far the most frequent neoplastic skin lesion. A subdivison into three grades has been proposed with increasing acceptance not least because of the therapeutic consequences. The transition to invasive squamous cell carcinoma is reported in 5-10 % and with immunosuppression in 30 % of patients.Bowen's disease is a variant of squamous cell carcinoma in situ of the skin and the mucocutaneous junction. The differentiation from bowenoid papulosis as a lesion associated with human papillomavirus (HPV), actinic (solar) keratosis grade III, intraepidermal poroid lesions and in cases of clonal type from clonal seborrhoic keratosis and Paget's disease is very important.Keratoacanthoma is currently uniformly interpreted as a variant of highly differentiated squamous cell carcinoma of the skin with clinical and histomorphological characteristics. Clinically keratoacanthoma erupts rapidly and is capable of resolving spontaneously. Histologically, there is a characteristic growth pattern and various stages of regression. The final histomorphological diagnosis needs the entire specimen.Squamous cell carcinoma of the skin is the second most common type of skin cancer following basal cell carcinoma. With respect to reccurrencies and risk of metastases the subtyping of cutaneous squamous cell carcinoma is very important. The classification system of the Union Internationale Contra le Cancer (UICC) is based solely on the anatomical spread and the classification system of the American Joint Committee on Cancer (AJCC) also considers so-called high-risk features in the staging between stages I and II.