Organ transplant patients (OTP) require lifelong immunosuppressive therapy and consequently are prone to develop skin tumors, i.e skin cancer is the most frequent malignancy in organ transplant recipients. OTP frequently develop extensive areas of actinic damage, epidermal dysplasia, wich accounts for increased risk of aggressive skin cancer development in susceptible patients, and are referred to as "field cancerisation". Therefore the whole area of field cancerisation has to be treated. In our study we will treat this areas with two different methods and not only the single visible lesions of actinic keratoses.In this open prospective randomized intraindividual study one half of the patients' scalp or face will be treated with Imiquimod 5% cream for 4 weeks, 3 times a week, and the other half with Methyl-aminolaevulinate 16% cream photodynamic therapy, two applications in two weeks interval. The pre- and post treatment extension of field cancerisation will be assessed by means of a highly sensitive digital fluorescence imaging system.