The purpose of this study is to compare two different therapies for actinic keratoses in organ transplant recipients with regard to efficacy and tolerability. The investiagtors are planning to examine treatment with Imiquimod 5% cream versus treatment with Methyl-aminolaevulinate 16% cream and subsequent irradiation with red light, so-called photodynamic therapy, in this patients' group. A secondary objective of our study is to investigate the reduction in the field cancerisation after both treatments using fluorescence diagnostic method and digital imaging.