The purpose of this research is to obtain a blood sample from patients with actinic keratoses undergoing routine Photodynamic Therapy, in order to measure biomarkers that are relevant to VitD and 5FU metabolism and might be predictive of PDT outcome. The biomarkers to be examined include serum VitD levels at the time of PDT, and the presence/absence of gene alleles that correlate with expression of several proteins involved in VitD and 5FU metabolism. The presence of these biomarkers will be correlated to the improvement in AK lesion counts at the patient's routine follow-up visit 3 months after PDT treatment.