Mark Wefers Bettink

Chapter 3 50 The COMET canmeasure in healthy skin as well as in skin lesions. After ALA application the measurement area needs to be covered, to avoid consumption of PpIX by light. A priming time for ALA, typically 4 hours or more, is needed to synthesize a suitable concentration of PpIX to enable measurements of mitoPO 2 and oxygen disappearance rate. After topical administration on healthy skin, PpIX is synthesized in the epidermis but not (significantly) in the dermis [24]. While ALA penetrates into the dermis the heme- synthesis pathway in most dermis cells is inactive. The conversion of ALA to PpIX requires energy and an intact heme cycle, thus PpIX is not synthesized in the metabolically inactive cells of the stratum corneum. In healthy skin this limits the intradermal measurement location and signal origin of the COMET to the epidermis with a thickness of about 0,1mm [25]. The recommended measurement location is the skin of the sternum seen in figure 2. This provides a central measurement location less influenced by temperature changes, movement and peripheral vasoconstriction [26]. Fig .2 COMET skin sensor position on the sternum