Portable Measurement of Protoporphyrin IX in the Skin

Photodynamic Therapy (PDT) has gained popularity as an effective, non-scarring treatment for thin, non-hyperkeratotic actinic keratoses (AKs). Similar to the topical agents 5-fluorouracil and imiquimod, PDT is particularly useful when utilized as a field-directed therapy for the treatment of areas with multiple AKs and extensive sun damage. PDT consists of two steps: 1) the topical application of a photosensitizer agent aminolevulinic acid (ALA), which is preferentially converted to the photosensitive protoporphyrin IX (PpIX) in precancerous and neoplastic cells, and 2) controlled exposure to a visible wavelength light source. Current methodologies utilize a "one-size-fits-all" approach with regard to duration of incubation with photosensitizing agents and illumination. Moreover, patients frequently experience pain after long PDT prodrug incubation. Better characterization of photosensitization in PDT can help tailor incubation and overall treatment time to minimize treatment duration and discomfort while maximizing clearance of the target lesions. The direct measurement of PpIX is a promising, yet rarely performed test that may help determine the appropriate PDT treatment time, the need for re-treatment or adjuvant therapy, and potential efficacy of treatment. Point-probe measurements have shown extreme heterogeneity between PpIX levels in different patients and among different lesions in the same patient. However, these point-probe measurements are unable to account for the variance in PpIX production in different parts of the skin because of their relatively limited field of view. The recent development of a low-cost, smart phone-based, wide-field fluorescence dosimetry imaging system to map PpIX levels onto a 2D image allows for handheld, real-time analysis of PpIX levels in human skin. Initial unpublished clinical results have shown its utility in human subjects. However, a more extensive characterization of the factors that influence baseline in PpIX has yet to be performed. This study intends to elaborate these inter-and intra-individual variances, including analysis of changes in PpIX concentrations based on anatomical location, age, diet, temperature, pigmentation, and previous skin damage.