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NOGUEIRA, M. S. Fluorescence lifetime spectroscopy for diagnosis of clinically similar skin lesions. 2016. 263 p. Master’s Thesis (Master of Science) – Sao Carlos Institute of Physics, University of Sao Paulo, Sao Carlos, 2016. The fluorescence spectroscopy and lifetime analysis in biological tissues has been presented as a technique of great potential for tissue characterization for diagnostic purposes. This potential is due to the main advantages of optical techniques based on fluorescence for diagnosis, which includes the possibility of evaluating the tissue metabolism in situ, without removal and processing of the biological sample, through a fast and non-invasive response. Skin lesions were the target interrogated tissue in the present study. They can be clinically classified into two major groups: pigmented and non-pigmented lesions. In each group, the clinical discrimination of benign and malignant lesions may be a complex task, especially for non-experienced clinicians. When these lesions have clinically similar features, the choice of the treatment modality becomes difficult. In this context, auxiliary diagnostic techniques are very important to improve the diagnostic resolution as well as treatment planning and success. Gold standard for skin diagnosis is obtained with the biopsy and further histological analysis. The information about these features is invasive and time consuming. When using a non-invasive procedure such as fluorescence lifetime measurements, the main interrogated fluorophores are NADH (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide), biomolecules involved in cellular respiration that may provide information on the metabolism of the cells. To differentiate each skin lesion, it is necessary to take into account the contribution of endogenous fluorophores emission such as collagen and elastin, and the absorption of chromophores such as melanin and hemoglobin. In addition to fluorescence decay analysis considering the contribution of fluorophores and chromophores, a stable and portable system is desired for clinical measurements and interrogation of biological tissue in vivo. In this study, we have assembled, calibrated, and characterized one of the world’s first portable time-resolved fluorescence spectroscopy system for singlepoint measurements. This system was designed to be robust and user-friendly for clinical applications. The system was calibrated and characterized in vitro before the clinical application. It was used for evaluation of the photoaging process in sunexposed and non-exposed skin and for discrimination of clinically similar skin lesions. Significant statistical differences were observed for 10 parameters when comparing normal and photoaged skin (student’s t-test, p < 0.001), and for all combinations of non-pigmented and pigmented lesions when using tri-exponential decay parameters (Wilcoxon rank sum test, p<0.05). Both in vivo measurements showed promising results and have potential for many applications in dermatology, oncology and aesthetics. Next steps include multivariate data analysis and the determination of the diagnostic resolution of fluorescence lifetime spectroscopy. Further investigation of optical processes related to fluorescence decay changes is necessary, since fluorescence lifetime values in biological tissues reported on the literature are very scarce and heterogeneous and not completely understood.