Malignant transformation of melanocytes is associated with changes in melanogenesis. Therefore, fluorescence of
melanin may be an informative
indicator of this process. But the conventionally excited autofluorescence of
melanin in skin tissue is ultra-weak and its main part in the visible spectral region is hidden by the much stronger fluorescence from other endogenous fluorophores. Here, using a new mode of stepwise two-photon excitation,
melanin-dominated fluorescence spectra of pigmented skin lesions are reported. From these, pure
melanin fluorescence spectra of normal pigmented skin,
melanocytic nevi and malignant pigmented
melanoma were analyzed. They show distinctly different spectral shapes:
melanoma gave a characteristic fingerprint with a fluorescence band peaking at 640 nm, independent of the
melanoma subtype. The
melanin fluorescence spectra peaked at 590 nm for all types of common
melanocytic nevi. These differences in the fluorescence spectra are probably based on different contents of
eumelanin and
pheomelanin. In a series of 167 cases with
melanocytic nevi and
melanomas, the sensitivity of this new method to diagnose
melanoma was 93.5%, the specificity 80.0% and the diagnostic accuracy 82.6%. The two-photon excitation fluorescence method is a new diagnostic tool which may in future supplement conventional dermatohistopathology.