Selective Melanin Fluorescence Provides Insight into Skin Cancer

LTB has developed a laser-based method that selectively “extracts” and measures the fluorescence of melanin from mixtures of fluorophores. Such mixtures of fluorophores are found in human skin, for example. In this context, melanin, which fluoresces very weakly, is one of at least 10 fluorescent species but plays a prominent role: it determines skin color (as well as hair and eye color), acts as a natural sunscreen, and can also serve as the starting point for tissue degeneration leading to skin cancer. Important information regarding the early detection of melanoma can be expected from melanin fluorescence. However, this fluorescence is typically overshadowed by the fluorescence of other fluorophores under standard fluorescence excitation conditions.

The new LTB method for selectively measuring melanin fluorescence is based on laser-induced two-photon excitation and utilizes the fact that typical fluorophores absorb both (NIR) photons simultaneously, while melanin fluoresces after the stepwise absorption of two photons. Theoretically, long pulse durations are recommended for the most selective melanin excitation. Experimentally, two-photon excited melanin fluorescence has previously only been observed with femtosecond pulses, as femtosecond pulses have been the method of choice in the broad field of autofluorescence investigations of the skin.

For the first time, LTB has measured two-photon excited melanin fluorescence in solution, in histological skin samples, and in vivo in skin tissue using nanosecond pulses with a highly sensitive measurement setup. The experiment confirmed the theoretical expectations regarding fluorophore discrimination. The high selectivity of melanin fluorescence measurement results in a significant increase in sensitivity for the early detection of melanoma. Interesting additional applications in dermatology, ophthalmology, pharmacy, and cosmetics are anticipated.

The shift from femtosecond to nanosecond lasers also brings beneficial side effects for the user, including cost advantages (fs vs. ns lasers) and, particularly, skin-friendly aspects.