Since their invention in 1960 lasers have been useful light sources for research and medical applications. As manufacturing techniques have been improved and lasers have become cheaper and more reliable, their field of applications rapidly increased. Examples of these applications are photodynamic therapy of tumours (PDT) and photothermal therapy (PTT).
The mechanisms behind PDT can be classified as photochemical processes (including the generation of singlet oxygen and free radicals, which, in turn, react with biomolecules and destroy cancer cells). The efficacy of PDT, therefore, depends on the tumour selectivity of the photosensitizer and on the quantum yield of production of reactive oxygen species in the target cells.
Several non-photosensitizing drugs, such as metallo-porphyrins or metallo-phthalocyanines, have been shown to possess tumour-selectivity as well. These dyes have low quantum yields of fluorescence and decay from the electronically excited state to the ground state primarily by non-radiative pathways, releasing their energy in several forms, including heat. The mechanisms behind PTT employ the heat deposited in the tissue during laser-tissue interaction mediated by either endogenous chromophore (haemoglobin, melanin) or externally added dyes, so-called photothermal sensitizers. In the photothermal mechanism, generation of local hyperthermal effects leads to specific damage in cells and tissues containing the photothermal sensitizers, while surrounding cells remain unaffected.
Since photosensitizers and biomolecules have broad absorption spectra and since the coherence of light is rapidly lost due to multiple scattering and tissue non-uniformity, the monochromacy of laser light is not necessary for PDT. However, laser light is a key element in PTT since it satisfies several physical conditions for photothermal tissue destruction, which will be discussed in this review.
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