▶ Previous Artlcle: #4-2. Laser in Medicine: Basic Laser Terminology
Selective Photothermolysis
The selective photothermolysis is a theory published by RR. Anderson and JA. Parrish in Science, an academic journal, in 1983. It reads as follows: “the rays which are properly short and selectively absorbed may selectively damage in vivo pigmented structures, cells, and organelles. There is no need to precisely target the laser because the intrinsic optical and thermal properties provide selectivity for the target.”
In order for the laser to make a selective photothermolysis on the target, there are the following three conditions: first, a wavelength that have selectivity for the target chromophore must be selected; second, the laser must have the same or a shorter irradiation time as or than the TRT of the target chromophore; third, the laser must have enough energy density (fluence) to destroy the target chromophore.
Under these conditions, only the target chromophore can be selectively destroyed and removed, while the surrounding normal tissue is left unharmed. Following laser therapy, as a result, the incidence of side effects becomes low, and the recovery duration decreases.
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The extended selective photothermolysis is an extended concept of selective photothermolysis. According to this theory, although a laser uses a wavelength matched to the chromophore and enough fluence to destroy the target chromophore, the heat generated is not confined to the inside of chromophore but diffuses beyond the chromophoreto the surroundings in order to treat the target lesion.
Hair removal and treatment of vascular lesions are typical examples. As for hair removal, the chromophore is melanin. However, the target that must be hit is stem cells around the hair. Therefore, a laser with melanin as a chromophore at a wavelength range which can penetrate deep into the hair roots should be used. In addition, the irradiation time should be sufficient to allow the generated heat to diffuse to the surrounding stem cells, which is more than the TRT of the chromophore.
The fluence must also increase to allow the heat to reach the surrounding target. The same is true of blood vessels. When a blood vessel is irradiated by laser with oxyhemoglobin as a chromophoreat a wavelength of 585nm or 595nm, the laser-absorbing oxyhemoglobin generates the heat, which damages the endotheliocytes in the lining of blood vessel, in order to destroy the blood vessel. In the case of 1064nm laser, the heat generated after being absorbed in the oxyhemoglobin diffuses beyond the blood vessels to the surrounding tissue, thereby decreasing the blood vessels, which is also an extended selective photothermolysis.
Subcellular Selective Photothermolysis
The subcellular selective photothermolysis is a concept related to selective photothermolysis but refers to a phenomenon where the selectivity works at the subcellular level. Laser toning is a typical example. Used with Q-switched Nd:YAG Laser to treat liver spots, laser toning selectively damages only melanin within the keratinocyte or the basal cell while allowing cells to remain relatively untouched.
-To be continued
