— Accurate diagnosis of pigmented lesions is mandatory before laser treatment.
— For some pigmented lesions, laser treatment may be the only treatment option.
— Tattoos respond well to Q-switched lasers.
— Amateur and traumatic tattoos respond more readily to treatment than do professional tattoos.
— Cosmetic tattoos should be approached with caution.
— Treatment of melanocytic nevi remains controversial, but worth pursuing.
The idea of treating cutaneous pigmented lesions with lasers was first tested in the early 1960s with the use of a normal mode ruby laser. This research indicated that the target was the melanosome. Unfortunately, due to laboratory difficulties, further research was halted.
In the past 15 years selective photothermo-lysis has largely transformed dermatologic laser surgery. The term selective photothermolysis describes site-specific, thermally mediated injury of microscopic tissue targets by selectively absorbed pulses of radiation (Anderson 1983). Three basic elements are necessary to achieve selective photothermolysis: (1) a wavelength that reaches and is preferentially absorbed by the desired target structures, (2) an exposure duration less than or equal to the time necessary for cooling of the target structures, and (3) sufficient fluence to reach a damaging temperature in the targets. When these criteria are met, selective injury occurs in thousands of microscopic targets, without the need to aim the laser at each one.
At wavelengths that are preferentially absorbed by chromophoric structures such as melanin-containing cells or tattoo-ink particles, heat is created in these targets. As soon as heat is created, however, it begins to dissipate by conduction. The most selective target heating is achieved when the energy is deposited at a rate faster than the rate for cooling of the target structures. In contrast to diffuse coagulation injury, selective photothermolysis can achieve high temperatures in structures or individual cells with little risk of scarring because gross dermal heating is minimized.
Pigmented Lesion Removal by Selective Photothermolysis
Because melanin absorbs light at a wide range of wavelengths - from 250 to 1200 nm - several lasers or intense pulsed light sources can effectively treat pigmented lesions. For tattoos, light absorption depends on the ink color, but the predominant color (blue-black) also absorbs well throughout the 532-1064-nm range. Almost any laser with sufficient power can be used to remove benign pigmented lesions of the epidermis. The selective rupture of skin melanosomes was first noted by electron microscopy in 1983, after 351-nm, submicrosecond excimer laser pulses of only about 1 J/cm2. At fluences damaging melanocytes and pigmented keratinocytes, epidermal Langerhans cells apparently escape injury.
With regard to wavelength, absorption by melanin extends from the deep UV through visible and well into the near-IR spectrum. Across this broad spectrum, optical penetration into skin increases from several micrometers to several millimeters. One would therefore expect melanosomes and the pigmented cells containing them to be affected at different depths across this broad spectrum.
A variety of thermally mediated damage mechanisms are possible in selective photo-thermolysis, including thermal denaturation, mechanical damage from rapid thermal expansion or phase changes (cavitation), and pyroly-sis (changes in primary chemical structure). Mechanical damage plays an important role in selective photothermolysis with high-energy, submicrosecond lasers for tattoo and pigmented lesion removal. The rate of local heating and rapid material expansion can be so severe that structures are torn apart by shock waves, cavi-tation, or rapid thermal expansion.
Grossly, the immediate effect of submi-crosecond near-UV, visible, or near-IR laser pulses in pigmented skin is immediate whitening. This response correlates very well with the melanosome rupture seen by electron microscopy and is therefore presumably a direct consequence of melanosome rupture. A nearly identical but deeper whitening occurs with Q-switched laser exposure of tattoos, which like melanosomes consist of insoluble, submicrom-eter intracellular pigments. Although the exact cause of immediate whitening is unknown, it is almost certainly related to the formation of gas bubbles that intensely scatter light. Over several to tens of minutes, these bubbles dissolve, causing the skin color to return to normal or nearly normal. In addition, pyrolysis may occur at the extreme temperatures reached within melano-somes or tattoo ink particles, directly releasing gases locally. Regardless of its cause, immediate whitening offers a clinically useful immediate endpoint that apparently relates directly to melanosome or tattoo ink rupture (Fig. 3.1).
Melanin in both the epidermis (as in cafe-au-lait macules and lentigines) and the dermis (as in nevus of Ota), as well as dermal tattoo particles, is an important target chromophore for laser selective photothermolysis. Clinically,
selective photothermolysis is highly useful for epidermal and dermal lesions in which cellular pigmentation itself is a cause. These include lentigines, cafe-au-lait macules (which display a high rate of recurrence), nevus spilus, Becker nevi, blue nevi, and nevus of Ota. However, selective thermolysis has only been variably effective for dermal melasma, postinflamma-tory hyperpigmentation, or drug-induced hyper-pigmentation.
Currently Available Technology
Lasers and Intense Pulsed Light Sources Used to Treat Pigmented Lesions and Tattoos
■ Continuous-Wave Lasers (CW Lasers)
Although Q-switched lasers are now the modality of choice for most pigmented lesions, continuous-wave and quasi-continuous lasers, when used properly, can also be effective (Tables 3.1, 3.2, 3.3). The lasers include the CW argon laser (488 and 514 nm), a CW dye laser (577 and 585 nm), a CW krypton (521-530 nm), a quasi-CW copper vapor laser (510 and 578 nm), an erbium (2940 nm) and CO2 (10,600 nm) laser.
The CW and quasi-CW visible light lasers can be used to selectively remove pigmented lesions. However, because of the shorter wavelengths of these lasers, they penetrate only superficially. Thus, they are effective only for
Light source Wavelength
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