TREATMENT OF PIGMENTED LESIONS
Laser treatment of pigmented lesions has evolved into a highly selective and relatively safe procedure being increasingly performed in dermatology practices. Although earlier attempts at treating pigmented lesions with ablative continuous wave CO2 and argon lasers were complicated by scarring and pigmentary alteration, the introduction of short-pulsed lasers, namely, quality-switched (QS) mode lasers, has enabled a more direct targeting of pigment with a low risk of damage to surrounding tissue. While these lasers remain the standard treatment for pigmented lesions, more recently, ablative and nonablative fractionated devices and lasers of picosecond pulse durations have been added to the armamentarium for the treatment of pigmented lesions.
Destruction of pigmented lesions through laser technology is based on the principle of selective photothermolysis. According to this theory, a target chromophore can be preferentially heated and destroyed without harm to adjacent tissues by selection of a wavelength of maximum absorption and a pulse duration that is shorter than the thermal relaxation time of the target.1
Although several lasers are currently available for the treatment of pigmented lesions and pigmentary disorders, it is of utmost importance that the clinical diagnosis of the lesion be certain.2 If not, biopsy is recommended to rule out a dysplastic nevus or melanoma, as the effect of laser irradiation on malignant degeneration is not fully understood, and therefore laser treatment of these lesions is not recommended by the authors. In fact, controversy still remains over laser treatment of nevocellular and congenital melanocytic nevi given their premalignant potential.
Choosing the Appropriate Light Source
Pigmented lesions can be epidermal, dermal, or combined, and can differ in both the overall quantity and the density of pigment distribution. The location of pigment guides the selection of the appropriate wavelength for treatment, as lesions with deeper pigment, in general, will be more amenable to treatment with longer wavelengths. The chromophore targeted in the treatment of pigmented lesions is the melanosome, with a thermal relaxation time ranging from 50 to 500 nanoseconds.1 Melanosomes are typically destroyed at pulse durations between 40 and 750 nanoseconds, with an absorption spectrum ranging from 351 to 1064 nm.3,4 QS mode lasers have pulse durations in the nanosecond range and are thus recommended for the treatment of pigmented lesions. Nonpulsed, quasi-continuous wave green light lasers such as copper vapor (511 nm), krypton (520–530 nm), and variable pulse with KTP (532 nm) lasers do not work consistently because thermal relaxation time of the melanosome is exceeded. While not always possible, all attempts should be made to avoid lasers with absorption peaks of other commonly targeted chromophores found in the skin, namely, hemoglobin and oxyhemoglobin.
While fractional ablative and nonablative devices tend to target water as their main chromophore, the resultant microscopic treatment zones (MTZs) create distinct columns of epidermal and dermal injury5...