Biomedical Engineering Reference
In-Depth Information
3
Laser treatment of benign pigmented lesions
Omar A. Ibrahimi and Suzanne L. Kilmer
introduction
Signifi cant advances in laser technology have increased the
options available for the treatment of benign pigmented
lesions. Lasers were fi rst used in the treatment of pigmented
lesions by Leon Goldman in the 1960s when he used the ruby
laser (694 nm) to treat nevi and tattoos (1,2). The focus then
shifted to the use of continuous-wave modalities such as CO
2
laser (10,600 nm) and argon laser (418 and 514 nm). These
continuous-wave lasers were used to treat pigmented lesions
via nonselective destruction. Due to the lack of selectivity, the
results were often unpredictable, with frequent complications
such as scarring and pigmentary changes.
A revolutionary advance in the laser treatment of pigmented
lesions was made by R. Rox Anderson and John Parrish of the
Wellman Center for Photomedicine at Massachusetts General
Hospital with the eludication of the theory of selective photo-
thermolysis (3), which permitted precise and selective microsur-
gery of pigmented lesions. The Wellman group also developed
fractional photothermolysis (4), which reintroduced the ability
to nonselectively improve the appearance of certain types of
undesirable pigment in a safe manner.
targeted with a QS laser with pulse widths in the nanosecond
domain, whereas longer pulse widths used in lasers for hair
removal better target clumped melanin such as hair shafts.
In both cases, when appropriate settings are used the risk of
dyspigmentation and scarring is rare.
clinical technique
Lasers and Light Sources
Continuous-Wave Lasers
The continuous-wave lasers used include argon (488 and
514 nm), green light (532 nm), and CO
2
(10,600 nm) lasers.
These continuous-wave lasers are useful only for epidermal
lesions because the thermal injury that accompanies their use
often leads to scarring when applied to dermal lesions. When
each individual lesion is treated, a nonspecifi c thermal damage
results in its destruction with subsequent denuding of the epi-
dermis. The lesion's destruction may be followed by erythema
as well as pigmentary and textural changes, but the skin gener-
ally heals with excellent cosmetic results.
QS Lasers
The QS laser systems used for the treatment of superfi cial pig-
mented lesions include the 532-nm frequency-doubled (FD)
QS Nd:YAG, the 694-nm ruby, and the 755-nm alexandrite
lasers. Strong absorption of light at these wavelengths by mela-
nin and the nanosecond pulse duration make these lasers an
excellent treatment modality for superfi cial and some dermal
pigmented lesions in which the melanin is fi nely distributed.
The QS ruby, alexandrite, and 1064-nm Nd:YAG lasers may be
useful for treating deeper pigmented lesions such as nevi of
Ota and tattoos (11). The 1064-nm QS Nd:YAG laser should
be used when treating darker skin types, because it greatly
reduces the risk of epidermal injury and pigmentary altera-
tion. In addition to QS lasers, current research is focused on
the development of picosecond lasers which may be benefi cial
in the treatment of pigmented lesions (12,13).
the principle of selective photothermolysis
Selective destruction of human epidermis with lasers using
melanin as the target chromophore was fi rst demonstrated in
the early 1960s (1,2) using a normal-mode ruby laser (wave-
length 694 nm and pulse width 500 µs). A subsequent study
with the Q-switched (QS) ruby laser using a 50-ns pulse width
showed the threshold radiant exposure to be 10-100 times
lower, suggesting a more selective effect of the shorter pulse
width (5). For the next 20 years, this work was largely over-
looked, until the emergence of Anderson and Parrish's theory of
selective photothermolysis (3,6-10). The theory of selective
photothermolysis redirected attention to the concept of treating
a specifi c target or chromophore (melanin and hemoglobin)
with specifi c laser parameters (wavelength, pulse width, and
energy level). Wavelength selectivity limits absorption to a
specifi c target chromophore. Thermal damage is then con-
fi ned to that target by limiting the pulse width to less than or
equal to the thermal relaxation time of the target chromo-
phore. Once the pulse width is determined, energy levels can
be optimized to achieve a desired effect. Application of the
theory of selective photothermolysis facilitates the selection of
the most appropriate laser for various pigmented lesions,
without the high rates of complications and recurrence noted
with destructive, continuous-wave modalities. For example,
melanocytes or melanin-containing keratinocytes are best
Long Pulsed Lasers
To better target hair follicles (which are larger melanin-
containing targets), lasers with longer (millisecond range) pulse
durations were developed. These systems include the long
-
pulsed ruby (694 nm), alexandrite (755 nm), diode (800 or
810 nm), and Nd:YAG (1064 nm) lasers. The millisecond pulse
width more closely matches the thermal relaxation time of
nested melanocytes, and the collateral thermal damage infl icts
a lethal injury to melanocytes that are adjacent to the target
area but that might not actually contain melanin at the time of
58
