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Surgery and Cosmetics

James M. Spencer, MD, MS

Excimer Lasers in Dermatology

James M. Spencer, MD, MS

Friday, October 01, 2004

The excimer lasers are a group of lasers operating in the ultraviolet range that have found wide application in a variety of medical specialties, including ophthalmology, cardiology, orthopedic surgery, and, most recently, dermatology. Examples include the 193-nm argon-fluoride laser, the 248-nm krypton-fluoride laser, the 351-nm xenon-fluoride laser, and, of particular interest to dermatology, the 308-nm xenon-chloride laser. Due to the high energy of the photons that are generated by these lasers, initial medical applications focused on the lasers' ability to ablate tissue without heat (cold ablation).1 But recent attention in dermatology has turned to nonablative phototherapy with the 308-nm xenon-chloride laser.

All of the excimer lasers function as a mixture of a noble gas and a halogen. Electrical stimulation results in the formation of unstable excited dimers of the two gases, hence the name excimer. Upon return to the ground state, these excited dimers create laser light in the UV range. The laser light created can be delivered by a fiberoptic cable and can function in an aqueous environment, which has allowed its use in a variety of medical situations.

Most lasers of medical interest work by a thermal mechanism: The target of the laser absorbs the light and gets hot. Selective photothermolysis is a way to spatially confine the heat, but the fundamental mechanism of the laser is the same: The target gets hot and is destroyed. The excimer lasers interact with tissue in a fundamentally different way. When used at high energies, they can cause tissue ablation without creating heat. It is theorized that the photons of the excimer lasers are of such high energy that, when delivered in short pulses, they can exceed the energy of the molecular bonds holding some organic molecules together. Thus, these lasers can cause some materials to simply decompose without becoming hot, causing no damage to surrounding structures. This has been termed ablative photodecomposition, and has been used to reshape the cornea, to ablate atherosclerotic plaques in coronary vessels, and to ablate cartilage intraoperatively in joints and in the spine. This same property of the excimer lasers was initially investigated for facial resurfacing. However, for technical reasons, it was deemed impractical, and resurfacing with thermally based systems, such as the ultrapulsed CO2 and the erbium/YAG lasers, was developed instead.

Dermatologic use of the excimer lasers was revisited in 1997 when Bonis and colleagues tested a novel application: Nonablative fluences were used as a form of UVB phototherapy.2 By simply turning the fluence down below the ablative threshold, UVB phototherapy in the form of laser light could be delivered. This had several theoretic advantages: First, the light could be precisely targeted, sparing normal surrounding skin from UV exposure. Second, very high fluences could be delivered in short periods of time. Third, the light-tissue interaction could be different with laser light versus incoherent light. Psoriasis is a common dermatologic disease known to respond well to UV phototherapy. It has long been known that the optimal wavelength to treat psoriasis is around 313 nm in the UVB range, and the 308-nm xenon-chloride excimer laser is very close to this ideal. In a small pilot study of 10 patients, Bonis and colleagues reported that psoriatic plaques could be cleared in 8 to 10 laser treatments, versus over 30 treatments required with conventional narrow-band UVB phototherapy. This pilot study suggested that excimer lasers might allow targeted, rapid phototherapy superior to conventional UV phototherapy with incoherent light.

This pilot study was followed by a second pilot study investigating dose-response.3 Thirteen patients with multiple plaques of psoriasis were treated with escalating multiples of the MED (minimal erythema dose), as determined on normal skin. This study reported that psoriatic plaques tolerated doses of UVB 3 to 4 times the MED of normal skin with no apparent sunburn reaction. Therapy with these fluences would be possible only with targeted therapy that spares the surrounding normal skin. It was also reported that very high multiples of the MED (8 to 16 times) cleared psoriasis in as little as 1 treatment, but produced a painful blistering sunburn that would be unacceptable in clinical practice.

The largest trial to date included 80 patients treated twice a week beginning with low multiples of the MED of normal skin and slowly escalating as tolerated.4 This study reported that 72% of patients achieved 75% or greater clearance in an average of 6.2 treatments. This is significantly faster than with conventional UVB or narrow-band UVB phototherapy, which gives whole-body exposure and requires around 30 treatments or more for this degree of improvement. However, the relatively small spot size limits the area that can realistically be treated by the laser, so excimer laser phototherapy is indicated only for those patients with a limited total area to be treated.

A variety of other skin diseases also respond to UVB phototherapy and thus are potential candidates for excimer laser therapy. Vitiligo is an acquired localized loss of melanocytes resulting in hypopigmented macules that may be amenable to UV phototherapy. Both PUVA and, more recently, narrow-band UVB have been extensively used to treat vitiligo, typically requiring months to years of treatment, with modest results. The most effective therapy for vitiligo with conventional phototherapy seems to be narrow-band UVB. Njoo and colleagues reported that 53% of children treated twice a week for up to 1 year achieved 75% or greater repigmentation.5 One year of twice-a-week treatment requires a substantial commitment to therapy on the patient's part. In a pilot study of the feasibility of excimer laser phototherapy for vitiligo, this author reported that at least some minimal repigmentation was seen in 57% of 23 patches of vitiligo treated 3 times a week for only 2 weeks, and in 82% of 11 patches treated 3 times a week for 4 weeks.6 The repigmentation was far from complete, but indicated that excimer laser therapy may be fruitful for vitiligo. Our group has now treated 55 patches of vitiligo in 32 patients for a maximum of 30 treatments or until 75% or greater repigmentation is achieved, whichever came first.7 Patients were treated twice a week with slowly escalating doses, as tolerated. Overall, 52.8% of treated patches repigmented 75% or greater in a mean of 23 treatments. Anatomic location was an important variable: Lesions on the face repigmented 75% or more in 71.5%, lesions on the neck and scalp in 60%, lesions in the genital area in 50%, lesions on the extremities in 46.7%, and lesions on the torso in 40%, whereas no lesions on the hands and feet achieved such repigmentation. Other important variables were skin phototype (darker skin responded better) and the age of the lesion (new lesions responded better than old lesions). In a separate study, we also found that use of an immune response modifier (tacrolimus ointment) potentiates the effect of the laser.8

In sum, excimer lasers are as effective or more effective than conventional phototherapy about one-third of the time. Uninvolved normal skin is spared UV exposure. Response follows the pattern seen with PUVA and narrow-band UVB: Response is greatest on the face, intermediate on the trunk and limbs, and poor on the hands and feet, and is more likely in dark-skinned patients than in fair patients. The response seems to be enhanced with the use of topical immune response modifiers. The use of a topical immune response modifier in areas receiving UV phototherapy does raise the theoretic concern of increased carcinogenesis. However, a paradox of vitiligo is that patients tend not to develop skin cancer in vitiliginous areas, which diminishes this concern. A limitation of the excimer laser is that only a limited skin area can be treated in a single session. Realistically, we have found a 15% surface area to be the upper limit. Other forms of hypopigmentation also appear to be amenable to excimer laser therapy, such as scars, leukoderma following laser resurfacing, and stretch marks. It is certainly possible that other UV-responsive diseases, such as atopic dermatitis and cutaneous T-cell lymphoma (CTCL), will respond as well, but the spot size of the laser will remain a limiting factor for use with widespread disease.

References

  1. Gossop ND, Jackson RW, Koort HJ, et al. The excimer laser in orthopedics. Clin Orthop. 1995;310:72-81.
  2. Bonis B, Kemeny L, Dobozy A, et al. 308nm UVB excimer laser for psoriasis [letter]. Lancet. 1997;350(9090):1522.
  3. Asawanonda P, Anderson R, Chang Y, et al. 308-nm excimer laser for the treatment of psoriasis: a dose-response study. Arch Dermatol. 2000;136(5):619-624.
  4. Feldman SR, Mellen BG, Housman TS, et al. Efficacy of the 308-nm excimer laser for treatment of psoriasis: results of a multicenter trial. J Am Acad Dermatol. 2002;46(6):900-906.
  5. Njoo MD, Bos JD, Westerhof W. Treatment of generalized vitiligo in children with narrow-band (TL-01) UVB radiation therapy. J Am Acad Dermatol. 2000;42(2 pt 1):245-253.
  6. Spencer JM, Nossa R, Ajmeri J. Treatment of vitiligo with the 308-nm excimer laser: A pilot study. J Am Acad Dermatol. 2002;46(5):727-731.
  7. Hadi SM, Spencer JM. 308nm excimer laser for the treatment of vitiligo. Dermatol Surg. 2004;30(7):1732-1739.
  8. Kawalek AZ, Spencer JM, Phelps RG. Combined excimer laser and topical Tacrolimus for the treatment of vitiligo: a pilot study. Dermatol Surg. 2004;30(2):130-135.
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