Abstract
Despite application of cryogen spray (CS) precooling, customary treatment of port wine stain (PWS) birthmarks with a single laser pulse does not result in complete lesion blanching for a majority of patients. One obvious reason is nonselective absorption by epidermal melanin, which limits the maximal safe radiant exposure. Another possible reason for treatment failure is screening of laser light within large PWS vessels, which prevents uniform heating of the entire vessel lumen. Our aim is to identify the parameters of sequential CS cooling and laser irradiation that will allow optimal photocoagulation of various PWS blood vessels with minimal risk of epidermal thermal damage. Light and heat transport in laser treatment of PWS are simulated using a custom 3D Monte Carlo model and 2D finite element method, respectively. Protein denaturation in blood and skin are calculated using the Arrhenius kinetic model with tissue-specific coefficients. Simulated PWS vessels with diameters of 30-150 µm are located at depths of 200-600 µm, and shading by nearby vessels is accounted for according to PWS histology data from the literature. For moderately pigmented and dark skin phototypes, PWS blood vessel coagulation and epidermal thermal damage are assessed for various parameters of sequential CS cooling and 532-nm laser irradiation, i.e. the number of pulses in a sequence (1-5), repetition rate (7-30 Hz), and radiant exposure. Simulations of PWS treatment in darker skin phototypes indicate specific cooling/irradiation sequences that provide significantly higher efficacy and safety as compared to the customary single-pulse approach across a wide range of PWS blood vessel diameters and depths. The optimal sequences involve three to five laser pulses at repetition rates of 10-15 Hz. Application of the identified cooling/irradiation sequences may offer improved therapeutic outcome for patients with resistant PWS, especially in darker skin phototypes.
Highlights
Treatment of cutaneous vascular lesions usually employs pulsed lasers with wavelengths of 532 nm or 585–595 nm, according to the principle of selective photothermolysis [1]
The radiant exposure per pulse is equal to the epidermal damage threshold, Hepi (i.e., 5.4 and 3.7 J/cm2 for the first and second case, respectively)
The maximum temperature of 57°C is reached with the multiple cryogen spurts (MCS)-multiple laser pulses (MLP) sequence, 4°C less than with the single laser pulse
Summary
Treatment of cutaneous vascular lesions usually employs pulsed lasers with wavelengths of 532 nm or 585–595 nm, according to the principle of selective photothermolysis [1]. In addition to nonselective absorption of laser light by epidermal melanin, which limits the maximal safe radiant exposure, one plausible reason for poor treatment outcome is optical screening in large PWS vessels, due to the limited penetration depth of the strongly absorbed laser light [5]. This effect prevents uniform coagulation of the entire vessel lumen with single short laser pulses, allowing the vessels to recover from such partial damage [6,7]. Conclusions—Application of the identified cooling/irradiation sequences may offer improved therapeutic outcome for patients with resistant PWS, especially in darker skin phototypes
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