Optimal laser parameters for port wine stain therapy: a theoretical approach.

The optimal parameters for laser therapy of port wine stains (PWS) have been deduced from temperature calculations on two models: (i) the four-layer model; and (ii) the tube model in which two plane parallel layers, representing the epidermis and dermis, and a dermal rectangular blood vessel are considered. The calculations were performed with a vessel of average cross section 0.06 mm X 0.08 mm located in the centre of the laser beam. The numerical calculations were performed by an alternate direction-implicit finite difference method. The optimal parameters were: wavelengths lambda = 415, 577 and 540 nm; pulse time, 1 ms less than t1 less than or equal to 10 ms; and beam radius W1 greater than 0.1 mm. The energy densities E1 (for t1 = 1 ms) required to coagulate blood vessels down to a depth of 0.65 mm in order to establish bleaching of the PWS were 0.5, 1.6 and 2 J cm-2 for the different lambda respectively. The value for the argon laser (lambda = 488 and 514.5 nm) was E1 = 6.5 J cm-2 (t1 = 1 ms). Because, for this pulse time, heat summation effects at the boundary of the laser beam cause no drastic increase in local temperature at optimal wavelengths, the stripe technique was again considered and compared with the separated spot technique. The Nd-YAG and CO2 lasers prove even less selective than the argon laser. The influence of cooling the skin with water shows that only for lambda = 577 nm and t1 = 0.1 s is there an increase in E1 from 2.5 to 6 J cm-2 for which dermal damage occurs.