THERAPEUTIC RESPONSE DURING PULSED-LASER TREATMENT OF PORT-WINE STAINS - DEPENDENCE ON VESSEL DIAMETER AND DEPTH IN DERMIS

Selective photothermolysis with pulsed lasers is presumably the most s uccessful therapy for port-wine stain birthmarks (flammeus nevi). Sele ctivity is obtained by using an optical wavelength corresponding to hi gh absorption in blood, together with small absorption in tissues. Fur ther on, the pulse length is selected to be long enough to allow heat to diffuse into the vessel wall, but simultaneously short enough to pr event thermal damage to perivascular tissues. The optical wavelength a nd pulse length are therefore dependent on vessel diameter, vessel wal l thickness and depth in dermis. The present work demonstrates that in the case of a 0.45 ms long pulse at 585 nm wavelength, vessels of 40- 60 mu m require minimum optical fluence. Smaller vessels require highe r fluence because the amount of heat needed to heat the wall becomes a substantial fraction of the absorbed optical energy. Larger vessels a lso require a higher dose because the attenuation of light in blood pr events the blood in the centre of the lumen from participating in the heating process. It is shown that the commonly used optical dose in th e range of 6-7 J cm(-2) is expected to inflict vessel rupture rather t han thermolysis in superficially located vessels. The present analysis might serve to draw guidelines for a protocol where the optical energ y, wavelength and pulse length are optimized with respect to vessel di ameter and depth in dermis.