Therapeutic response during pulsed laser treatment of port-wine stains: Dependence on vessel diameter and depth in dermis

Abstract

Abstract. Selective photothermolysis with pulsed lasers is presumably the most successful therapy for port-wine stain birthmarks (flammeus nevi). Selectivity is obtained by using an optical wavelength corresponding to high absorption in blood, together with small absorption in tissues. Further on, the pulse length is selected to be long enough to allow heat to diffuse into the vessel wall, but simultaneously short enough to prevent thermal damage to perivascular tissues. The optical wavelength and pulse length are therefore dependent on vessel diameter, vessel wall 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 40~0 Fm require minimum optical fluence. Smaller vessels require higher fluence because the amount of heat needed to heat the wall becomes a substantial fraction of the absorbed optical energy. Larger vessels also require a higher dose because the attenuation of light in blood prevents the blood in the centre of the lumen from participating in the heating process. It is shown that the commonly used optical dose in the range of 6-7 J cm-2 is expected to inflict vessel rupture rather than thermolysis in superficially located vessels. The present analysis might serve to draw guidelines for a protocol where the optical energy, wavelength and pulse length are optimized with respect to vessel diameter and depth in dermis.

Wavelengths for port wine stain laser treatment: influence of vessel radius and skin anatomy

“…Appropriate choice of this parameter is important to ensure heat deposition remains confined to the vessel volume and has been discussed previously (Svaasand et al 1995, de Boer et al 1996.…”

“…An explanation for the difference between the models is that RBCs at the centre of the lumen contribute less to light attenuation than those adjacent to the vessel wall, particularly if the radius is much larger than the blood absorption length (Svaasand et al 1995. Therefore, as blood absorption increases, the number of RBCs per vessel that contribute to heat production decreases.…”

“…Thus, light attenuation is higher in the homogeneous than discrete vessel model. A correction factor should be used to reduce the number of RBCs in the homogeneous model so the fluence rate distribution obtained is equal to that from the discrete vessel model (Svaasand et al 1995. However, although use of the correction factor explains the different results from the two models, it does not explain the wavelength-dependent effects observed in PWS patients by Tan et al (1990).…”

Laser surgery of port wine stains using local vaccum pressure: Changes in calculated energy deposition (Part II)

“…PWS vessel diameters range from 10 to 100 mm, and from 200 to 1,000 mm in depth [11]; therefore, vessel diameter and depth affect response to laser irradiation. Several studies have shown that specific pulse durations effectively target vessels within a specific diameter range, while smaller and larger vessels require higher fluences (for the same pulse durations) to reach the same average blood temperatures [12][13][14]. In recent clinical studies, Svaasand et al [15] used a pressure cuff on the upper arm of PWS patients to increase the blood volume fraction thereby reducing by 40% the fluence needed to reach the same degree of purpura in the absence of pressure.…”

Synergistic photodynamic and photothermal treatment of port‐wine stain?

“…Unlike photothermolysis, which spares microvessels (5-20 mm diameter) from PDL-induced photocoagulation [19], PDT destroys all vessels containing photosensitizer. A sub-therapeutic PDT exposure, using yellow light (l ¼ 576 nm) absorbed by BPD, could conceivably be used to make PWS blood vessels more vulnerable to subsequent photothermal damage.…”