A COMPUTER SIMULATION MODEL FOR ANALYSIS OF RISE IN SKIN TEMPERATURE FOLLOWING IRRADIATION WITH THE GaAlAs DIODE LASER

Abstract

A popular benchmark for defining a laser therapy systcm is an output power of less than 100 mW, and low reactive-level laser therapy (LLLT) is commonly regarded as ‘safe’ for irradiating skin. However, with very small spot sizes in the order of 200 μm, the power density at tissue of a 100 mW laser therapy system is in the order of 318 W/cm2. If target tissue is painted with black ink or a similar pigment, this power density is quite capable of causing a burn injury. This has obvious implications in the use of such LLLT systems in treating dark-coloured naevi such as naevus cell naevus, or in LLLT on black patients. In order to examine this problem in detail, the authors designed a computer model with a program to calculate and analyze the temperature rise in an irradiated hemispherical block of homogeneous tissue at various powers in the range of 6 mW- 60 mW, and at spot sizes of 200 μm, 400 μm, 1.0 mm and 2.0 mm following 5 seconds irradiation with an 830 nm GaAlAs diode laser, after which the laser was ‘turned off’ The laser was deemed to be applied in the contact pressure mode, thereby compressing superficial blood vessels in the irradiated area so that blood flow cooling could be discounted. At maximum absorption, it was predicted that the 60 mW beam of the GaAlAs diode laser at 830 nm and a 200 μm spot size has the potential for causing thermal injury. The authors would like to stress that this is only a theoretical computer model, with hypothesized constants and a homogeneous target material. However, this model certainly demonstrates the error of applying arbitrary limits as to what constitutes the output power of an LLLT system, and would also appear to limit the application of LLLT at the above parameters on dark-colored naevi or for darker-skinned patients.