Application of computer modelling to predict the effect of melanin concentration on treatment time

摘要

The South African population consists of individuals from European, Asian and African descent whopresent a wide range of skin phototypes. This poses a problem when treating patients with lasers due to adifference in melanin content and the associated absorption of light in the epidermal layer for the differentindividuals. Suitably calibrated computer models can be used to predict the fluence rate transmitted throughspecified skin layers. A computer model (allowing for different skin layers) was developed in the ASAPsoftware environment. The epidermal absorption coefficients for 28 individuals was measured with a diffusereflectance probe. Epidermal absorption coefficients between 0.002 and 3 mm-1 were measured and used in thecomputer model. Photodynamic Therapy (PDT) is one of the treatments currently under investigation to treatskin cancer, specifically squamous cell carcinomas (SCC). The fluence rate reaching the tumour (at a depth of200 μm below the surface) was calculated for different absorption coefficients (present in the South Africanpopulation). For an epidermal thickness of 90 μm with an absorption coefficient of 3 mm-1, less than 30 % ofincident light reached the tumour as opposed to the 54% for a very low absorption coefficient (0.002 mm-1). Foreffective treatment a dose of 4.5J/cm2 is required. Therefore for a constant laser power of 50 mW (at the skinsurface) the higher absorbing skin needs to be treated for 413 s and the low absorbing skin needs to be treated foronly 222 s. This variation highlights the need for in-situ calibration and modelling during treatment planning.