The Council for Scientific and Industrial Research (CSIR) in South Africa is one of the leading scientific and technology research, development and implementation organisations in Africa. It undertakes directed research and development for socio-economic growth.

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February 2009 edition

Laser technology

Studying the effect of laser therapy on different skin tones

"For laser treatment to be effective in South Africa, it is important to take skin tone into consideration," CSIR National Laser researcher Aletta Karsten says, "During the past two decades the use of lasers as a treatment modality has increased dramatically. One of the applications is photodynamic therapy, which is a cancer treatment modality where a photosensitiser (PS) is administered to the patient. The PS accumulates mainly in the cancerous tissue and after a period of time the tumour is irradiated by a laser or another light source." She says, "The wavelength is carefully chosen to coincide with one of the absorption peaks. The penetration of skin is wavelength dependent. The energy of the photons is absorbed by the PS and singlet and triplet oxygen is formed inside the cells. This is lethal to the cells and causes cell death. "

Karsten, who is involved in studies into the penetration depth of lasers on different skin tones, has determined that in order to achieve the desired effect, it is important to supply enough laser light to kill the tumour cells without damaging the healthy surrounding tissue. For embedded tumours this becomes more difficult due to the absorption of light in the first layers before the tumour. She says, "In my study five different skin tones from very light to dark were evaluated. I have determined that while most of the absorption takes place in the first 0.15 mm into the skin, which is inside the epidermal layer, light still propagates to deeper levels. In dark skin, the absorption can be at least three times more than in very fair skin." As a result Karsten warns, "Care should be taken to ensure that no harm is done to healthy skin during laser treatments by depositing too much laser energy on darker skin tones. When treatment parameters are specified, it is important to note the amount of light that is absorbed in the epidermis to ensure that enough light will reach the deeper levels where the tumour is embedded."

In addition, she says, "When comparing the different laser wavelengths, it also became clear that the amount of absorption is dependent on the wavelength. Thus, the effect can differ significantly if the evaluations are not conducted at the correct wavelength." Although the penetration of laser light is a function of the wavelength, the laser wavelength used for a specific photodynamic therapy treatment is determined by the absorption features of the PS.

"While this work illustrated the effect of two parameters (wavelength and absorption properties) on the effective dose at different depths into the skin, this is not a completed study. The effect of parameters such as the shape of the input beam and the oxygen concentration in the skin or tumour still needs to be investigated and modelled. This will form the basis of our future work in this area," she says.

Enquiries: CSIR Communication

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