Phototherapy light therapy is one of the oldest forms of treatment where matter is exposed to daylight or certain wavelengths of light through the use of various light sources such as lasers, light emitting diodes (LEDs), fluorescent/dichroic lamps (1)
Niels Finsen is thought to be the pioneer of modern phototherapy. He developed the first artificial UV light source for the treatment of lupus vulgaris, a serious tuberculosis skin infection (2). Phototherapy then advanced to utilising LEDs and lasers as the light sources which are still used to effectively treat the world population today. Lasers are used on a daily basis for surgery, dentistry and dermatology (3). More importantly the use of lasers and LEDs to help with early detection of breast cancer, non-invasive measurement of blood glucose and early detection of tooth decay are being investigated and developed (4).
Since light was first used in medical applications there have been great advances and so the question no longer remains does light have biological effects, but significantly how can energy from lasers and LEDs working at cellular and organism levels improve treatments, and what optimal light parameters are needed for different uses of these light sources.
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To explore some examples of biological and medical applications of UV, visible and IR light we must first understand the interaction of light with a single atom; molecules; liquids and solids.
Light (also known as electromagnetic radiation) is energy and is made up of photons or discrete bundles of electromagnetic energy and transfers energy through space. The energy one individual photon is dependant only on wavelength, the 'energy content per photon' is a way of characterising different ranges of light that construct the electromagnetic spectrum (3).
Figure : The electromagnetic spectrum showing the difference in wavelength, colour and energy of the different types of electromagnetic waves (4).
As can be seen from Figure 1, blue photons have greater energy than green ones, and they in turn have greater energy than red ones, that have greater energy than NIR onesâ€¦etc. It can therefore be concluded that the energy of a "dose" of light is dependant solely upon the number of photons and their specific wavelength or colour.
Photons that are delivered into living tissue experience either absorption or scattering. The scattered photons are eventually absorbed or if not, they can escape from the tissue via diffuse reflection. Once absorbed the photons interact with water or other absorbers called chromophores, both of which are located in the tissue. Most of the photons tend to have wavelengths that confine them to the red/NIR regions of the spectrum, so the chromophores that absorb them have a tendency of having delocalized electrons in molecular orbitals which are excited from the ground state up into a first excited state by a quantum of energy provided by the photon (5). In addition to these electronic transitions we must also consider the transitions due to the vibration and rotation of the molecule. The energy difference between the electronic transitions is much larger than any difference between vibrational and rotational transitions. This is because vibrational and rotational transitions occur in the same energy state whereas electronic transitions occur between two different energy states. The excited electrons are unstable and decay giving their energy to the tissue in the form of heat, and based on the first law of thermodynamics, the energy transferred to the tissue remains conserved.
Examples of medical application of light
Photodynamic therapy (PDT)
This form of treatment can be used to treat shallow cancers. A photosensitising drug (for example porfimer sodium) is injected into the bloodstream of the patient and is taken up by all cells in the body, but remains in the cancer cells for a longer length of time. The tumour is then exposed to a specific wavelength of light, the photosensitizer drug in the tumour absorbs the light, and a form of oxygen known as singlet oxygen is produced. It is a very aggressive chemical species, so reacts with and destroys the nearby cancer cells. The specific wavelength of light is produced from either a laser or an LED and controls the distance that the light can travel into the tissue of the patient (7).
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PDT has limitations in that the light can't pass through greater thickness of skin so large tumours can't be treated, thus research into stronger photosensitizers that are activated by light with greater penetration availability are being investigated (8).
Lasers are widely used in medicine today. A list of commercially available ones can be seen in Table 1 below.
Figure : Commercially available lasers and their medical applications (3)
In surgery, a highly collimated laser beam is aimed at the tissue needed to be cut, light is absorbed and heats the tissue to boiling point. Continuous evaporation of this tissue occurs and so the next layer of tissue is heated to boiling pointâ€¦etc. This can be achieved with a CO2 laser.
In dentistry surgical operations in the small area of the mouth are made easier by the ease of use and precision of laser beams. The prevention of cavities is achieved by the surface vitrification (converted to a glassy substrate) of the enamel of the tooth by a laser pulse. After melting has occurred the enamel exposed to the laser solidifies without microholes, which are responsible for infections that cause cavities. Once again a CO2 laser is used (3).
Different lasers are chosen for different purposes due to their specific characteristics. For example in general the longer the wavelength of the light, the deeper the penetration into tissue is achieved. Another example is the use of CO2 and Nd:YAG lasers to destroy internal tumours as the light from these layers can travel through endoscopes that allows doctors to see parts of the inside of the body that could otherwise not be reached.
Interaction of UV light with the skin
There are many other uses of light in medical applications but the area in which I am most interested in is the use of UV light to treat the skin condition psoriasis.
Psoriasis is an immune-inflammatory and proliferative skin disease that is triggered by abnormal lymphocytes from blood. In 1.5-3% of the population (10) it results in red, dry, thick and itchy patchy of skin anywhere on the body and can be mild, moderate or severe (10). Narrow band UV-B (280-315nm) treatment is the typical light therapy utilised for treating more severe cases of psoriasis, and a cream for lesser cases. The length, type and intensity of treatment are specific to each individual patient.
UV-B helps with psoriasis as it increases the levels of vitamin D in the patients (11). Vitamin D is mostly obtained by skin production after exposure to UV-B from the sun, and as it is not common in food, less than 15% is obtained from foods such as oily fish.
Melanin pigment present in our skin competes for and then absorbs UV-B photons responsible for the photolysis of the precursor 7-dehydrocholesterol (found in the skin) to previtamin D3. This process only occurs at wavelengths 290-315nm, but the optimum wavelength for both in vivo and in vitro vitamin D3 synthesis is 300 Â± 5nm. This previtamin D3 is then quickly converted to vitamin D3 (Cholecalciferol) (13) (14).
There are some disadvantages to UV-B treatment of psoriasis, the first being photoaggravation of psoriasis through the Koebner phenomenon. This phenomenon is when psoriatic lesions appear on traumatised areas of skin triggered by sunburn. These cases are worse when using Broadband UV-B (BB UV-B) than NB UV-B. Erythema reactions result in red inflamed skin and can be caused by many things including insect bites, exposure to heat, sunlight and UV (15). Studies have revealed that with NB UV-B, erythema reactions are much more painful than predicted for the patients' degree of erythema. The advantage of using NB UV-B over BB UV-B appears to be that effectiveness in psoriasis removal is disassociated from the ability to create erythema, so treatments that are less likely to cause 'burning' than the 'nearerythemogenic' treatments, traditionally used with BB-UVB, are appropriate (15).
In conclusion using light in biological and medical applications has been recognized for hundreds of years. These applications rely on basic physical principles (especially when using lasers) and are now being used to revolutionize how diseases can be detected, treated and monitored.
1. The history of phototherapy: something new under the sun? Roelandts, R. s.l.Â : J Am Acad Dermatol , 2002, Vol. 46.
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