Photodynamic therapy…

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What is photodynamic therapy used to treat?

This article is for medical professionals

Professional reference articles are designed for the use of health professionals. They are written by UK doctors and are based on research evidence, UK and European guidelines. You can find one of our health articles more useful.

What is photodynamic therapy?

Photodynamic therapy (PDT) is a treatment method that has been developing rapidly within various medical specialties since the 1960s. Photoreactive chemicals are injected into the patient and irradiated with enough light to activate the chemicals, emitting free radicals and destroying the targeted abnormal cells. An understanding of the principles of photobiology and tissue optics is necessary to understand how this works. Originally used to treat some malformations, it is currently being used in the treatment of some forms of macular degeneration, and various skin conditions such as basal cell carcinomus (BCC), squamous cell carcinomus (SCC) , Actinic keratose, Bowen’s disease, psoriasis, cutaneous) photorejuvenation of T-cell lymphoma, pimples and wrinkles.

How does it work?

Photodynamic effects require:

A chemical photosensitizer

  • Most have a heterogeneous ring structure similar to chlorophyll or heme in hemoglobin.
  • Adequate tissue levels are required to function.
  • The drug can be administered systematically, topically or directly to the organ. There is a low risk of systemic side effects in topical administration for superficial lesions.
  • The selectivity of drugs varies for different hyper-proliferating tissues. For example, sodium porfimer is distributed to connective tissue and blood vessels, while 5-aminolevulinic acid (ALA) is concentrated in the mucosal layers.
  • Thus the choice of photosensitizing drug depends on the nature of the wound being treated.

Light of appropriate wavelength

Each photosensitizer has a special spectrum, requiring light of the appropriate wavelength for maximum absorption and impact.

The clinically used sensitizer operates between 420 nm (blue) and 780 nm (dark red). Longer wavelengths penetrate deeper (blue 1–2 mm and red more than 5 mm). Newer agents are likely to work with clinically longer wavelengths.

Oxygen

This is essential for the effect and therefore the well-scented and oxygen-rich tissue is required for the technique to work.

In oxygen molecules, energy is transferred as light through an intracellular fotensitizer. Oxygen then forms a highly reactive mediator. They have a short half-life (a fraction of a second) and thus have a very localized tissue-damaging effect. The energy involved is low (so that hyperthermia is unlikely to occur) and does not harm the surrounding organs.

What are the possible benefits of photodynamic therapy?

The potential benefits of PDT are:

  • Scarring is unlikely, as collagen and elastin are unaffected, leaving organs intact.
  • Highly selective tissue necrosis. It is obtained by:
  • Localization of drug to proliferating tissue.
  • Selective uptake of photocentritis, particularly to the tissue layers.
  • Accurate directing of laser light using optical fiber.
  • Resistance to treatment does not develop with repeated treatment.

What are the limitations of photodynamic therapy?

The technique has several limitations:

  • The direction of light requires an appropriate site and tissue depth to be effective.
  • It is ablative and does not provide material for histological diagnosis. Diagnosis must be made before treatment.
  • It is complex, requires optimal light distribution with the laser, and there is collaboration and coordination between practitioners.
  • Frequent skin photo sensitivity, permanent weeks with some photosensitis, range usage.
  • Availability of required light sources has been a problem. Now low-cost portable light sources are more readily available.

Side effects

The experience of PDT will vary from one person to another. How treatment is given and its side effects vary:

  • Which part of the body is treated.
  • Type of photosensitizing drug.
  • The time between giving the medicine and applying light.
  • Skin sensitivity to light after treatment.