Skin lesions suitable for photodynamic therapy
What is PDT and how does it work?
PDT utilises photosensitising agents, oxygen and light, to create a photochemical reaction that selectively destroys cancer cells. Photosensitising agents are drugs that are administered into the body through topical, oral or intravenous methods. In the body, they concentrate in cancer cells and only become active when light of a certain wavelength is directed onto the area where the cancer is. The photodynamic reaction between the photosensitising agent, light and oxygen kills the cancer cells.
Listed below are several photosensitising agents currently being used in PDT.
|Methyl aminolevulinic acid (Metvix™) cream||
|Aminolevulinic acid hydrochloride topical solution (Levulan® Kerastick®)||
|Porfimer sodium (Photofrin™)||
|Benzoporphyrin derivative monacid ring A||
|Tin ethyl etiopurpurin|
Light sources used in PDT include laser or nonlaser light. Laser light has the advantages of being:
- monochromatic (exactly one colour/wavelength that corresponds with the peak absorption of the photosensitising agent)
- coherent (able to focus lightwaves to specific site)
- intense (high irradiance allowing for shorter treatment times)
Laser light is suitable for small skin lesions whilst nonlaser light is better for the treatment of large skin lesions as the field of illumination is larger. Nonlaser light that emits polychromatic light is also suitable when using different photosensitisers with different absorption maxima.
Recently, natural daylight has been used successfully as a light source for the treatment of actinic keratoses.
What is PDT used for?
PDT is currently being used or investigated as a treatment for the following skin conditions:
- Actinic keratoses on the face and scalp
- Basal cell carcinomas
- Intraepidermal squamous carcinoma (squamous cell carcinoma in situ, Bowen disease)
- Squamous cell carcinoma
- Mycosis fungoides (cutaneous T-cell lymphoma)
- Kaposi sarcoma
- Viral warts
How is PDT given?
For the treatment of skin cancers PDT is performed in the rooms of your dermatologists or practitioner:
What are the possible side effects of PDT?
Side effects from PDT are due to the treated area being sensitive to light. The photosensitivity usually lasts about 24 hours (depending on the specific agent). Side effects may include:
- Burning/stinging sensation
- Swelling and redness
- Peeling and blisters
- Skin infections
The treated area should be protected from light exposure using a dressing. A local anaesthetic such as lignocaine (lidocaine) spray may be applied to the treatment area before or during Stage 2 of the procedure to help relieve pain.
The treated skin lesion may blister and ulcerate as the cancer cells die off. This may take several weeks to heal. Scarring is generally minimal (but can be moderate). Loss of pigmentation may occur sometimes and can be permanent.
Although photosensitising drugs concentrate in cancer cells, they can also make healthy cells more sensitive to light. This is not a problem when photosensitising creams are used as they are localised to the treatment site. It is more of a problem when photosensitising drugs are given by mouth or injected intravenously. These patients may find all parts of their body sensitive to light and should take precautions to protect themselves from light for the necessary period of time (may be days or weeks depending on the photosensitising drug used).