Photodynamic therapy (PDT) is a method of treating sun damage and certain types of skin cancer and pre-cancerous lesions, as well as augmenting the effect of some other light-based treatments e.g. for hair removal and for acne. A substance is used which is taken up into dividing cells. As cancerous and pre-cancerous lesions are growing, they will take up more of this substance than normal skin. This applies also to inflamed areas such as acne lesions. The substance is a photosensitizing agent, so areas that have taken it up become more sensitive to light.
When bright light is shone on such lesions, they are selectively affected more than the normal surrounding areas. As the normal surrounding area will also take up some of the substance, but more slowly, bright light must be avoided afterwards for 48 hours. This means one must stay indoors, or wear covering clothing, or a dressing for 2 days afterwards, otherwise blistering or swelling may occur. The treatment process involves a quick microdermabrasion for facial areas (or aggressive loofahing at home for body areas) to remove dead cell build-up, followed by application of the substance.
This is generally left on for an hour and then washed off. A bright red light is then shone on the treatment area for 20 minutes. As the substance absorbs the light energy, it can feel quite painful, so to reduce this, refrigerated air is blown over the area. After the treatment, the area is covered to protect it from further light exposure and this protection/light avoidance continues for 48 hours. Lesions that have taken up more of the substance will become inflamed and may form sores. These will then heal, leaving the skin more healthy than before. Healing time varies, with the face being quicker than the body and young, healthy patients healing more quickly than older people.
PDT does not help all types of skin cancer (eg. melanoma), so it does not replace all other forms of therapy such as excision. It is very useful for actinic/solar keratoses and many forms of basal cell carcinoma, and can treat large areas of sun damage where it is sometimes difficult to see where one lesion starts and another finishes. It can also be useful in treating lesions before they become visible to the naked eye.
Another use for PDT is to augment light-based treatments for acne or hair removal. Due to the 48 hour light-avoidance issue, this would only be used in stubborn, resistant cases which had not responded sufficiently to the standard therapy.