Photodynamic therapy (PDT) involves the administration of a “photoactive” agent to the patient followed by exposure of the affected area to light. The photoactive agent that has been most used belongs to a group of substances known as synthetic porphyrins. When illuminated, the agent will be involved in a complex chemical reaction leading to the destruction of the cells (see figure below).

The great advantage of PDT is that the therapeutic effect will only take place in the illuminated area and only during illumination. This allows for a high degree of therapeutic control and a significant reduction of side effects.

However, injection of synthetic porphyrin into a patient results in a period of general photosenitivity which can last for weeks. During this period the patient should not be exposed to sunlight in order to avoid phototoxic reactions (severe sunburns).

Illustration of the mechanism of Metvix PDT treatment: