How to determine an appropriate dose when using various light sources in photodynamic therapy

Photodynamic therapy (PDT) using 5-Aminolaevulinic Acid (5-ALA) has emerged as a very effective treatment for superficial skin lesions. PDT has been traditionally performed using a laser. However, the availability of broad-band non-laser sources is challenging the role of the laser. Unfortunately, dosimetry of broad-band sources presents some practical difficulties. The effectiveness of a light source for PDT depends on the incident spectral irradiance of the light, tissue transmission to the desired depth, and sensitiser absorption which depends on the particular absorbing chromophore. These elements are combined mathematically to produce the ''total effective fluence rate'' at a specified depth. On applying the model to several light sources, it would appear that green light is much more effective than red light for treating down to a depth of 2.0 mm in dermis but red light appears to give better superficial skin sparing. The model may be easily applied to any light source provided the incident spectral irradiance is known. It is believed that this concept has considerable practical value.