Photodynamic therapy with pulsed light sources: a theoretical analysis.

The effectiveness of photodynamic therapy using pulsed sources was evaluated using a mathematical model describing the time-dependent excitation and de-excitation of the photosensitizer molecule. Using the various numerical data available in the literature on haematoporphyrin we calculated that the effectiveness of pulsed excitation in PDT is identical to that of CW excitation for peak fluence rates below 4 x 10(8) W m-2. Above this threshold the effectiveness drops significantly. In practice this effect will occur with sources with high pulse energy and low repetition frequency. The commonly used dye lasers pumped by either a Cu vapour laser or a frequency doubled Nd:YAG laser have a PDT effectiveness identical to that of a CW source of the same wavelength and the same average fluence rate.