Effect of photosensitizer concentration in tissue on the penetration depth of photoactivating light

The additional optical absorption in tissue resulting from the uptake of exogenous photosensitizers increases the effective attenuation of photoactivating light. This may be significant for the irradiation of solid tumours in photodynamic therapy, since it reduces the depth or volume of tissue treated. The effect has been studied in vitro by using dihaematoporphyrin ether (DHE) and 630 nm light in tissues representing a wide range of absorption and scattering conditions. While the attenuation may be markedly changed by small concentrations of DHE in pure scattering media, tissues with significant inherent light absorption are little affected by the additional absorption of DHE at concentrations relevant to clinical photodynamic therapy. However, it is shown that for other potential photosensitizers such as the phthalocyanines, which have substantially greater absorption at the treatment wavelength than DHE, the penetration of light in tissues may be significantly reduced.