Light dosimetry in animal models: application to photodynamic therapy in otolaryngology

Photodynamic therapy (PDT) for treatment of head and neck cancer uses a photoactive compound that is illuminated with 630 nm (red) light. The effectiveness of PDT depends on the penetration of light into tissue that is both tissue and wavelength dependent. The characterization of the optical properties of an animal oral mucosa and skin has been done to determine the amount of light below these tissues available to be used for photodynamic therapy. The tissue absorbance of visible light from 400 nm to 700 nm has been determined in vitro for hamster check pouch mucosa and for athymic mouse skin. The pattern of absorbance is similar for both tissues and demonstrates greater transmission at the longer wavelengths. The diffuse transmittance of light in vivo for these animal models was measured with an interstitial fiberoptic probe. At 630 nm the diffuse transmittance for nude mouse skin averages 10% of the incident light energy, and that for the hamster mucosa almost 50% of the incident light energy.