The importance of in situ light dosimetry for photodynamic therapy of oral cavity tumors

Background. Photodynamic therapy (PDT) is being evaluated for treatment of localized head and neck cancer. 'Light dose' is usually prescribed as incident fluence, which takes no account of reflected and scattered light. This study investigates variations in total tissue fluence for a given incident fluence in the oral cavity. Methods. Light dosimetry was performed in 19 patients treated with PDT for cancers in the oral cavity and in 5 volunteers. Illumination was with 652 nm laser light delivered via a microlens. In situ dosimetry was performed with isotropic probes held against the tissue in the illuminated area. Results. Tissue fluences of 254% to 305% of the incident fluence were measured in the illuminated area in healthy volunteers. In the patient population tissue fluences were 133% to 545% of the incident fluence. Conclusion. The relationship between incident and total tissue fluence depended on the location and pigmentation of the target area and was not predictable. In situ dosimetry during cavity illumination allows for more controlled tissue illumination and should be employed as the basis for light dose prescription in PDT for head and neck cancer.