In Vivo Monitoring of Photodynamic Therapy: from lab to clinic

Photodynamic therapy (PDT) is an emerging clinical treatment modality, which utilizes light, oxygen and a light sensitive drug (the photosensitizer), for curative and palliative treatment of a variety of malignant and non-malignant conditions tumors. PDT is frequently used in the clinic for treatment of superficial skin lesions by superficial illumination of the lesion after the topical administration of a photosensitizer or its precursor. However, PDT is also under investigation for treatment of larger tumors volumes in regions such as the head and neck1,2 and the prostate3 by inserting opticfibers in the tumor volume for the delivery of the treatment light. The therapeutic effect in PDT is induced by the interaction between the tissue and reactive oxygen radicals. These reactive oxygen radicals, predominantly singlet oxygen, are formed by the interaction of photosensitizer, light (of an appropriate wavelength) and oxygen in the tissue. The deposited PDT dose is the amount of light that actually interacts with the photosensitizer that leads to formation of reactive oxygen species responsible for inducing tissue response. Note that this is different from the actual amount of light delivered to the tissue since not all of the light delivered interacts with the photosensitizer scattering and absorption of the tissue. In addition, based on the photosensitizer’s ability to form reactive oxygen species (ROS) only a percentage of light that interacts with the photosensitizer lead to formation of ROS. Also there is a difference between the actual delivered light dose and the intended delivered light dose. Where the intended delivered light dose is set by the clinician to be delivered. Only by measuring the amount of light in situ it is possible to determine the actual delivered light dose. So although this intended light dose is kept the same in individuals undergoing treatment, the actual delivered light dose and the deposited PDT dose can vary due to biological variation and the dynamic interaction between light, photosensitizer and oxygen in tissue. Inter individual variations in deposited PDTdose yield variations in induced tissue response and treatment outcome. For this reason it is necessary to determine and monitor the deposited PDT dose during therapeutic illumination.

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