Influence of the photosensitizer photobleaching in the propagation of light during Photodynamic Therapy

Photodynamic Therapy (PDT) is an optical treatment modality used to destroy malignant tissues. Nowadays there are fixed clinical PDT protocols that make use of a particular optical dose, photosensitizer amount and drug-light interval. However the treatment response varies depending on the type of pathology and the patient. In order to adjust current dosimetry to get an optimal treatment outcome, the development of accurate predictive models has emerged as the ideal tool to achieve new personal protocols. Several attempts have been made in this way although the influence of the photosensitizer distribution on the optical parameters has not been taken into account until this moment. We present a first approach to predict the spatial-temporal variation of the absorption coefficient during the photodynamic process applied to a dermatological disease taking into account the photobleaching of a topical photosensitizer. The model presented also takes into account an inhomogeneous initial distribution of the photosensitizer, the propagation of light in the biological media and the evolution of the molecular concentrations of different components involved in the photochemical reactions. The obtained results permit us to investigate how the depletion of the photosensitizer during the photochemical reactions affects to the light absorption as it propagates within the target tissue.