Optimization of physiological parameter for macroscopic modeling of reacted singlet oxygen concentration in an model.

Singlet oxygen (O) is generally believed to be the major cytotoxic agent during photodynamic therapy (PDT), and the reaction between O and tumor cells define the treatment efficacy. From a complete set of the macroscopic kinetic equations which describe the photochemical processes of PDT, we can express the reacted O concentration, [O], in a form related to time integration of the product of O quantum yield and the PDT dose rate. The production of [O] involves physiological and photophysical parameters which need to be determined explicitly for the photosensitizer of interest. Once these parameters are determined, we expect the computed [O] to be an explicit dosimetric indicator for clinical PDT. Incorporating the diffusion equation governing the light transport in turbid medium, the spatially and temporally-resolved [O] described by the macroscopic kinetic equations can be numerically calculated. A sudden drop of the calculated [O] along with the distance following the decrease of light fluence rate is observed. This suggests that a possible correlation between [O] and necrosis boundary may occur in the tumor subject to PDT irradiation. In this study, we have theoretically examined the sensitivity of the physiological parameter from two clinical related conditions: (1) collimated light source on semi-infinite turbid medium and (2) linear light source in turbid medium. In order to accurately determine the parameter in a clinical relevant environment, the results of the computed [O] are expected to be used to fit the experimentally-measured necrosis data obtained from an animal model.