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

Singlet oxygen ( ¹O ₂) is generally believed to be the major cytotoxic agent during photodynamic therapy (PDT), and the reaction between ¹O ₂ 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 1O2 concentration, [ ¹O ₂]rx, in a form related to time integration of the product of ¹O ₂ quantum yield and the PDT dose rate. The production of [ ¹O ₂]rx 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 [ ¹O ₂]rx to be an explicit dosimetric indicatorfor clinical PDT. Incorporating the diffusion equation governing the light transport in turbid medium, the spatially and temporally-resolved [ ¹O ₂]rx described by the macroscopic kinetic equations can be numerically calculated. A sudden drop of the calculated [ ¹O ₂]rx along with the distance following the decrease of light fluence rate is observed. This suggests that a possible correlation between [ ¹O ₂]rx 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 [ ¹O ₂]rx are expected to be used to fit the experimentally-measured necrosis data obtained from an in vivo animal model.