Investigating the impact of oxygen concentration and blood flow variation on photodynamic therapy

Type II photodynamic therapy (PDT) is used for cancer treatment based on the combined action of a photosensitizer, a special wavelength of light, oxygen (³O₂) and generation of singlet oxygen (¹O₂). Intra-patient and inter-patient variability of oxygen concentration ([³O₂]) before and after the treatment as well as photosensitizer concentration and hemodynamic parameters such as blood flow during PDT has been reported. Simulation of these variations is valuable, as it would be a means for the rapid assessment of treatment effect. A mathematical model has been previously developed to incorporate the diffusion equation for light transport in tissue and the macroscopic kinetic equations for simulation of [³O₂], photosensitizers in ground and triplet states and concentration of the reacted singlet oxygen ([¹O₂]_rx) during PDT. In this study, the finite-element based calculation of the macroscopic kinetic equations is done for 2-(1-Hexyloxyethyl)-2-devinyl pyropheophorbide (HPPH)-mediated PDT by incorporating the information of the photosensitizer photochemical parameters as well as the tissue optical properties, photosensitizer concentration, initial oxygen concentration ([³O₂]₀), blood flow changes and ϕ that have been measured in mice bearing radiation-induced fibrosarcoma (RIF) tumors. Then, [¹O₂]_rx calculated by using the measured [³O₂] during the PDT is compared with [¹O₂]_rx calculated based on the simulated [³O₂]; both calculations showed a reasonably good agreement. Moreover, the impacts of the blood flow changes and [³O₂]₀ on [¹O₂]_rx have been investigated, which showed no pronounced effect of the blood flow changes on the long-term ¹O₂ generation. When [³O₂]₀ becomes limiting, small changes in [³O₂] have large effects on [¹O₂]_rx.