Explicit dosimetry for photodynamic therapy: Macroscopic singlet oxygen modeling

Singlet oxygen (¹O₂) is the major cytotoxic agent responsible for cell killing for type-II photodynamic therapy (PDT). An empirical four-parameter macroscopic model is proposed to calculate the "apparent reacted ¹O₂ concentration", [¹O₂]_rx, as a clinical PDT dosimetry quantity. This model incorporates light diffusion equation and a set of PDT kinetics equations, which can be applied in any clinical treatment geometry. We demonstrate that by introducing a fitting quantity "apparent singlet oxygen threshold concentration" [¹O₂]_{rx, sd}, it is feasible to determine the model parameters by fitting the computed [¹O₂]_rx to the Photofrin-mediated PDT-induced necrotic distance using interstitially-measured Photofrin concentration and optical properties within each mouse. After determining the model parameters and the [¹O₂]_{rx, sd}, we expect to use this model as an explicit dosimetry to assess PDT treatment outcome for a specific photosensitizer in an in vivo environment. The results also provide evidence that the [¹O₂]_rx, because it takes into account the oxygen consumption (or light fluence rate) effect, can be a better predictor of PDT outcome than the PDT dose defined as the energy absorbed by the photosensitizer, which is proportional to the product of photosensitizer concentration and light fluence.