Abstract
Photodynamic therapy (PDT) is a well-established treatment modality for cancer and other malignant diseases; however, quantities such as light fluence and PDT dose do not fully account for all of the dynamic interactions between the key components involved. In particular, fluence rate (ϕ) effects, which impact the photochemical oxygen consumption rate, are not accounted for. In this preclinical study, reacted reactive oxygen species ([ROS](rx)) was investigated as a dosimetric quantity for PDT outcome. The ability of [ROS](rx) to predict the cure index (CI) of tumor growth, CI = 1 - k / k(ctr), where k and k(ctr) are the growth rate of tumor under PDT study and the control tumor without PDT, respectively, for benzoporphyrin derivative (BPD)-mediated PDT, was examined. Mice bearing radiation-induced fibrosarcoma (RIF) tumors were treated with different in-air fluences (Φ = 22.5 to 166.7 J / cm(2)) and in-air fluence rates (ϕ(air) = 75 to 250 mW / cm(2)) with a BPD dose of 1 mg / kg and a drug-light interval (DLI) of 15 min. Treatment was delivered with a collimated laser beam of 1-cm-diameter at 690 nm. Explicit measurements of in-air light fluence rate, tissue oxygen concentration, and BPD concentration were used to calculate for [ROS](rx). Light fluence rate at 3-mm depth (ϕ(3 mm)), determined based on Monte-Carlo simulations, was used in the calculation of [ROS](rx) at the base of tumor. CI was used as an endpoint for three dose metrics: light fluence, PDT dose, and [ROS](rx). PDT dose was defined as the product of the time-integral of photosensitizer concentration and ϕ(3 mm). Preliminary studies show that [ROS](rx) best correlates with CI and is an effective dosimetric quantity that can predict treatment outcome. The threshold dose for [ROS](rx) for vascular BPD-mediated PDT using DLI of 15 min is determined to be 0.26 mM and is about 3.8 times smaller than the corresponding value for conventional BPD-mediated PDT using DLI of 3 h.