Type II photodynamic therapy (PDT) is based on the use of photochemical reactions mediated through an interaction
between a tumor-selective photosensitizer, photoexcitation with a specific wavelength of light, and production of
reactive singlet oxygen. However, the medical application of this technique has been limited due to inaccurate PDT
dosimetric methods. The goal of this study is to examine the relationship between outcome (in terms of tumor growth
rate) and calculated reacted singlet oxygen concentration [1O2]rx after HPPH-mediated PDT to compare with other PDT
dose metrics, such as PDT dose or total light fluence. Mice with radiation-induced fibrosarcoma (RIF) tumors were
treated with different light fluence and fluence rate conditions. Explicit measurements of photosensitizer drug
concentration and tissue optical properties via fluorescence and absorption measurement with a contact probe before and
after PDT were taken to then quantify total light fluence, PDT dose, and [1O2]rx based on a macroscopic model of singlet
oxygen. In addition, photobleaching of photosenitizer were measured during PDT as a second check of the model.
Changes in tumor volume were tracked following treatment and compared to the three calculated dose metrics. The
correlations between total light fluence, PDT dose, reacted [1O2]rx and tumor growth demonstrate that [1O2]rx serves as a
better dosimetric quantity for predicting treatment outcome and a clinically relevant tumor growth endpoint.