Background and Objective: In order to understand the mechanisms of photodynamic therapy (PDT) it is important to monitor parameters during illumination that yield information on deposited PDT dose. The aim of this study is to investigate the possibility of monitoring implicit parameters, such as photobleaching, in addition to monitoring explicit parameters (fluence (rate), oxygenation, photosensitizer concentration) directly or indirectly. These parameters are monitored during PDT without interrupting the therapeutic illumination. Materials and Methods: Rats were injected with 0.3 mg kg-1m-THPC. Sixteen hours after administration the abdominal muscle in rats was irradiated for 1,500 seconds using clinically relevant fluence rates of 50, 100, and 250 mW cm-1of diffuser length at 652 nm. In addition to the linear diffuser for delivering treatment light, isotropic fiber-optic probes and fiber-optic probes for differential path-length spectroscopy (DPS) were placed on both sides of the muscle to monitor tissue physiological parameters, fluence rate, and fluorescence. Results: The m-THPC treatment groups show a decrease in fluence rate throughout PDT of 16%, 19%, and 27% for the 50, 100, and 250 mW cm-1groups, respectively. Both during and post-PDT differences in vascular response between treatment groups and animals within the same treatment group are observed. Furthermore we show fluence rate dependent bleaching of m-THPC up to a measured fluence rate of 100 mW cm-1. Conclusion: The data presented in this study show the possibility of simultaneously monitoring fluence (rate), fluorescence, hemoglobin oxygen saturation, and blood volume during PDT without interruptions to the therapeutic illumination. Differences in saturation profiles between animals and treatment groups indicate differences in vascular response during illumination. Furthermore, the relationship between fluence rate andm-THPCfluorescence photobleaching is complex in an interstitial environment.

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Keywords Fluorescence spectroscopy, PDT, Rat muscle, Reflectance spectroscopy, m-THPC
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Kruijt, B., van der Ploeg Van Den Heuvel, A., de Bruijn, H.S., Sterenborg, H.J.C.M., Amelink, A., & Robinson, D.J.. (2009). Monitoring interstitial m-THPC-PDT in vivo using fluorescence and reflectance spectroscopy. Lasers in Surgery and Medicine, 41(9), 653–664. doi:10.1002/lsm.20845