Local physiological changes during photodynamic therapy

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Abstract
Background and Objective Herein an overview is provided of the causes, consequences, and significance of photodynamic therapy (PDT)‐mediated effects on tumor oxygenation and blood flow during illumination. Study Design/Materials and Methods Techniques particularly valuable to this research have included tissue oxygen tension measurement by the Eppendorf pO2 Histograph; spatial quantification of hypoxia by EF3 and EF5; and tissue oxygenation/blood flow monitoring by diffuse reflectance/correlation spectroscopy. Results Severe hypoxia was measured in vivo during PDT and is shown to be a consequence of photochemical oxygen consumption and/or compromised vascular perfusion. Oxygen depletion can be controlled by treatment regimen, occurs in a spatially‐definable pattern, and is therapy‐limiting. PDT‐induced changes in tumor oxygenation during illumination are correlated with outcome. In PDT‐treated tissues, blood flow also is determined by treatment regimen and correlates with treatment response. Conclusions Photodynamic therapy creates distinct, measurable changes in tumor oxygen and blood flow during illumination. These physiological changes may ultimately affect treatment efficacy. Lasers Surg. Med. 38:494–499, 2006.