Purpose: Delayed fluorescence (DF) from the endogenous molecule protoporphyrin IX (PpIX) has been shown to be a truly unique reporter of the local oxygen partial pressure in tissue. During FLASH-RT, oxygen is consumed in the targeted tissues and oxygen depletion is commonly known to occur. Understanding oxygen consumption dynamics during the treatment is therefore crucial to understand the mechanisms leading to the FLASH effect.
Methods: An intensified time-gated camera was synchronized with a high power pulsed 635 nm laser diode with >1 Watt output, as well as with an external FPGA board. The FPGA was used to control the camera’s intensifier, allowing for expanded gating flexibility. A ratiometric approach utilizing DF and prompt fluorescence signal was used to reach video frame rates. In a first time, pO2 changes were monitored using PpIX in aqueous solution and murine models to parametrize the imaging system. In a second time, the system was used to monitor pO2 in mice during flash radiation therapy.
Results: pO2 images were obtained at video rates reaching 5 fps. In vitro Calibration proved to be hardly achievable due to PpIX's environment dependency. Using established in vivo oxygen methods as controls, we were able to relate PpIX signal to pO2 values. We found that intracellular oxygen consumption is more significant than interstitial oxygen depletion during FLASH-RT.
Conclusion: The ability to image pO2 is a unique way to understand complex biochemistry of oxygen supply and consumption during RT. Time-gated imaging of PpIX DF allows for wide field direct monitoring of oxygen changes during FALSH-RT. This novel technique will bring further clarification on the mechanism leading to the FLASH effect and will contribute to FLASH-RT optimization.
Hypoxia, Fluorescence, Radiation Effects