(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: effect occurred in ultrahigh dose rate radiotherapy has attracted a lot of attentions. Oxygen depletion is one of the most popular explanation to this phenomenon. The explanation estimated the effect based on a constant oxygen consumption rate (OCR). However, it is expected that the rate would vary with oxygen concentration. In this work, we employed our GPU-based MC tool gMicroMC to investigate this issue from the first principles, which was a computationally challenge problem for conventional MC packages.
Methods: randomly sampled N (N=10, 100, 1000) electrons with initial kinetic energy of 4.5 keV within a water sphere of 1 um in radius, approximately corresponding to the instantaneous dose rate of 10^5~10^7 Gy/s if we terminate chemical stage at 1 µs. We simulated the physical and physicochemical stages of water radiolysis triggered by electrons and the chemical stage under different oxygen concentrations.
Results: an oxygen concentration of 9% (86 µM), the OCRs were 0.21, 0.19 and 0.13 µM/Gy, for N=10, 100 and 1000 at 1 µs, respectively. This rate is not high enough to deplete oxygen in normal tissue for tens of Gy. As N increases, initial radical distribution becomes denser, but ROD reduces due to competitions between reactions of oxygen and other molecules and reactions among other molecules. The increased reactions among radicals reduces reactions with oxygen, hence reducing the chances of oxygen depletion. For a low oxygen condition of 0.5% (4.8µM), the OCRs were 0.027, 0.024 and 0.015 µM/Gy , which still cannot deplete the oxygen at a dose of tens of Gy.
Conclusion: high computational efficiency of gMicroMC allowed us to investigate oxygen effect in FLASH setting. Results showed that the high dose rate is unlikely to deplete oxygens and hence oxygen depletion may not the reason for FLASH effect.
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