Modeling the effect of oxygen on the chemical stage of water radiolysis using GPU-based microscopic Monte Carlo simulations, with an application in FLASH radiotherapy

Abstract

Abstract Oxygen plays a critical role in determining the initial DNA damages induced by ionizing radiation. It is important to mechanistically model the oxygen effect in the water radiolysis process. However, due to the computational costs from the many body interaction problem, oxygen is often ignored or treated as a constant continuum radiolysis-scavenger background in the simulations using common microscopic Monte Carlo tools. In this work, we reported our recent progress on the modeling of the chemical stage of the water radiolysis with an explicit consideration of the oxygen effect, based upon our initial development of an open-source graphical processing unit (GPU)-based MC simulation tool, gMicroMC. The inclusion of oxygen mainly reduces the yields of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>e</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">h</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:mrow> <mml:mo>.</mml:mo> </mml:msup> </mml:math> chemical radicals, turning them into highly toxic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mo>.</mml:mo> <mml:mo>−</mml:mo> </mml:mrow> </mml:msup> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:mrow> <mml:mn>2</mml:mn> <mml:mo>.</mml:mo> </mml:msubsup> </mml:math> species. To demonstrate the practical value of gMicroMC in large scale simulation problems, we applied the oxygen-simulation-enabled gMicroMC to compute the yields of chemical radicals under a high instantaneous dose rate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>˙</mml:mo> </mml:mover> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">i</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> to study the oxygen depletion hypothesis in FLASH radiotherapy. A decreased oxygen consumption rate (OCR) was found associated with a reduced initial oxygen concentration level due to reduced probabilities of reactions. With respect to dose rate, for the oxygen concentration of 21% and electron energy of 4.5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">k</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> <mml:mi mathvariant="normal">V</mml:mi> </mml:mrow> </mml:mrow> </mml:math> , OCR remained approximately constant (∼0.22 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>μ</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> </mml:mrow> <mml:mtext> </mml:mtext> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">y</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ) for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>˙</mml:mo> </mml:mover> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">i</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ’s of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>6</mml:mn> </mml:msup> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>7</mml:mn> </mml:msup> </mml:mrow> <mml:mtext> </mml:mtext> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> <mml:mi mathvariant="normal">y</mml:mi> </mml:mrow> </mml:mrow> <mml:mr

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