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Abolfath, R.M.* ; Baikalov, A.* ; Bartzsch, S. ; Afshordi, N.* ; Mohan, R.*

The effect of non-ionizing excitations on the diffusion of ion species and inter-track correlations in FLASH ultra-high dose rate radiotherapy.

Phys. Med. Biol. 67:105005 (2022)

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PURPOSE: We present a microscopic mechanism that accounts for the outward burst of ``cold" ion species (IS) in a high-energy particle track due to coupling with ''hot" non-ion species (NIS). IS refers to radiolysis products of ionized molecules, whereas NIS refers to non-ionized excitations of molecules in a medium. The interaction is mediated by a quantized field of acoustic phonons, a channel that allows conversion of thermal energy of NIS to kinetic energy of IS, a flow of heat from the outer to the inner core of the track structure. METHODS: We perform Monte Carlo (MC) simulations of ionizing radiation track structures in water to score the spatial coordinates and energy depositions that form IS and NIS at atto-second time scales. We calculate temperature profiles of the tracks with MC track structure simulations and verify the results analytically using the Rutherford scattering formulation. These temperature profiles are then used as boundary conditions in a series of multi-scale atomistic molecular dynamic (MD) simulations that describe the sudden expansion and enhanced diffusive broadening of tracks initiated by the non-equilibrium spectrum of high-energy IS. RESULTS: We demonstrate the coexistence of ``hot" NIS with ``cold" IS in the radiation track structures right after their generation. NIS, concentrated within nano-scales volumes wrapping around IS, are the main source of intensive heat-waves and the outward burst of IS due to femto-second time scale IS-NIS coupling. By comparing the transport of IS coupled to NIS with identical configurations of non-interacting IS in thermal equilibrium at room temperature, we demonstrate that the energy gain of IS due to the surrounding hot nanoscopic volumes of NIS significantly increases their effective diffusion constants. CONCLUSIONS: The much higher diffusion constants predicted in the present model suggest higher inter-track chemical reaction rates at FLASH-UHDR, as well as lower intra-track reaction rates.

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Publication type Article: Journal article

Document type Scientific Article

Corresponding Author

Keywords Flash ; Molecular Dynamics ; Monte Carlo ; Radiation Therapy ; Radiobiology

ISSN (print) / ISBN 0031-9155

e-ISSN 1361-6560

Journal Physics in Medicine and Biology

Quellenangaben Volume: 67, Issue: 10, Article Number: 105005

Publisher Institute of Physics Publishing (IOP)

Publishing Place Bristol

Non-patent literature Publications

Reviewing status Peer reviewed

Institute(s) Institute of Radiation Medicine (IRM)