High-linear energy transfer (LET) radiation, such as heavy ions is associated with a higher relative biological effectiveness (RBE) than low-LET radiation, such as photons. Irradiation with low- and high-LET particles differ in the interaction with the cellular matter and therefore in the spatial dose distribution. When a single high-LET particle interacts with matter, it results in doses of up to thousands of gray (Gy) locally concentrated around the ion trajectory, whereas the mean dose averaged over the target, such as a cell nucleus is only in the range of a Gy. DNA damage therefore accumulates in this small volume. In contrast, up to hundreds of low-LET particle hits are required to achieve the same mean dose, resulting in a quasi-homogeneous damage distribution throughout the cell nucleus. In this study, we investigated the dependence of RBE from different spatial dose depositions using different focused beam spot sizes of proton radiation with respect to the induction of chromosome aberrations and clonogenic cell survival. Human-hamster hybrid (AL) as well as Chinese hamster ovary cells (CHO-K1) were irradiated with focused low LET protons of 20 MeV (LET = 2.6 keV/µm) beam energy with a mean dose of 1.7 Gy in a quadratic matrix pattern with point spacing of 5.4 × 5.4 µm2 and 117 protons per matrix point at the ion microbeam SNAKE using different beam spot sizes between 0.8 µm and 2.8 µm (full width at half maximum). The dose-response curves of X-ray reference radiation were used to determine the RBE after a 1.7 Gy dose of radiation. The RBE for the induction of dicentric chromosomes and cell inactivation was increased after irradiation with the smallest beam spot diameter (0.8 µm for chromosome aberration experiments and 1.0 µm for cell survival experiments) compared to homogeneous proton radiation but was still below the RBE of a corresponding high LET single ion hit. By increasing the spot size to 1.6–1.8 µm, the RBE decreased but was still higher than for homogeneously distributed protons. By further increasing the spot size to 2.7–2.8 µm, the RBE was no longer different from the homogeneous radiation. Our experiments demonstrate that varying spot size of low-LET radiation gradually modifies the RBE. This underlines that a substantial fraction of enhanced RBE originates from inhomogeneous energy concentrations on the µm scale (mean intertrack distances of low-LET particles below 0.1 µm) and quantifies the link between such energy concentration and RBE. The missing fraction of RBE enhancement when comparing with high-LET ions is attributed to the high inner track energy deposition on the nanometer scale. The results are compared with model results of PARTRAC and LEM for chromosomal aberration and cell survival, respectively, which suggest mechanistic interpretations of the observed radiation effects.
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February 2024
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January 12 2024
Enhanced RBE of Particle Radiation Depends on Beam Size in the Micrometer Range
K. Ilicic;
K. Ilicic
a Institute of Radiation Medicine (IRM), Helmholtz Zentrum München – German Research Center for Environmental Health, Neuherberg, Germany
b Department of Radiation Oncology, School of Medicine, Technische Universität München, Germany
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G. Dollinger;
G. Dollinger
c Institute for Applied Physics and Measurement Technology, Universität der Bundeswehr München, Neubiberg, Germany
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A. Dombrowsky;
A. Dombrowsky
a Institute of Radiation Medicine (IRM), Helmholtz Zentrum München – German Research Center for Environmental Health, Neuherberg, Germany
b Department of Radiation Oncology, School of Medicine, Technische Universität München, Germany
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C. Greubel;
C. Greubel
c Institute for Applied Physics and Measurement Technology, Universität der Bundeswehr München, Neubiberg, Germany
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S. Girst;
S. Girst
c Institute for Applied Physics and Measurement Technology, Universität der Bundeswehr München, Neubiberg, Germany
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M. Sammer;
M. Sammer
c Institute for Applied Physics and Measurement Technology, Universität der Bundeswehr München, Neubiberg, Germany
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C. Siebenwirth;
C. Siebenwirth
b Department of Radiation Oncology, School of Medicine, Technische Universität München, Germany
c Institute for Applied Physics and Measurement Technology, Universität der Bundeswehr München, Neubiberg, Germany
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E. Schmid;
E. Schmid
d Department for Anatomy and Cell Biology, Ludwig-Maximilians Universität München, Germany
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T. Friedrich;
T. Friedrich
e GSI Helmholtz Center for Heavy Ion Research, Department of Biophysics, Darmstadt, Germany
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P. Kundrát;
P. Kundrát
f Department of Radiation Dosimetry, Nuclear Physics Institute of the Czech Academy of Sciences, Prague, Czech Republic
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W. Friedland;
W. Friedland
a Institute of Radiation Medicine (IRM), Helmholtz Zentrum München – German Research Center for Environmental Health, Neuherberg, Germany
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M. Scholz;
M. Scholz
e GSI Helmholtz Center for Heavy Ion Research, Department of Biophysics, Darmstadt, Germany
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S.E. Combs;
S.E. Combs
a Institute of Radiation Medicine (IRM), Helmholtz Zentrum München – German Research Center for Environmental Health, Neuherberg, Germany
b Department of Radiation Oncology, School of Medicine, Technische Universität München, Germany
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T.E. Schmid;
T.E. Schmid
a Institute of Radiation Medicine (IRM), Helmholtz Zentrum München – German Research Center for Environmental Health, Neuherberg, Germany
b Department of Radiation Oncology, School of Medicine, Technische Universität München, Germany
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J. Reindl
J. Reindl
1
c Institute for Applied Physics and Measurement Technology, Universität der Bundeswehr München, Neubiberg, Germany
1 Corresponding author: Prof. Dr. Judith Reindl, Universität der Bundeswehr München, Institute für Applied Physics and Measurement Technology, Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany, email: [email protected].
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Radiat Res (2024) 201 (2): 140–149.
Article history
Received:
October 31 2023
Accepted:
December 01 2023
Citation
K. Ilicic, G. Dollinger, A. Dombrowsky, C. Greubel, S. Girst, M. Sammer, C. Siebenwirth, E. Schmid, T. Friedrich, P. Kundrát, W. Friedland, M. Scholz, S.E. Combs, T.E. Schmid, J. Reindl; Enhanced RBE of Particle Radiation Depends on Beam Size in the Micrometer Range. Radiat Res 1 February 2024; 201 (2): 140–149. doi: https://doi.org/10.1667/RADE-23-00217.1
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