Direct measurement of the 3-dimensional DNA lesion distribution induced by energetic charged particles in a mouse model tissue

Abstract

Charged particles are increasingly used in cancer radiotherapy and contribute significantly to the natural radiation risk. The difference in the biological effects of high-energy charged particles compared with X-rays or γ-rays is determined largely by the spatial distribution of their energy deposition events. Part of the energy is deposited in a densely ionizing manner in the inner part of the track, with the remainder spread out more sparsely over the outer track region. Our knowledge about the dose distribution is derived solely from modeling approaches and physical measurements in inorganic material. Here we exploited the exceptional sensitivity of γH2AX foci technology and quantified the spatial distribution of DNA lesions induced by charged particles in a mouse model tissue. We observed that charged particles damage tissue nonhomogenously, with single cells receiving high doses and many other cells exposed to isolated damage resulting from high-energy secondary electrons. Using calibration experiments, we transformed the 3D lesion distribution into a dose distribution and compared it with predictions from modeling approaches. We obtained a radial dose distribution with sub-micrometer resolution that decreased with increasing distance to the particle path following a 1/r 2 dependency. The analysis further revealed the existence of a background dose at larger distances from the particle path arising from overlapping dose deposition events from independent particles. Our study provides, to our knowledge, the first quantification of the spatial dose distribution of charged particles in biologically relevant material, and will serve as a benchmark for biophysical models that predict the biological effects of these particles.C harged particles, including protons, α-particles, and heavy ions, are increasingly used in cancer radiotherapy and represent a significant component of the natural background irradiation on earth and in space (1-3). Their biological effect is often very different from that of photons (X-or γ-rays), largely because charged particles deposit their energy along a track, whereas photons produce a fairly homogeneous dose distribution. Linear energy transfer (LET; typically given in units of keV/μm) has been introduced as a parameter to describe the amount of energy that charged particles deposit along their track. Particles with high LET are densely ionizing and typically biologically more effective than photons or low-LET particles. Energy deposition along a particle track is not restricted to the path itself (the so-called "track core"), but extends laterally into an area known as the penumbra of the particle, which can reach considerable distances for high-energy particles. Energy deposition in the penumbra arises from energetic secondary electrons, so-called δ-electrons, which are generated by ionization events of the charged particles and carry energy away from the immediate path into the penumbra. According to classical track structure theory, ∼50% of the total energy is deposited in the...

In vitro and in vivo assessment of direct effects of simulated solar and galactic cosmic radiation on human hematopoietic stem/progenitor cells

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A Process of Resection-Dependent Nonhomologous End Joining Involving the Goddess Artemis

“…X rays are low-LET radiation, which generate DSBs of modest complexity. In contrast, high-LET radiation, such as heavy ions or α particles, induces highly complex DSBs 65 , 66 . Significantly, most DSBs induced by high-LET radiation are repaired with similar kinetics to the X-ray-induced slow DSB repair component 38 , 67 ( Figure 1 B).…”

Differential Repair Protein Recruitment at Sites of Clustered and Isolated DNA Double-Strand Breaks Produced by High-Energy Heavy Ions

“…Live cell imaging of protein recruited to the track core has been used by us and others to measure the movement of the DSB lesions in the cell nucleus [13][14][15] . We have previously shown that the yield of heavy ion-induced DSBs as a function of the distance from the primary track can be well described by the physics of the energy deposition of the primary ion and δ-rays 16 . To measure the repair kinetics in single DSB sites we have now exposed to swift heavy ions human osteosarcoma (U2OS) cells stably expressing the GFP-tagged DSB repair factors NBS 17 and 53BP1 18 , two different DSB surrogate markers which were recruited in all cell cycle phases.…”