Induction and repair of DNA double-strand breaks assessed by gamma-H2AX foci after irradiation with pulsed or continuous proton beams

Abstract

In particle tumor therapy including beam scanning at accelerators, the dose per voxel is delivered within about 100 ms. In contrast, the new technology of laser plasma acceleration will produce ultimately shorter particle packages that deliver the dose within a nanosecond. Here, possible differences for relative biological effectiveness in creating DNA double-strand breaks in pulsed or continuous irradiation mode are studied. HeLa cells were irradiated with 1 or 5 Gy of 20-MeV protons at the Munich tandem accelerator, either at continuous mode (100 ms), or applying a single pulse of 1-ns duration. Cells were fixed 1 h after 1-Gy irradiation and 24 h after 5-Gy irradiation, respectively. A dose-effect curve based on five doses of X-rays was taken as reference. The total number of phosphorylated histone H2AX (gamma-H2AX) foci per cell was determined using a custom-made software macro for gamma-H2AX foci counting. For 1 h after 1-Gy 20-MeV proton exposures, values for the relative biological effectiveness (RBE) of 0.97 ± 0.19 for pulsed and 1.13 ± 0.21 for continuous irradiations were obtained in the first experiment 1.13 ± 0.09 and 1.16 ± 0.09 in the second experiment. After 5 Gy and 24 h, RBE values of 0.99 ± 0.29 and 0.91 ± 0.23 were calculated, respectively. Based on the gamma-H2AX foci numbers obtained, no significant differences in RBE between pulsed and continuous proton irradiation in HeLa cells were detected. These results are well in line with our data on micronucleus induction in HeLa cells.

Dose-controlled irradiation of cancer cells with laser-accelerated proton pulses

“…Both radiobiological results are in good agreement with a complementary experiment performed at the Munich Tandem Van-de-Graaf accelerator [37,38] and a recent study of the RBE of intense single pulses of LDPR, where the dose applied to the cells was varying across the probe and analyzed retrospectively for individual irradiated areas [26]. Making use of different pulse modes of the Tandem accelerator, the first study focused on the dependence of the RBE on the peak dose rate by comparing the effect of short-pulses (few nanoseconds) and continuous beams of 20 MeV protons, while the latter directly made use of the intrinsically high peak dose rates of LDPR of up to few Gy per pulse.…”

The Effectiveness of 20 MeV Protons at Nanosecond Pulse Lengths in Producing Chromosome Aberrations in Human-Hamster Hybrid Cells

“…These data are of high relevance for clinical use of laser-accelerated proton beams. Previous experiments on monolayer and 3D tissue cultures (in vitro) did not provide evidence for significantly altered radiobiological effectiveness in terms of cytogenetic damage or DNA repair (8)(9)(10)(11)(12). The RBE values determined for pulsed and continuous irradiation modes were always comparable to the RBE of 1.1, which is achieved by conventional proton therapy (20).…”

“…However, based on our previous in vitro experiments (8)(9)(10)(11)(12) we do not expect a relevant difference between pulsed and conventional proton irradiation.…”

“…With respect to these endpoints no indication for a significantly altered efficiency of pulsed protons compared to a continuous irradiation was observed (8)(9)(10)(11). Further biological experiments were performed using a human skin tissue model to account for the 3D geometry and the cell interaction, again without statistically significant changes in RBE according to dose rate (12).…”

The radiobiology of laser-driven particle beams: focus on sub-lethal responses of normal human cells

“…Neither approach did find significant radiobiological differences between laser-driven electrons or protons and conventionally accelerated beams or reference photon radiations. In fact, the majority of in vitro studies making use of different sources confirm that in the therapeutically relevant dose range of a few Gy, even if applied in a single pulse of only few nanoseconds duration, non-linear radiobiological effects due to simultaneous multiple damages in cells and, thus, below any timescale of repair mechanisms, are unlikely to arise [30][31][32][33][34][35]. Only two relevant exceptions exist: Achayra et al [36], who reported a decrease in genetic damage measured as micronucleus formation after a single pulse of electrons but not after multiple pulses (10 6 to 10 8 Gy/s), hypothesising more efficient radical recombination; Schmid et al [37], who found a slight decrease in effectiveness at causing (some types of) chromosome aberrations after nanopulsed protons (conventionally accelerated).…”