Abstract Body

There is currently high interest in new radiotherapy approaches employing significantly higher dose rates than used in conventional practice, where radiation is typically delivered at a few Gy/minutes. This follows from the recent demonstration of the normal tissue sparing effects of radiation delivered at high-dose rate (10s to 1000s Gy/s), which have highlighted the promise of the so-called FLASH approach as the next step towards an advanced and more effective form of cancer radiotherapy. While this is motivating a large number of in-vitro and in-vivo studies aimed to clarify the mechanisms behind the FLASH effect, it has also revived a broader fundamental interest in the investigation of the dose rate dependency of the biological effects of radiation.

Amongst existing radiation sources, particle beams accelerated by high power lasers provide unique capabilities for these studies, as the particles are emitted in intrinsically ultrashort and very bright bursts. Of particular relevance are laser-driven ion beams: these are typically accelerated on picosecond timescales, resulting in distinctive properties, which differ strongly from those of the beams produced by conventional Radio-Frequency (RF) accelerators. A number of experiments have demonstrated that, with a suitable selection and transport system, it is possible to deliver to a sample multi-Gy doses in single nanosecond, or sub-ns, pulses, i. e. at dose rates exceeding 10⁹ Gy/s, which allow extending radiobiological investigations to new, extreme irradiation regimes.

The talk will review the main laser-based acceleration mechanisms as well as the beam properties which have been demonstrated in experiments. While the so-called Target Normal Sheath Acceleration mechanism is the established method for delivering proton beams and has been used in numerous irradiation experiments, opportunities exist for accelerating and delivering other species, e.g. carbon, through emerging mechanisms such as Radiation Pressure Acceleration, which act on the bulk of an ultrathin foil.

We will also discuss the results of experimental campaigns where the laser-driven ions have been used to irradiate biological samples at ultra-high dose rate, and to highlight, through a number of suitable assays, similarities and differences with known biological effects at conventional dose rates. Data dependencies on dose, dose-rate, particle LET, cell type, sample dimensionality and oxygenation highlight a complex scenario which challenges established understanding and requires new hypotheses and models.