Background and Aims

In a recent experiment at the HZDR research electron accelerator ELBE the influence of dose rate and partial oxygen pressure on the Flash effect was studied in a zebrafish embryo model. High mean electron dose rates of 10⁵ Gy/s (max regime) and partial oxygen pressure below 5 mmHg were found to protect zebrafish embryo from radiation damage compared to continuous reference irradiation (0.11 Gy/s) and higher oxygen pressure (Pawelke et al. Radiother Oncol 2021). However, the influence of beam pulse structure remains unanswered in this experiment.

Methods

In addition to the above mentioned two pulse regimes, the ELBE accelerator was used to mimic the pulse structure of a clinical electron linac delivering a dose of 28 Gy by 5 pulses at a frequency of 250 Hz. For comparison, a fourth regime of similar mean dose rate, but continuous beam (280 Gy/s) mimicking Flash irradiation at a proton isochronous cyclotron (Beyreuther et al. Radiother Oncol 2019) was applied.

Results

First results indicate a clear difference between the "max regime" and the other three electron pulse regimes. Further analysis is under way and the results for different endpoints will be presented.

Conclusions

The ELBE electron accelerator can be applied to study the influence of beam dose rate and pulse structure on the Flash effect by varying both parameters over several orders of magnitude. Hence, it is the ideal tool for systematic studies on optimal electron beam parameters for Flash.

This work was supported by EMPIR 18HLT04UHDpulse project.

Background and Aims

In a recent experiment at the HZDR research electron accelerator ELBE the influence of dose rate and partial oxygen pressure on the Flash effect was studied in a zebrafish embryo model. High mean electron dose rates of 10⁵ Gy/s (max regime) and partial oxygen pressure below 5 mmHg were found to protect zebrafish embryo from radiation damage compared to continuous reference irradiation (0.11 Gy/s) and higher oxygen pressure (Pawelke et al. Radiother Oncol 2021). However, the influence of beam pulse structure remains unanswered in this experiment.

Methods

In addition to the above mentioned two pulse regimes, the ELBE accelerator was used to mimic the pulse structure of a clinical electron linac delivering a dose of 28 Gy by 5 pulses at a frequency of 250 Hz. For comparison, a fourth regime of similar mean dose rate, but continuous beam (280 Gy/s) mimicking Flash irradiation at a proton isochronous cyclotron (Beyreuther et al. Radiother Oncol 2019) was applied.

Results

First results indicate a clear difference between the "max regime" and the other three electron pulse regimes. Further analysis is under way and the results for different endpoints will be presented.

Conclusions

The ELBE electron accelerator can be applied to study the influence of beam dose rate and pulse structure on the Flash effect by varying both parameters over several orders of magnitude. Hence, it is the ideal tool for systematic studies on optimal electron beam parameters for Flash.

This work was supported by EMPIR 18HLT04UHDpulse project.

Background and Aims

The recent rediscovery of the “Flash-effect” revived the interest in high dose-rate radiation effects throughout the radiobiology community, promising protection of normal tissue, while simultaneously not altering tumour control. Systematic preclinical studies resulted in a “recipe” of necessary beam parameters for inducing an electron Flash effect (https://doi.org/10.3389/fonc.2019.01563). For protons, the Flash effect was confirmed in a few animal experiments using the beam parameters available at clinical cyclotrons. Extending the clinical parameter range, the “Dresden platform for high-dose rate radiobiology” enables proton experiments with dose-rates of up to 10⁹ Gy/s.

Methods

The general applicability of the different proton beams for radiobiological studies was proven using biological models of increasing complexity, from cellular models to zebrafish embryo to mouse, at the Draco laser accelerator and, for comparison, at the University Proton Therapy Dresden (UPTD).

Results

A proof-of-principle irradiation campaign was performed using a mouse ear tumour model (https://doi.org/10.1371/journal.pone.0177428) to study the effects of the continuous beam delivery at UPTD and the pulsed beam delivery at Draco with peak dose-rates of 10⁸ Gy/s. Moreover, to investigate the interplay of oxygen consumption and proton dose-rate up to 300 Gy/s and 10⁹ Gy/s, respectively, were applied at UPTD and Draco to study the radiation response of zebrafish embryos.

Conclusions

The successful performance of comparison experiments at Draco laser accelerator and UPTD cyclotron paves the way for upcoming in vivo experiments at both machines. At the conference, we will provide an overview of our radiobiological experiments and the obtained results.

Background and Aims

Dosimetry for FLASH radiotherapy, VHEE radiotherapy as well as for laser-driven beams cause significant metrological challenges due to the ultra-high dose rates and pulsed structure of these beams, in particular for real time measurements with active dosimeters. It is not possible to simply apply existing Codes of Practice available for dosimetry in conventional external radiotherapy here. However, reliable standardized dosimetry is necessary for accurate comparisons in radiobiological experiments, to compare the efficacy of these new radiotherapy techniques and to enable safe clinical application. UHDpulse aims to develop the metrological tools needed for reliable real-time absorbed dose measurements of electron and proton beams with ultra-high dose rate, ultra-high dose per pulse or ultra-short pulse duration.

Methods

Within UHDpulse, primary and secondary absorbed dose standards and reference dosimetry methods are developed, the responses of available state-of-the-art detector systems are characterised, novel and custom-built active dosimetric systems and beam monitoring systems are designed, and methods for relative dosimetry and for the characterization of stray radiation are investigated.

Results

Prototypes of different active dosimetry systems show promising results for real-time dosimetry for particle beams with ultra-high pulse dose rates. The results of the UHDpulse project will be the input data for future Codes of Practice.

Conclusions

A brief overview of the progress in the UHDpulse project and the involved institutions will be given.

Acknowledgement: This project 18HLT04 UHDpulse has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

Background and Aims

In FLASH radiotherapy (RT), a protective effect of healthy tissue was observed, while tumor control remains comparable to conventional RT. One possible explanation is the oxygen depletion hypothesis, in which radiolysis of water/cytoplasma causes the production of radicals that then react with O2 dissolved in water. This would cause a reduction in O2, which results in a hypoxic target and thus a radio-protective effect. In a previous study (Jansen et al. 2021), we measured O2 depletion using an optical sensor in sealed water phantoms during irradiation at high dose rates (<300 Gy/s) for protons, carbon ions and photons.

Methods

In the study presented here, this experiment was conducted further to ultra-high dose rates (10^9 Gy/s) with protons at DRACO and electrons at ELBE, where also the impact of different pulse structures on O2 depletion in water was tested. In addition, various settings were tested in order to irradiate zebrafish embryos with FLASH while simultaneously measuring O2.

Results

We were able to confirm the results of our previous study even at ultra-high dose rates and with electrons. Furthermore, it was possible to measure O2 depletion during zebrafish embryo irradiation making a simultaneous study of biological response and O2 depletion possible.

Conclusions

Not enough O2 was depleted at clinical doses to explain a FLASH effect based on radiation-induced hypoxia. The amount of O2 depleted per dose depends on dose rate, and higher dose rates deplete slightly less O2. The experimental set up allows for future joint experiments of biological samples and oxygen monitoring.

Background and Aims

Since the oxygen depletion hypothesis has been recently challenged, the setup of a new joint project dedicated to an alternative mechanistic explanation of the FLASH effect, presently submitted to the Italian Association for Cancer Research (AIRC), will be presented. Our approach is a bottom up analysis linking radiation chemical based radicals description and DNA damage modeling studies. This should enable us to predict the irradiation parameters of absolute dose, and dose rate for which the effect could be verified.

Methods

We will develop point-like and two dimensional optical based methods for FLASH real time dosimetry using Cerenkov and radioluminescence light.

In a second phase a multiscale mechanistic description of ultrahigh dose rate induced damage including oxygen interplay and reactive oxygen species production and reactions will be developed. Our approach is based on chemical track structure Monte Carlo simulations and dedicated extensions of analytical biophysical models.

Results

We expect to obtain:

-Real time dosimetric methods to monitor FLASH beams for cells and mice irradiations. The same methods can be also translated to monitor FLASH delivery to human patients.

-A refined mechanistic description of the FLASH effect starting from basic radiation chemistry concepts in a biological environment. We will also provide in vitro and in vivo validation tests using two types of FLASH beams.

Conclusions

This work will contribute to unraveling the basic biological mechanisms of the FLASH effect and, at the same time, it will provide accurate real time dosimetric tools not available at the moment.

Background and Aims

After the rediscovery of the normal tissue sparing FLASH effect of high dose rate radiation, research activities on this topic have been revived. But especially for protons, the portfolio of accelerators capable of performing studies at ultra-high dose rates is limited. Laser-plasma accelerators (LPA) can generate extremely intense proton bunches of many 10 MeV kinetic energy. In combination with dedicated dose delivery systems, LPA proton sources facilitate peak dose rates well above 10⁸ Gy/s in a pulse structure regime complementary to conventional accelerators.

Methods

The reliable generation of proton spectra beyond 60 MeV at DRACO-PW [Ziegler et al, SciRep2021], combined with a dedicated energy selective pulsed magnet beam transport system [Brack et al, SciRep2020], allows tailored sample-specific dose distributions. Adapted on-shot dosimetry enables the required spectral monitoring of every proton bunch. Two irradiation series on volumetric biological samples were performed at DRACO-PW, accompanied by reference irradiations at the University Proton Therapy Dresden.

Results

The first small animal pilot study at a laser-driven proton source was conducted successfully. The mouse-ear tumor model’s requirements [Beyreuther et al, PLoS One2018] were fulfilled and verified at high precision (+/- 5%) concerning predefined dose value and conformity. Complementary, a study investigating dose-rate effects such as FLASH was performed irradiating zebrafish embryos with above 10⁹ Gy/s.

Conclusions

We present a laser-based irradiation platform at the DRACO-PW facility that enables systematic radiobiological studies, laying the foundations for further studies at LPA sources exploring ultra-high dose-rate effects, such as FLASH, over previously unreachable parameter space.

Background and Aims

Development of proton therapy is strongly driven by in-vivo experiments, which are, however, subject to increasingly strict ethical and technical requirements. Proton irradiation of small animals demands for versatile experimental setups and intelligent protocols. To realize precise positioning and treatment, on-site imaging with proton radiography was integrated into an existing beam setup for mouse brain sub-volume irradiation at University Proton Therapy Dresden.

Methods

A flat panel detector was installed on proton beam axis behind mouse position. Transmission radiographic images were acquired at high energy (200 MeV) with a single-scattered proton beam. Image quality was optimized regarding resolution, contrast and minimal dose deposition in the animal. The hippocampus as target region for current mouse irradiation experiments was determined by registration of mouse brain atlas data with pre-treatment off-site CT scans and x-ray images.

Results

The developed workflow allows precise brain irradiation with lateral target positioning accuracy <0.2 mm. Imaging with dose depositions <20 mGy in mice was achieved. For accurate irradiation, the designated target volume (right hippocampus) was aligned with the collimated treatment beam by registering the radiography image with off-site x-ray images with custom-made software (Figure 1). Immunohistochemically staining of DNA damage on histological whole-brain tissue sections validated successful positioning and irradiation (Figure 2).

Conclusions

Proton radiography enables effective high-precision lateral alignment of proton beam and target volume in mouse irradiation experiments with limited dose exposure.

This work was supported by EU-Horizon2020 grant 730983 (INSPIRE).

Figure 1. Workflow with x-ray image and radiography overlay for accurate positioning

Figure 2. Tissue section after irradiation

Background and Aims

We present the current status and outcomes of the joint radiobiological experiment performed at eight European proton therapy centers or research institutes. The study aims to spot the potential differences in the in vitro proton RBE values measured by different groups sharing a similar setup and identify its causes.

Methods

A phantom and a protocol for sample preparation and post-processing are shared among the participants to ensure minimal differences in the biological part of the experimental procedure. In this phantom, V79 cells grow on the polyester slides that can be inserted at different depths, which enables their simultaneous irradiation at multiple positions within the radiation field. The setup is irradiated with proton beams with two SOBP configurations (6 cm, 6 Gy, and 4 cm, 8 Gy), followed by the reference photon irradiation (LINAC or x-ray), and the biological effect is evaluated using a colony-forming assay.

Results

The study is still ongoing, and the spread of data for measured cell survival is yet to be evaluated. However, some non-obvious differences in the experimental procedures and setups are already revealed, e.g. post-processing timing or varying dose distributions in the beam plateau/fall-off regions.

Conclusions

As an outcome of the experiment, we plan to summarize the details of the experimental procedure for biological experiments with proton beams, differing between the centers across Europe. Accounting for these details would help to harmonize future studies in the field.

This work was supported by EU Horizon2020 grant 730983 (INSPIRE).

Background and Aims

FLASH radiotherapy requires the development of new detectors to be able to cope with ultra-high-pulse-dose-rates (UHDpulse) beams. This work aims to test customized Timepix3 detectors to identify the most suitable sensor and settings to be used for the characterization of stray radiation produced in UHDpulse proton beams (PB).

Methods

Dose rates (DR) exceeding 160Gy/s were delivered by a pencil proton beam of 220MeV energy at the University Proton Therapy Dresden, Germany. For data collection two customized semiconductor pixel detectors, Timepix3 ASIC chip, with electronics placed on a flexible cable (50mm distance from the sensor, Fig. 1) were immersed, in turns, inside a water-phantom. The detectors were moved laterally at different depth during irradiation.

Figure 1. Experimental setup with Minipix-Timepix3-Flex detector placed in a waterproof cadge.

Results

The Minipix-Timepix3-Flex detector (Fig. 1) provides ns timing resolution at the pixel level together with quantum-imaging sensitivity with 100% detection efficiency for heavy-charged particles. The integrated per-pixel deposited energy, number of registered events, and DR were measured (Fig. 2).

Figure 2. The plots show the spatial distribution of integrated deposited energy at 50mm (left) and 100mm (right) behind Bragg Peak for a 2ms proton pulse measured with Minipix-Timepix3 with 100µm (top) and 650µm (bottom) thick silicon sensor.

Conclusions

A detector equipped with a thinner silicon sensor, 100µm, is more suitable for UHDpulse PB measurements due to the reduced amplitude of the signal and pixel size, allowing to register higher event rates.

Acknowledgements: This work was supported by the 18HLT04UHDpulse project founded by EMPIR-programme and EU INSPIRE (730983) project.