Background and Aims

To characterize the biological "FLASH effect", it is necessary to have dosimeters available for reliable real-time measurements at ultra-high dose rates (> 40 Gy/s) and ultra-high dose-per-pulse (DPP > 0.6 Gy/pulse). A promising approach are detectors based on synthetic single crystal diamond working as Schottky photodiodes like the microDiamond T60019. The aim of this work is to investigate the dose response linearity of this detector type at ultra-high DPP.

Methods

Several different microDiamond detectors were investigated at PTB's research electron accelerator (20 MeV, 5 Hz, 2.5 µs pulse duration). To determine the DPP reference, the beam current monitor was calibrated against alanine.

Results

All microDiamonds respond linearly at low DPP (Figure 1). The response deviates from linearity with increasing DPP and finally reaches saturation. The DPP value at which non-linear behavior becomes significant varies between 0.1 and 2 Gy/pulse for the commercially available microDiamonds and is exemplar-dependent (SN). However, prototypes (B1, C1) demonstrated that the linear range can be extended.

Figure 1. Measured vs. actual DPP for different microDiamonds

Conclusions

The dose response of various microDiamond exemplars was investigated as a function of DPP. Commercially available microDiamonds have limitations in ultra-high DPP range. However, it has been shown with microDiamond prototypes that the linear range can be extended to the ultra-high DPP range. This shows that the microDiamond is in principle suitable for FLASH-RT dosimetry.

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

Vented ionization chambers (IC) are used as the standard dosimeter for clinical reference dosimetry. For ultra-high dose per pulse (DPP) in the range of 0.6 – 10 Gy, as under investigation for FLASH radiotherapy with electrons, available ICs show large deviations due to ion recombination. However, it is desirable to use ICs also under ultra-high DPP conditions as a secondary standard.

Methods

Parallel plate prototype ICs with different electrode distances d manufactured by PTW were investigated at PTB's research electron accelerator (20 MeV, 5 Hz, 2.5 µs pulse duration). To determine the DPP reference, the beam current monitor was calibrated against alanine. The measurements were compared to a numerical approach by solving a system of partial difference equations, taking into account charge creations by the radiation, their transport and reaction in an applied electric field.

Results

As the electrode distance decreases, the deviations due to ion recombination become smaller. For the prototype with d=0.25mm, almost no deviation is detectable anymore. There is a good agreement between measurements and simulations.

Figure 1: Measured and simulated DPP vs. reference DPP for ICs with different electrode distances d and a voltage of 250V.

Conclusions

Our investigation confirms the previous findings of Gomez et al. presented on this conference. Parallel plate ICs with very small electrode distances are a promising tool for real time dosimetry in FLASH radiotherapy.

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

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

Ionization chambers (IC) have been and remain the secondary standard of choice in the vast majority of hospitals all over the world. When an IC is irradiated at dose rates that exceed the conventional limits the ion recombination correction factor of these chambers starts to increase. Working in these regimes is unfeasible as the required correction factors exceed dosimetry standards. Analytical theories describing ion recombination effect fail to actually describe their behavior in the ultra high dose rate (UHDR).

Methods

Ultra-thin gap plane-parallel ionization chambers exhibit a submillimetric electrode distance. This geometry enhances the collection of the charge carriers by increasing the electric field strength inside the chamber and reducing the charge carrier densities between electrodes. The free electron fraction is much higher than in a conventional chamber, decreasing the amount of ion-recombination. The experimental measurements have been performed in UHDR electron beams (7 MeV and 9 MeV) at SIT ElectronFLASH accelerator.

Results

Figure 1: Response of an ultra-thin plane parallel ionization chamber prototype with a 0.27 mm gap polarized at -250 V in an electron beam from a SIT ElectronFLASH accelerator without application of any ion recombination correction.

Conclusions

Ultra thin gap plane-parallel ionization chambers are a promising option as secondary standard dosimeters for the FLASH radiotherapy quality assurance.

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

To characterize the biological "FLASH effect", it is necessary to have dosimeters available for reliable real-time measurements at ultra-high dose rates (> 40 Gy/s) and ultra-high dose-per-pulse (DPP > 0.6 Gy/pulse). A promising approach are detectors based on synthetic single crystal diamond working as Schottky photodiodes like the microDiamond T60019. The aim of this work is to investigate the dose response linearity of this detector type at ultra-high DPP.

Methods

Several different microDiamond detectors were investigated at PTB's research electron accelerator (20 MeV, 5 Hz, 2.5 µs pulse duration). To determine the DPP reference, the beam current monitor was calibrated against alanine.

Results

All microDiamonds respond linearly at low DPP (Figure 1). The response deviates from linearity with increasing DPP and finally reaches saturation. The DPP value at which non-linear behavior becomes significant varies between 0.1 and 2 Gy/pulse for the commercially available microDiamonds and is exemplar-dependent (SN). However, prototypes (B1, C1) demonstrated that the linear range can be extended.

Figure 1. Measured vs. actual DPP for different microDiamonds

Conclusions

The dose response of various microDiamond exemplars was investigated as a function of DPP. Commercially available microDiamonds have limitations in ultra-high DPP range. However, it has been shown with microDiamond prototypes that the linear range can be extended to the ultra-high DPP range. This shows that the microDiamond is in principle suitable for FLASH-RT dosimetry.

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

Vented ionization chambers (IC) are used as the standard dosimeter for clinical reference dosimetry. For ultra-high dose per pulse (DPP) in the range of 0.6 – 10 Gy, as under investigation for FLASH radiotherapy with electrons, available ICs show large deviations due to ion recombination. However, it is desirable to use ICs also under ultra-high DPP conditions as a secondary standard.

Methods

Parallel plate prototype ICs with different electrode distances d manufactured by PTW were investigated at PTB's research electron accelerator (20 MeV, 5 Hz, 2.5 µs pulse duration). To determine the DPP reference, the beam current monitor was calibrated against alanine. The measurements were compared to a numerical approach by solving a system of partial difference equations, taking into account charge creations by the radiation, their transport and reaction in an applied electric field.

Results

As the electrode distance decreases, the deviations due to ion recombination become smaller. For the prototype with d=0.25mm, almost no deviation is detectable anymore. There is a good agreement between measurements and simulations.

Figure 1: Measured and simulated DPP vs. reference DPP for ICs with different electrode distances d and a voltage of 250V.

Conclusions

Our investigation confirms the previous findings of Gomez et al. presented on this conference. Parallel plate ICs with very small electrode distances are a promising tool for real time dosimetry in FLASH radiotherapy.

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.