Physikalisch-Technische Bundesanstalt (PTB)
6.21 "High-energy photon and electron radiation"
Dr. Andreas Schüller is experimental physicist. He works as scientist in the working group “High-energy photon and electron radiation” of the department for “Dosimetry for radiation therapy and diagnostic radiology” of the Physikalisch-Technische Bundesanstalt, the National Metrology Institute of Germany. He is coordinator of the European Joint Research Project UHDpulse – Metrology for advanced radiotherapy using particle beams with ultra-high pulse dose rates.

Moderator of 5 Sessions

Session Type
FLASH Teaching Lecture
Date
Thu, 02.12.2021
Session Time
08:30 - 09:00
Room
Hall C
Session Type
FLASH Modalities Track
Date
Thu, 02.12.2021
Session Time
09:10 - 09:40
Room
Hall C
Session Type
FLASH Modalities Track
Date
Thu, 02.12.2021
Session Time
09:40 - 10:10
Room
Hall C
Session Type
FLASH in the Clinic Track
Date
Thu, 02.12.2021
Session Time
10:10 - 10:40
Room
Hall C
Session Type
Plenary Session
Date
Fri, 03.12.2021
Session Time
18:00 - 18:30
Room
Hall C
Session Description
Announcement of the Best Oral Awards & Best Poster Awards

Presenter of 3 Presentations

Welcome Words

Session Type
Plenary Session
Date
Wed, 01.12.2021
Session Time
08:00 - 09:00
Room
Hall C
Lecture Time
08:00 - 08:15

OVERVIEW AND CURRENT STATUS OF THE JOINT RESEARCH PROJECT UHDPULSE - “METROLOGY FOR ADVANCED RADIOTHERAPY USING PARTICLE BEAMS WITH ULTRA-HIGH PULSE DOSE RATES”

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Wed, 01.12.2021
Session Time
14:50 - 15:50
Room
Room 2.15
Lecture Time
14:50 - 15:00

Abstract

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.

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MULTI-LEAF FARADAY CUP FOR QUALITY ASSURANCE IN RADIATION THERAPY WITH ELECTRON AND ION BEAMS AT CONVETIONAL AND ULTRA-HIGH DOSE RATE

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Fri, 03.12.2021
Session Time
14:30 - 15:30
Room
Room 2.31
Lecture Time
15:00 - 15:10

Abstract

Background and Aims

For quality assurance in radiation therapy with high-energy electrons, protons or carbon ions, the beam energy has to be checked in clinical routine. This test is time-consuming and requires expensive equipment. If the dose rate is varied by orders of magnitude to compare ultra-high dose rate with conventional irradiation, one should be able to measure the energy quick and reliable and independently of the accelerator readouts or manufacturer's specifications. For this purpose, a portable Multi-Leaf Faraday Cup (MLFC) was developed (Fig.1), with a readout electronics whose is not influenced by the dose rate.

fig1.jpg

Methods

The MLFC was irradiated with protons with energies between 64 – 252 MeV from a synchrotron source. Aluminium disks were placed in front of the MLFC as range shifter. Charge and energy were measured per synchrotron spill (Fig.2). Furthermore, the MLFC was tested in electron beams of conventional and ultra-high dose per pulse.

fig2.png

Results

For monoenergetic electron or proton beams energy differences of 80 keV (0.8%) or 60 keV (0.1%), respectively, can be clearly distinguished.

Conclusions

Due to the measurement principle, which is independent of the dose rate, the MLFC is suitable for quality assurance of charged particle radiation with conventional as well as ultra-high dose rate, for short pulses as well as for continuous radiation. Energy and intensity can be measured quick and reliable.

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.

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Author Of 11 Presentations

Welcome Words

Session Type
Plenary Session
Date
Wed, 01.12.2021
Session Time
08:00 - 09:00
Room
Hall C
Lecture Time
08:00 - 08:15

OVERVIEW AND CURRENT STATUS OF THE JOINT RESEARCH PROJECT UHDPULSE - “METROLOGY FOR ADVANCED RADIOTHERAPY USING PARTICLE BEAMS WITH ULTRA-HIGH PULSE DOSE RATES”

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Wed, 01.12.2021
Session Time
14:50 - 15:50
Room
Room 2.15
Lecture Time
14:50 - 15:00

Abstract

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.

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ULTRA THIN PLANE-PARALLEL IONIZATION CHAMBERS: EXPANDING THE RANGE OF AIR IONIZATION CHAMBERS INTO ULTRA-HIGH DOSE RATE.

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Wed, 01.12.2021
Session Time
14:50 - 15:50
Room
Room 2.15
Lecture Time
15:00 - 15:10

Abstract

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

utic_vs_dpp.png

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.

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ION COLLECTION EFFICIENCY IN ULTRA-HIGH DOSE PER PULSE ELECTRON BEAMS

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Wed, 01.12.2021
Session Time
14:50 - 15:50
Room
Room 2.15
Lecture Time
15:40 - 15:50

Abstract

Background and Aims

Ionometry is challenging in ultra-high dose per pulse (DPP) beams due to high level of ion recombination and it does not follow the current theoretical model. This investigation aims to measure the ion collection efficiency for different types of ionization chambers.

Methods

The response of three plane-parallel ionization chambers has been measured in an ultra-high DPP (0.5 to 2 Gy/pulse) 20 MeV electron beam. To measure the pulse charge, a non-destructive integrating current transformer (ICT) is used. The ICT signal was calibrated against the absorbed dose to water measured with alanine.

Results

The dependence of the ion collection efficiency is not linear with the DPP. The intra-type variations were evaluated to be in the 2-5% range as illustrated in figure 1. The ion collection efficiency of the Advanced Markus chamber has been compared to the Petersson et al. (DOI: 10.1002/mp.12111) empirical model. The difference between the measured and expected values from their model is consistent with the intra-type variation observed.

ionometry.png

Figure 1: Ion collection efficiency in ultra-high dose per pulse electron beams

Conclusions

The investigation has shown that the ion collection efficiency of all ion chambers used for dosimetry in ultra-high DPP beams should be evaluated to improve uncertainty. It is important to develop a new theoretical model for ion recombination in ultra-high DPP beams.

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.

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LINEARITY OF DIAMOND DETECTORS IN ULTRA-HIGH DOSE-PER-PULSE ELECTRON BEAMS

Session Type
FLASH in the Clinic Track (Oral Presentations)
Date
Wed, 01.12.2021
Session Time
18:00 - 19:00
Room
Hall C
Lecture Time
18:00 - 18:10

Abstract

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.

dppd microdiamond.png

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.

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NEW R&D PLATFORM WITH UNIQUE CAPABILITIES FOR ELECTRON FLASH AND VHEE RADIATION THERAPY AND RADIATION BIOLOGY UNDER PREPARATION AT PITZ

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Thu, 02.12.2021
Session Time
11:00 - 12:00
Room
Hall C
Lecture Time
11:30 - 11:40

Abstract

Background and Aims

At the Photo Injector Test facility at DESY in Zeuthen (PITZ, near Berlin, Germany), the realization of an R&D platform for electron FLASH radiation therapy, VHEE radiation therapy and radiation biology is under preparation. The name of the new platform is HP²eFLASH-RT@PITZ, which stands for high power, high performance electron FLASH radiation therapy at PITZ.

Methods

The beam parameters that PITZ can provide are unique in the world: ps scale electron bunches with up to 5nC bunch charge at MHz repetition rate in bunch trains of up to 1 ms in length.

pitz_parameter_space.jpg

Results

The individual bunches can produce dose rates up to 1014 Gy/s and dose deposition up to 1000 Gy. Upon demand, each bunch of the bunch train can be guided to a different transverse location on the tumor, so that either a “painting” with micro beams, or a cumulative increase of absorbed dose using a wide beam distribution can be realized at the tumor. Full tumor treatment can hence be finished in a time interval of 1 ms, mitigating organ movement issues. Together with 20 years of operational experience at PITZ, and availability of detailed beam characterization and extremely flexible beam manipulation capabilities, this R&D platform will cover current parameter range of successfully demonstrated FLASH effects and extend the parameter range towards yet unexplored and unexploited short treatment times and high dose rates.

Conclusions

A summary of the plans for HP²eFLASH-RT@PITZ and the status of the preparations will be presented, with as goal to stimulate interest and broaden out our cooperation.

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PERFORMANCE OF A PROBE-TYPE GRAPHITE CALORIMETER (AERROW) IN ULTRA-HIGH DOSE PER PULSE ELECTRON BEAMS

Session Type
FLASH in the Clinic Track (Oral Presentations)
Date
Thu, 02.12.2021
Session Time
11:00 - 12:00
Room
Room 2.31
Lecture Time
11:20 - 11:30

Abstract

Background and Aims

One of the challenges in investigating FLASH radiation therapy is the reliable measurement of the absorbed dose at ultra-high dose per pulse (DPP). This work aims to evaluate a probe-type graphite calorimeter (Aerrow) at ultra-high DPP.

Methods

The DPP was varied between 0.5 to 5.5 Gy. The signal was compared to the pulse charge measured by a non-destructive Integrating Current Transformer (ICT). In addition, a depth dose curve was measured and compared to an ionization chamber measurement and a Monte-Carlo simulation.

Results

The response of the calorimeter was proportional to the ICT signal. On average, the standard deviation of the calorimeter response was 0.2 % in high dose rate and reached 1% at the lower rate. In figure 1, the depth dose measurement with calorimeter and ionization chamber is compared to Monte-Carlo simulation. The half-value depths in water R50 obtained from measurement and simulation agree within 1 mm.

calorimeter.png

Figure 1: Depth dose in water measured and simulated.

Conclusions

Calorimetry is showing promising results for absorbed dose measurements. Calorimetry gets simpler at FLASH dose rates as the dose delivery is in a few seconds or less. The preliminary results show that advanced thermal insulation of the calorimeter is not required, nor the use of a heat lost correction factor.

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.

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SECONDARY STANDARD DOSIMETRY: UNDERSTANDING THE IONIZATION CHAMBERS FOR THE FUTURE ULTRA-HIGH DOSE RATE APPLICATIONS

Session Type
FLASH in the Clinic Track (Oral Presentations)
Date
Thu, 02.12.2021
Session Time
11:00 - 12:00
Room
Room 2.31
Lecture Time
11:30 - 11:40

Abstract

Background and Aims

Ionization chamber models developed during the last century by Boag have been and continue to be of great interest for daily
clinical practice. However, its simplicity is achieved at the cost of ignoring higher order effects that do not play
an important role in the dose rates found in conventional radiotherapy. However, when we get into the ultra
high dose rate regime, these models have shown important discrepancies.

Methods

A computational model was developed whose main objective is to solve the differential equations governing
charge transport along the ionization chambers. Charge collection efficiency and instantaneous current of different ionization chambers obtained from the model are currently evaluated with a set of measurements carried
out in collaboration with the PTB.

Results

figure1_simulation.png

Figure 1: Charge collection efficiency (CCE) for different analytical models currently used for saturation correction of ionization chambers and their simulation counterpart for a PPC40 chamber. CCE was evaluated for a plane-parallel ionization chamber with 2 mm gap at 300 V and a pulse duration of 2.5 μs at reference temperature and pressure conditions.

figure2_simulation.png

Figure 2: Comparison between instantaneous current from simulation and measured at 4 Gy/pulse and 400 V using a ROOS chamber.

Conclusions

Our computational model, which is still under development, shows better agreement to experimental data than existing analytical models and a preliminary version of this software has been released for testing.


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.

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PLASTIC SCINTILLATOR UNDER ULTRA-HIGH DOSE RATE ELECTRON BEAM: LONG TERM DAMAGES AND CHANGES IN OPTICAL RESPONSE

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Fri, 03.12.2021
Session Time
14:30 - 15:30
Room
Room 2.31
Lecture Time
14:40 - 14:50

Abstract

Background and Aims

Background and aims: Scintillation detectors have advantages that could benefit dosimetry for FLASH radiotherapy, but radiation damages could alter their properties. This study aims at determining long term effects of ultra-high dose rate electron radiation on scintillation dosimeters.

Methods

Methods: Samples of clear and scintillating fibers (1 mm diameter, 15 mm long) were irradiated under electron beams with ultra-high dose per pulse (1 Gy/pulse, 20 MeV, 5 Hz, 2.5 µs pulse length) up to a total dose of 20 kGy. Scintillation and transmission of irradiated samples were assessed using the Hyperscint RP200 platform (Medscint inc., Canada), around 3 months after irradiation.

Results

Results: Emission spectrum remained stable for all doses (figure 1), with a maximum RMS difference of 0.002 between irradiated and unirradiated samples. The total light production and transmission seems minimally affected from irradiation, up to 20 kGy (figure 2). The error bars result from the differences due to sample manufacturing. However, further investigations will have to be made to confirm this statement.

clear_spectra.pngscint_spectra.png

Figure 1: Emission spectra of the irradiated samples.

intensity.png

Figure 2: Total intensity measured as a function of total dose.

Conclusions

Conclusions: Scintillating and clear plastic fibers do not show significant long term alterations when irradiated up to 20 kGy with ultra-high dose rate electron beams.


Acknowledgements: 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. Special thanks to the team of Medscint inc. for their help in analysing the irradiated samples.

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VENTED IONIZATION CHAMBERS FOR ULTRA-HIGH DOSE PER PULS CONDITIONS

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Fri, 03.12.2021
Session Time
14:30 - 15:30
Room
Room 2.31
Lecture Time
14:50 - 15:00

Abstract

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.microplate_dvsdref_u=250v.png

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.

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MULTI-LEAF FARADAY CUP FOR QUALITY ASSURANCE IN RADIATION THERAPY WITH ELECTRON AND ION BEAMS AT CONVETIONAL AND ULTRA-HIGH DOSE RATE

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Fri, 03.12.2021
Session Time
14:30 - 15:30
Room
Room 2.31
Lecture Time
15:00 - 15:10

Abstract

Background and Aims

For quality assurance in radiation therapy with high-energy electrons, protons or carbon ions, the beam energy has to be checked in clinical routine. This test is time-consuming and requires expensive equipment. If the dose rate is varied by orders of magnitude to compare ultra-high dose rate with conventional irradiation, one should be able to measure the energy quick and reliable and independently of the accelerator readouts or manufacturer's specifications. For this purpose, a portable Multi-Leaf Faraday Cup (MLFC) was developed (Fig.1), with a readout electronics whose is not influenced by the dose rate.

fig1.jpg

Methods

The MLFC was irradiated with protons with energies between 64 – 252 MeV from a synchrotron source. Aluminium disks were placed in front of the MLFC as range shifter. Charge and energy were measured per synchrotron spill (Fig.2). Furthermore, the MLFC was tested in electron beams of conventional and ultra-high dose per pulse.

fig2.png

Results

For monoenergetic electron or proton beams energy differences of 80 keV (0.8%) or 60 keV (0.1%), respectively, can be clearly distinguished.

Conclusions

Due to the measurement principle, which is independent of the dose rate, the MLFC is suitable for quality assurance of charged particle radiation with conventional as well as ultra-high dose rate, for short pulses as well as for continuous radiation. Energy and intensity can be measured quick and reliable.

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.

Hide