Background and Aims

As the development of suitable detectors in ultra high Dose Per Pulse (DPP) beams is on-going, temporary protocols solving limitations with standard tools are required for the acceptance of new dedicated machines. We present a protocol for system commissioning using clinically available dosimeters, optimizing the use of the detectors. Extra attention was given to beam characterization when varying the DPP.

Methods

Measurements were performed on the ELF, a dedicated research linac for Flash radiotherapy with electrons (ElectronFlash, SIT, Italy) at 7 and 9 MeV using standard dosimetric tools: a PTW Advanced Markus (AM) plane-parallel ion chamber, Gafchromic EBT-XD films, and alanine pellets. The system is highly tunable for pulse length, pulse repetition frequency and number of pulses. Collaboration with the developer allowed further tuning. The DPP was varied using pulse length, distance from the linac and e-gun current.

Results

Performance tests assessing dose, dose stability, PDD, profiles, and output linearity to system settings were measured cross-referencing at least two detectors (comparable within 5%). Energy and dose outputs, monitored with films and AM, were stable within 5% in several months. The AM was used for daily QA for doses up to 0.5 Gy/pulse (model proposed in abstract #211). Changing the DPP using the above-mentioned parameters resulted in a linear output change and minimal variation (<1 MeV) of the spectrum.

Conclusions

We characterized the ELF using standard dosimetric tools. We propose the combination of alanine and Gafchromic films as absolute dosimetric standard. The AM can be employed as real-time tool for daily QA.

Background and Aims

As the development of suitable detectors in ultra high Dose Per Pulse (DPP) beams is on-going, temporary protocols solving limitations with standard tools are required for the acceptance of new dedicated machines. We present a protocol for system commissioning using clinically available dosimeters, optimizing the use of the detectors. Extra attention was given to beam characterization when varying the DPP.

Methods

Measurements were performed on the ELF, a dedicated research linac for Flash radiotherapy with electrons (ElectronFlash, SIT, Italy) at 7 and 9 MeV using standard dosimetric tools: a PTW Advanced Markus (AM) plane-parallel ion chamber, Gafchromic EBT-XD films, and alanine pellets. The system is highly tunable for pulse length, pulse repetition frequency and number of pulses. Collaboration with the developer allowed further tuning. The DPP was varied using pulse length, distance from the linac and e-gun current.

Results

Performance tests assessing dose, dose stability, PDD, profiles, and output linearity to system settings were measured cross-referencing at least two detectors (comparable within 5%). Energy and dose outputs, monitored with films and AM, were stable within 5% in several months. The AM was used for daily QA for doses up to 0.5 Gy/pulse (model proposed in abstract #211). Changing the DPP using the above-mentioned parameters resulted in a linear output change and minimal variation (<1 MeV) of the spectrum.

Conclusions

We characterized the ELF using standard dosimetric tools. We propose the combination of alanine and Gafchromic films as absolute dosimetric standard. The AM can be employed as real-time tool for daily QA.

Background and Aims

The radiobiological study of the FLASH effect requires accurate and reliable dosimetry. As scintillators are promising candidates, this work presents a first characterization of their response at UHDR, suggesting solutions to account for possible saturating effects.

Methods

Five scintillating fibers, one clinical and two experimental Y₂O₃:Eu-based scintillators (DoseVue N.V., Belgium) and two experimental Al₂O₃:C-based scintillators (SCK CEN, Belgium), were irradiated using the ElectronFlash system (SIT, Italy). Linearity with dose was validated by varying the number of pulses for both experimental Y₂O₃:Eu-based scintillators. Dose per pulse (DPP) linearity was investigated in all scintillators by varying pulse duration and the distance from the linac exit window (SSD). Pulse scheme stability was investigated by changing of the pulse repetition frequency (PRF).

Results

Good linearity with dose (R²>0.99) was observed in both experimental Y₂O₃:Eu-based scintillators. Linearity with increasing pulse duration (R²>0.95) and an inverse squared relation between DPP and SSD (R²>0.95) were observed in 4 out of 5 scintillators. However, a concave curvature in response, suggesting saturation, was observed for all scintillators. This effect was more pronounced for the smaller applicator diameter. Our results showed reduced saturation effects when increasing integration time as well as when reducing signal intensity. All scintillators showed a decreasing response with increasing PRF.

Conclusions

The promising characteristics of scintillators as on-line dosimeters for UHDR were validated and possible solutions to reduce saturation effects have been evaluated. Further research on the response by varying PRF is needed.

This work is part of the 18HLT04 UHDpulse project which received funding from the EMPIR 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

Commercially available real time response dosimetric systems were shown to be unsuitable for Flash therapy, in which ultra-high Dose Per Pulse (DPP) is of interest. There is a need for novel, reliable and fast response detectors for such an application. In the present work, a comprehensive investigation is reported on the properties of diamond-based detectors specifically designed for Flash therapy.

Methods

Several diamond Schottky diode prototypes were produced at Rome Tor Vergata University, in cooperation with PTW Freiburg. Quite a few relevant design and electronical parameters were systematically varied, tuning the device’s overall performance and meet the stringent requirements of ultra-high DPP irradiation. The resulting prototypes were tested by using an ElectronFllsh linac (SIT S.P.A.), by using a 9 MeV electron beam and DPPs up to 12.5 Gy/pulse.

Results

The performance of the prototypes, investigated under a wide range of irradiation parameters, was found to depend on their specific structural properties and designs, showing a response linearity up to a DPP of 12.5 Gy/pulse in the best case.

Conclusions

First results of this systematic investigation, clearly demonstrate the feasibility of a diamond based detector for Flash therapy applications. The analysis of the obtained experimental results will be used as an input for further development and optimization procedure of the investigated prototypes.

The present work is part of the 18HLT04 UHDpulse project which 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) represent the standard for performing the commissioning of medical linacs. Nevertheless, their use in the UHDR range is not currently possible, due to the amount of charge produced by each pulse.

For dose-per-pulse (dpp) above 0.5 cGy/p, the approach implemented by international protocols for modelling ion recombination failed, because the free electron fraction p contribution must be considered. We modify the approach of Di Martino (2005) in order to obtain p by means of ionometric measurements only.

Methods

According to the proposed model:

where

q^col is the charge collected by IC;

V is the voltage applied to IC;

q^gen is the charge generated by the pulse;

A and λ are parameters depending on the IC..

By varying the voltage applied V, such function can be determined versus the unknown parameters (q^gen, A and λ ); then, such parameters can be determined by means of the non-linear regression method.

Once all parameters are known, p is calculated and and then k_sat is determined as:

being α = A / V .

Results

The method has been adopted for estimating k_sat for the Adv. Markus, both with the beam produced by ElectronFlash and by LIAC HWL.The fit provided an agreement better than 1% within IORT range and better than 5% with 0.6 Gy/p.

Conclusions

Once k_sat is known, ionometric measurements at the larger distance might become the central element of a reliable Quality Assurance program for any Flash linac.