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Displaying One Session

Thu, 01.01.1970

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Wed, 01.12.2021
Session Time
14:50 - 15:50
Room
Room 2.15
Session Description
Oral presentations and live Q&A.

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 RECOMBINATION CORRECTION FACTORS AND BENCHMARK OF DETECTORS IN A VERY-HIGH DOSE RATE PROTON SCANNING BEAM

Session Type
FLASH Modalities Track (Oral Presentations)
Date
Wed, 01.12.2021
Session Time
14:50 - 15:50
Room
Room 2.15
Lecture Time
15:10 - 15:20

Abstract

Background and Aims

Accurate dosimetry is paramount to study the FLASH effect since dose and dose-rate are critical dosimetric parameters governing its underlying mechanisms. With the goal of assessing the suitability of standard clinical dosimeters in a very high dose rate (VHDR) experimental setup, we evaluated the ion collection efficiency of several commercially available air-vented ionization chambers (IC) in conventional and VHDR proton irradiation conditions.

Methods

The Proteus 235 cyclotron (IBA) at the Orsay Proton therapy Centre was used to deliver VHDR pencil beam scanning irradiation. Ion recombination correction factors (ks) were determined for several detectors (Advanced Markus, PPC05, RAZOR Nano, CC01) using Jaffé plots. Dose rate independent detectors such as a Faraday cup and alanine dosimetry were employed to cross calibrate absolute dose measurements of the ICs.

Results

Mean dose rates at isocenter ranged from 2Gy/s to 230Gy/s, and instantaneous dose rates were up to 1000Gy/s. The IC and Faraday cup agreed within 3% and differences between alanine and reference measurements were smaller than 1% for doses above 20Gy. Recombination correction factors below 1.5% were obtained for all chambers at VHDR with significant variations among detectors, while ks values were significantly smaller (0.3%) for the conventional dose rates.

Conclusions

While the collection efficiency of the ICs in VHDR proton therapy is comparable to that in the conventional regime or for dose rates smaller than 150Gy/s, the reduction in collection efficiency cannot be ignored and varies significantly between irradiation conditions and detectors.

This work resulted from the project 18HLT04 UHDpulse which received funding from the EMPIR programme.

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FRICKE DOSIMETRY AS A PRIMARY STANDARD AND REFERENCE FOR ABSORBED DOSE TO WATER IN ULTRA HIGH PULSE DOSE RATE 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
15:20 - 15:30

Abstract

Background and Aims

The calibration of a dosimeter system to be used in ultra-high pulse dose rate electron beams - allowing FLASH radiotherapy - requires a traceable measurement of absorbed dose to water.
METAS has been using Fricke solution for dosimetry purpose for over twenty years. This chemical dosimeter is based on a closely water-equivalent ferrous ammonium sulfate solution. Irradiation with ionizing radiation causes oxidation of Fe2+ to Fe3+. The resulting concentration of Fe3+ in the Fricke solution is proportional to the absorbed dose to water and can be determined by analyzing the change in absorbance at well-defined wavelengths in the UV spectral range.

Methods

The primary standard is realized by means of the total absorption technique. A thin and monoenergetic electron beam with known charge is totally absorbed in a large volume of Fricke solution. The well-known deposited energy of the beam and the mass of the liquid is used to determine the radiation chemical yield of the Fricke dosimeter.

Results

This factor allows calibrating secondary standards like ionization chambers in reference fields by comparison with small bags filled with Fricke solution used as transfer standard. We will present the status and results of the total absorption experiment.

Conclusions

We show that the Fricke dosimeter is suitable for dosimetry in the (ultra-high) pulse dose rate regime, by comparing Fricke dosimetry to Alanine and ionization chambers.

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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DEVELOPMENT AND TEST OF A SMALL PORTABLE GRAPHITE CALORIMETER FOR USE IN ULTRA-HIGH DOSE RATE PARTICLE 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
15:30 - 15:40

Abstract

Background and Aims

The FLASH effect occurs in tissue when therapeutic radiation dose is delivered at ultra-high dose-rates (UHDR), greater than 40 Gy/s. At these dose-rates, the probability of sparing healthy tissue is greatly enhanced whilst damage to cancerous tissue remains devastating. In the clinic, accurate determination of absorbed-dose delivered to the target region at UHDR is challenging due to inefficient collection of charge within ionisation chambers (IC), and over-response of radiochromic film (RCF).

National Physical Laboratory (NPL), scientists re-purposed a simple portable graphite calorimeter (SPGC), for use with UHDR particle beams.

Methods

Measurements were carried out using a clinical 250 MeV scanned-proton beam system adapted to deliver FLASH proton radiotherapy beams and compared with the NPL primary-standard level graphite proton calorimeter, IC, RCF and alanine pellets, all in terms of dose-to-water.

Results

Preliminary analysis indicates agreement between the SPGC and primary-standard level calorimeter is within the overall combined measurement uncertainties of 1.5%, k=1. Calculation of calorimeter perturbation factors using Monte Carlo simulations are ongoing, as well as analysis of RCF, IC and alanine data.

Conclusions

This presentation will explain the measurement protocol carried out, present the results obtained and the associated levels of measurement uncertainty to show how a simple, portable calorimeter can be an effective tool in the clinic to accurately determine absorbed-dose delivered to the target at a significantly lower value of measurement uncertainty than current IC measurement protocols, typically >2%, k=1, or to determine correction factors for IC and RCF measurements in the clinic.

This project was funded by EMPIR 18HLT04UHDpulse

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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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