Background and Aims

To present alternatives, which rely on a Faraday Cup (FC) as reference, to the use of ionization chambers (ICs) for dosimetry in mm-small proton beams at ultra-high dose rates (UHDRs). Indeed, significant ion recombination combined with the volume averaging effect severely challenge the use of ICs in UHDR small-field dosimetry. Three distinct applications of a FC are presented: i) Prediction of the delivered dose; ii) Response characterization of field detectors up to UHDR; iii) On-line verification of delivered dose to biological samples.

Methods

250MeV transmission pencil beams can be delivered to small biological samples and detectors at currents up to ~700nA (~9000Gy/s on beam axis). For i) FC, beam width and integral depth-dose measurements are used to model the delivered dose. For ii) and iii) the FC is positioned downstream from the detectors or samples to be examined, which are then exposed to a wide range of dose rates. Following detectors have been studied: PTW IC 7862, PTW microDiamond 60019, EBT3 Gafchromic films, scintillating screens.

Results

EBT3 films and scintillating screens are dose rate independent, as well as microDiamond detectors (within +/-0.7%) over the range considered. The PTW IC 7862, though reproducible, exhibits a drop in response larger than 30% at ~9000Gy/s. Reproducibility of delivered dose for the proposed setup better than 1%.

Conclusions

FC are versatile dosimetry instruments that can be employed for dose prediction, field detector characterization and on-line dose verification for pre-clinical experiments at UHDR. microDiamond detectors showed promising results for their suitability for UHDR experiments for proton beams.

Background and Aims

The objective of this study was to assess the use of optically stimulated luminescence detectors (OSLDs) to support radiobiological experiments for ultra-high dose rates (FLASH) proton beams.

Methods

Two experimental setups were tested to accommodate either biological samples or multiple mm²-sized Al₂O₃:C OSLDs. The OSLDs were read out using a protocol with a reference irradiation under known conditions to account for material differences. The experiments were conducted in a single pencil beam at the PSI Gantry 1 at a wide dose rate range of (1-3800) Gy/s. A third experiment assessed the spot reconstruction at 9000 Gy/s.

Results

The OSLDs were demonstrated to be dose rate independent with a negligible signal fading. The OSLD evaluated doses were on average (n=66) within 1 % of the nominal dose for (3 – 33) Gy for dose-rates (1 – 1000) Gy/s. The discrepancy between the OSLDs and the nominal dose was higher for the (3800-9000) Gy/s dose rates due to averaging effects of the narrow pencil beam over the OSLD surface, where a correction was demonstrated. An OSLD dose measurement was overall found to be reproducible within 1 %. The use of an OSLD grid enabled an estimation of the beam spot size and position in agreement (deviation < 2%) with radiochromic film measurements.

Conclusions

The results demonstrate that the almost point-like OSLDs are applicable for accurate proton dosimetry in ultra-high dose rates and suitable to support radiobiological experiments in water and air.

Background and Aims

To present alternatives, which rely on a Faraday Cup (FC) as reference, to the use of ionization chambers (ICs) for dosimetry in mm-small proton beams at ultra-high dose rates (UHDRs). Indeed, significant ion recombination combined with the volume averaging effect severely challenge the use of ICs in UHDR small-field dosimetry. Three distinct applications of a FC are presented: i) Prediction of the delivered dose; ii) Response characterization of field detectors up to UHDR; iii) On-line verification of delivered dose to biological samples.

Methods

250MeV transmission pencil beams can be delivered to small biological samples and detectors at currents up to ~700nA (~9000Gy/s on beam axis). For i) FC, beam width and integral depth-dose measurements are used to model the delivered dose. For ii) and iii) the FC is positioned downstream from the detectors or samples to be examined, which are then exposed to a wide range of dose rates. Following detectors have been studied: PTW IC 7862, PTW microDiamond 60019, EBT3 Gafchromic films, scintillating screens.

Results

EBT3 films and scintillating screens are dose rate independent, as well as microDiamond detectors (within +/-0.7%) over the range considered. The PTW IC 7862, though reproducible, exhibits a drop in response larger than 30% at ~9000Gy/s. Reproducibility of delivered dose for the proposed setup better than 1%.

Conclusions

FC are versatile dosimetry instruments that can be employed for dose prediction, field detector characterization and on-line dose verification for pre-clinical experiments at UHDR. microDiamond detectors showed promising results for their suitability for UHDR experiments for proton beams.

Background and Aims

Seven proton therapy centers collaborating within INSPIRE project participated in the mailed dosimetry audit of proton therapy treatment units utilizing spot scanning beams organized by EURADOS WG9 'Radiation dosimetry in radiotherapy'.

Methods

Prototype assembly for positioning the detectors in the water phantom has been developed and tested during simultaneous irradiation at IFJ PAN in Krakow and PTC in Prague in June 2020. Alanine, radiophotoluminescent and thermoluminescent detectors from five European institutes have been irradiated with 10 x 10 cm² single layer field and 10 x 10 x 10 cm³ plan.

Results

Very good agreement between the two centers has been obtained for alanine, RPL and TL detectors. Based on the results of the initial experimental campaign, the assembly for detector positioning has been modified. As a next step, one assembly for each participating center has been produced including the corresponding number of water-proof detector holders. In the frame of INSPIRE join research activity, audit manuals, detectors and holder assemblies were distributed to participating centers. Audit irradiations took place in all centers during March-April 2021.

Conclusions

The obtained results provide an indication of dosimetric agreement between centers and bases for definition of standardization procedures in dosimetry of active scanning proton beams. After a successful initial experience, mailed audit of scanning proton beams may become a part of clinical routine helping in standardization of active proton beam dosimetry.

This work was partially supported from the European Union's H2020 Research and Innovation Programme, under Grant Agreement No: 730983 (INSPIRE project).