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

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

Real-time in vivo verification enables immediate action in case a dose delivery is not according to plan. In vivo verification techniques based on prompt gamma rays will be very challenging or impossible during FLASH irradiations due to the extremely high instantaneous prompt radiation flux. Positron emission tomography of positron emitters created by the therapeutic beam in the patient is more promising as the signal is delayed by the radioactive half-life and can thus be measured in-between beam pulses. Experimental results will be presented and a technological development path to enable positron emission tomography during FLASH therapy will be presented.

Methods

We are investigating imaging of the positron emitter nitrogen-12 (with a very short half-life of just 11 ms) for real-time in vivo verification of proton therapy. The behaviour of scintillation detectors during the irradiation of graphite and PMMA phantoms with a pulsed proton beam, as well as the effect of switching off the detectors during the beam pulse, was investigated. Biograph mCT PET scanner detectors as well as NaI detectors were used.

Results

Switching on/off of the detectors by enabling/disabling the charge multiplication in their photomultiplier tubes allows recovery of the detector operation within about 1 ms after the beam off/detector on moment. Beyond this time, some residual recovery is observed, which can be software-corrected on list-mode data.

Conclusions

When switched on after a beam pulse, the recovery of scintillation detectors is sufficiently quick to take positron emission tomography data of the very short-lived nitrogen-12 and any other positron emitter.

Background and Aims

We are developing a novel technology for precise, real-time beam monitoring (e.g., X-ray photons, electrons, protons, helium-ions, carbon-ions, etc.) for all modalities of FLASH radiotherapy (FLASH-RT) based on a recently patented, transparent, large-area beam imaging detector. The technology provides beam profiling and dose-rate verification during delivery to ensure patient safety and a beam termination signal if necessary.

Methods

Our FLASH-RT monitor is based on a new, highly efficient, semi/micro-crystalline scintillator film surrounded by an ultrafast, multi-camera machine vision system operating at ≥10,000 frames/second, that continuously streams and analyzes each image during treatment. The system incorporates an innovative folded-optical system with an active scintillator area of 26 x 30 cm. It features a thin profile, rapid internal calibration, and analyzes images every 100 µs for beam position, profile, and dosimetry.

Results

A proof-of-concept was first demonstrated in 2019 with funding from the National Cancer Institute and DOE Office of Science and Office of Nuclear Physics. True 2D-position and beam profile spatial resolutions of <10 µm have been achieved. Besides FLASH-RT, the system with modifications can be adapted for hypofractionated-RT, spatially-fractionated-RT, and BNCT real-time dosimetry and monitoring. It can also monitor and analyze sporadic dose-rate spikes of <100 µs from synchrotron accelerators for proton and carbon-ion therapy. The new scintillator material is highly radiation hard, as demonstrated after 400,000 Gy at dose-rates of ≥3,000 Gy/s.

Conclusions

The described system provides a large-area, precise, ultrafast beam profile analysis and dosimetry system with real-time verification within 100 µs. It is a FLASH-RT-enabling technology.

Background and Aims

A graphite calorimeter with a small core (5 mm diameter, 7 mm height) has been designed with the objective to characterize solid-state detectors in new radiotherapy modalities with respect to response changes resulting from changes in LET, dose rate and other parameters. Using a replica of the calorimeter, one can, for example, substitute the graphite core with the detector under investigation to obtain paired measurements with the graphite core and the detector under investigation under near-identical scatter conditions. In this study, we used the calorimeter to test alanine dosimetry for dose-rate effects in proton FLASH beams.

Methods

Measurements were performed at the Varian ProBeam system at the Danish Center of Particle beam using 250 MeV PBS beams with nozzle currents ranging from 4 nA to 215 nA (FLASH). For the main tests, we compared alanine pellets placed in the beam entrance with the temperature increase in the graphite core placed about 8 cm downstream.

Results

14 proton irradiations delivered doses of 10-11 Gy at different nozzle currents using a fixed 7 x 7 spot pattern (30 mm x 30.6 mm field size with a 50 Gy/s field dose rate for 215 nA nozzle current). The alanine doses correlated strongly with the temperature increases in the graphite core. Within experimental uncertainty, the ratio between alanine dose and temperature increase was found to be independent of the nozzle current in the tested dose rate range.

Conclusions

This study supports that alanine can be used for proton FLASH dosimetry without correction for dose-rate effects.