Background and Aims

Background: There are few measurement devices that can produce real-time cumulative and per-pulse measurements of FLASH beams at shortened SSDs.

Aim: This study sought to characterize the performance of a Sun Nuclear EDGE diode detector system for dosimetry at mean dose rates in excess of 1000Gy/s with respect to mean- and instantaneous-dose rate linearity.

Methods

Measurements were made using a Varian Trilogy Linac that was converted to produce FLASH beams by removing the target and flattening filter from the beamline of the 10MV photon mode, giving access to the pristine 10MeV electron beam. A Sun Nuclear EDGE diode detector paired with a FLASH capable electrometer was evaluated by measuring a PDD in Solid Water and across various dose ranges. The uppermost slice of the Solid Water was positioned against the faceplate (SSD = 60cm) and 10±1 pulses were delivered for each measurement. An ion chamber with a bias of 450V was placed at a depth of 20cm to take measurements in the bremsstrahlung tail to prevent saturation. Gafchromic film was used for absolute dosimetry to calibrate the Ion chamber and to verify the PDD by taking measurements at all depths of interest from a single delivery of 10 pulses. Dose linearity was investigated by running the Linac with per-pulse gating from 4 to 50 pulses. Dose rate (Instantaneous and time averaged) was evaluated from the PDD measurements.

Results

The EDGE and Gafchromic film measured PDDs saw strong agreement within the variance of film. Per-pulse dosimetry and dose-rate responses are still being evaluated.

Conclusions

The agreement of the film and diode in the PDD indicates no dose, dose rate, or instantaneous dose rate dependencies for this system. Ongoing studies will quantify each parameter independently. This system can be reliably used for rapid dosimetry under mean dose rates in excess of 1kGy/s.

Background and Aims

Ultra-high dose rate FLASH proton radiotherapy (F-PRT) has been shown to have advantages in increasing the therapeutic window compared to standard proton radiotherapy (S-PRT) in both shoot-through and spread-out Bragg peak double scattered (DS) whole abdomen mouse irradiation with a dedicated research beam line. However, most of the current proton therapy delivery systems utilize pencil beam scanning (PBS) techniques. Additionally, PBS has the capability to deliver ultra-high dose rates to larger fields. The goal of this study was to determine the capabilities of a gantry room with PBS for P-FRT and compare normal tissue sparing capabilities with DS beams for various definitions of dose rate.

Methods

The FLASH PBS beam was characterized for a gantry room. A treatment plan was designed with 4x4 pencil beam spots with 5mm spot spacing. A 2x2cm² collimator was placed after the range shifter to ensure a sharp field penumbra. Field uniformity and alignment was verified using EBT3 Gafchromic film. Effective field dose rate (EFDR) was determined using a NIST-traceable Advanced Markus Chamber. Dose delivery characteristics were analyzed using machine log files. Whole abdomen of female C57BL/6J mice were irradiated to a dose of 15 Gy with one of four groups: (1) PBS F-PRT 167 Gy/s EFDR, (2) PBS F-PRT 112 Gy/s EFDR, (3) PBS S-PRT 1.6 Gy/s EFDR, (4) DS S-PRT 1.0 Gy/s EFDR. Using methods previously developed, EdU-positive proliferating cells and regenerated intestinal crypts were quantified.

Results

A maximum EFDR of 167 Gy/s was achieved using a cyclotron current of 800nA with spot durations of 3ms. Log files show beam stability between irradiations with maximum variation in field delivery time of 2.75ms.

Conclusions

PBS was demonstrated for use with F-PRT in a clinical treatment room, demonstrating key evidence needed for safe and effective clinical translation of FLASH radiotherapy.

Background and Aims

Radiochromic plastic dosimeter PRESAGE has been used for 3D dosimetry for many years. In this study, the feasibility of utilizing PRESAGE phantoms and an optical CT scanner for 3D dose measurements in ultra-high-dose-rate FLASH electron beams was assessed.

Methods

Experiments were performed using a Varian 2100 C/D linear accelerator, converted to deliver ultra-high-dose-rate 10 MeV electron beam. The LINAC delivered approximately 0.7 Gy/pulse for FLASH irradiations. Dose rate was varied from about 40 Gy/s to 240 Gy/s by changing the repetition rate. PRESAGE phantoms were irradiated en face at six FLASH dose rates: 40 Gy/s, 80 Gy/s, 120 Gy/s, 160 Gy/s, 200 Gy/s, and 240 Gy/s. EBT film and scintillator measurements were used to verify dose. Optical response of PESAGE phantom versus delivered dose was evaluated with various known doses. A novel parallel-beam optical CT scanner, utilizing fiber optic taper for collimated images, was developed for fast, high resolution, and accurate readout of 3D dosimeters. Percent depth dose curves for various FLASH dose rates and regular dose rate were generated and compared based on the optical response versus dose measurements. Percent depth dose curves from Eclipse Monte Csarlo calculation were also generated.

Results

The optical density of PRESAGE phantom was confirmed to be linear with absorbed dose, consistent with the observation at regular treatment dose rates. At depths past D90, percent depth dose as a function of depth for six FLASH dose rates (240-40 Gy/s) are nearly identical, indicating that optical response of PRESAGE is dose-rate independent. At depths near to and shallower than Dmax there was increased uncertainty in the results due to unevenness in the phantom surface and low signal-to-noise ratio.

Conclusions

PRESAGE phantoms show dose-rate independence for a wide range of ultra-high-dose-rate electron beams, indicating these phantoms can be useful for relative 3D dose measurements in FLASH electron beams.

Background and Aims

Background:The development of new dosimetric procedure for ultra-high dose-rate (uHDR) for charged particles radiotherapy is underway aiming a swift clinical translation of this promising modality. However, the characterization of new detectors in challenging active-scanning delivery with a mixture of high and low linear energy transfer (LET) beams in FLASH-radiotherapy is still missing.
Aim:To characterize new diamond-detectors for uHDR charged particles delivery, providing accurate dose and dose-rate measurements without LET-dependencies.

Methods

Methods:Two types of diamond-detectors were investigated: a microDiamond-detector (PTW-Freiburg) and a diamond-detector prototype specifically designed for operation in uHDR (flashDiamond). The detectors were irradiated with a helium beam (145.7MeV/u) under conventional or uHDR delivery. Two electronic chains were utilized for dose and instantaneous dose-rate measurements. For dose acquisition the detectors were coupled to a UNIDOS electrometer (conventional/uHDR delivery). Instantaneous dose-rate measurements were performed under uHDR conditions by coupling the detectors to a transconductance amplifier (DLPCA200–FEMTO). Dose-rate delivery structure similarity between monitoring chamber and diamond-detectors were studied for single-spot irradiations. Dose linearity at 4cm depth (single-spot delivery) and in-depth dose-response (2x2cm² field) from 2 to 16cm were investigated in water tank with the two measurement chains and detectors.

Results

Results:Diamond-detectors allowed reproducing the delivery structure recorded from the monitoring chamber (correlation-slope=1.002). Both diamonds-detectors show an excellent linearity response for each delivery mode (correlation-slope=0.997). Figure1 shows the dose-rate structure acquired with a microDiamonds at 14cm depth in water for a 2x2cm² field. In-depth dose responses did not show any LET-dependences (helium LET range of 4 to 50keV/µm).

Figure1:micro-diamonds instantaneous dose-rate on a 2x2cm² field

Conclusions

Conclusion:Diamond-detectors promise accurate dose and dose-rate response in conventional/uHDR delivery. Accurate dose-rate measurement is crucial in understanding and sharing results of potential FLASH-effect. Such measurements are a step forward benchmarking tools predicting dose-rate of scanned ion beams, which will be urgently needed for further treatment planning studies.

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 developed metrological tools needed for reliable real-time absorbed dose measurements of electron and proton beams with ultra-high dose rate and ultra-high dose per pulse. The UHDpulse project will be finished in February 2023.

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 achievements of the involved institutions within the framework of UHDpulse 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.