Background and Aims

A new non-destructive very low beam current measurement system suitable for online monitoring during FLASH experiments has been developed as part of a collaboration between Bergoz Instrumentation, Paul Scherrer Institute (PSI) and Instrumentation Technologies. The beam current measurement system is based on the prototype development validated with real beam measurements at PSI’s proton therapy facility, PROSCAN.

Methods

The beam current measurement system consists of a re-entrant cavity resonator and a Libera Digit 500 implementation. The re-entrant cavity resonator, operating at the fundamental resonance mode matched to a given harmonic of the CW beam repetition rate, provides a fast response for a given excitation. The re-entrant cavity resonator provides a position- and beam-size-independent response. The re-entrant cavity resonator, a narrow-band device, provides a high signal-to-noise ratio that enhances the application for online monitoring.

Figure 1: (Simulation) Signal from the re-entrant cavity resonator for a 0.28 microsecond macro-pulse with a micro-bunch repetition rate of 14 nanosecond. Each micro-bunch has a charge of 0.14 attocoulomb. The average output power is approximately 0.3 femtowatt.

The Libera Digit 500, representing the digital end, implements a Digital Down-Conversion (DDC) block to provide amplitude and phase of the measured signal at the resonance frequency with a 1 ms time resolution.

Results

We report on the real beam measurements (0.1-10 nA) of the first industrialised non-destructive beam current measurement system performed at PROSCAN and on the online-monitoring capabilities for FLASH intensities.

Conclusions

A new non-destructive current measurement system for FLASH has been industrially developed and validated with beam measurements.

Background and Aims

Verifying a treatment machine’s dose and dose rate in FLASH radiation therapy is essential for assurance of accurate treatments. However, determining the absolute standard and verifying the dose rate of the delivered beam for patient treatment is challenging. For these reasons, we aimed to find the possibility of using machine log files to confirm the dose and dose rate for proton line scanning beams.

Methods

Scanning plans delivering 3, 6, and 9 Gy of 2x2, 3x3-cm square and a round shape of 23 mm diameter with a proton beam (Sumitomo Heavy Industries, Japan) were created for tests. An 8x8 cm mini ridge filter was used for creating a 2 cm of SOBP for a 230 MeV proton beam. Dose and dose rates were measured in the mid of SOBP with a PPC05 ionization chamber (IBA) in a high-density polyethylene phantom. The log files generated from the dose monitor in the scanning nozzle by plan irradiations were exported. The parameters converting ionization counts to dose were measured at 2cm depth in a water phantom with a conventional dose rate beam. Ionization count values and irradiation time recorded in the log files were used to compute dose and dose rates and compared with measurements.

Results

The calculated dose and dose rate agree with measurements within 3% of the difference. The observed differences are considered to come from the parameter inaccuracy converting the ionization counts to dose in a phantom.

Conclusions

We demonstrated the feasibility of calculating dose rate in FLASH radiation therapy using log files which reflects machine errors. In a future study, we will develop an dose calculation engine with log files and Monte-Carlo simulation.

Acknowledgements: This work was supported by the National Research Foundation of Korea grant funded by the Korea government. (No. 2021ME2E8A1048108)

Background and Aims

Often, FLASH therapy will be hypofractionated so developing a dose verification feedback loop is critical. We investigate feasibility of real-time thermoacoustic range verification during FLASH delivery by a Mevion Hyperscan proton therapy system.

Methods

A Mevion Hyperscan S250i in physics mode delivered 15-20pC/pulse to a 6”x6”x3” Lexan block into which two transducers were embedded at 90-degree angles relative to the proton beamline. During each experiment, one transducer was distal to the beam and the other was positioned lateral (Fig.1a-b).

Nuclear emissions were detected by a large (0.5m) plastic scintillator+PMT assembly and compact (3cm) radiation detector. The large assembly provided a measure of beam intensity as a function of time; the compact detector provided a trigger.

Thermoacoustic emissions were detected simultaneously using a 4-channel digital oscilloscope (Fig.1c). Results are not signal averaged.

The beam profile was characterized using Gafchromic film, which provided estimates of σ_x=29.0mm and σ_y=27.4mm. Therefore, the “lateral” transducer was in the beam whenever WE range exceeded 7.62cm=3”.

The beamlets in Lexan were modeled using TRIM software to estimate initial pressure. Thermoacoustic emissions were simulated in kWave assuming point receivers. The Grüneisen and soundspeed of Lexan were assumed to be Γ=2 and ν_s=2.035mm/μs.

Fig 1. left: dose maps, right: measured(color) vs simulated(black).

Results

Thermoacoustic signals were visible on the oscilloscope, despite broadband noise that caused shot-to-shot variations. After bandpass filtering, however, DC levels were consistent and shot-to-shot variations seemed to be primarily in amplitude. Moreover, signal detected by the distal transducer agreed with simulations (Fig. 1d). However, lateral placement within the beam induced strong EMI (Fig.1e).

Conclusions

Thermoacoustic signals can be detected at distal transducer locations on a pulse-by-pulse basis during FLASH radiotherapy. For 10.7 WE range, accuracy was 0.7+/-0.4mm. Experiments should be repeated in clinical mode with small diameter beamlets to determine utility of lateral transducer locations.

Background and Aims

Ultra-High Dose-per-Pulse (UHDP) dosimetry is one of the most challenging topics in FLASH radiotherapy. Saturation effects occur in conventional ionization chambers due to the large charge density after UHDP irradiation. This leads to a non-negligible perturbation of the electric field inside the gas that can assume high values, causing uncontrolled discharge, or nullify the bias field, causing complete recombination. Therefore, a new kind of online dosimeter for online dose measurements in UHDP regime is needed. Recently, a new model of gas-based online dosimeter, the ALLS chamber, has been proposed (Fig.1, Fig.2). The filling gas Argon at a pressure of 100 Pa eliminates the electron capture of Oxygen and both saturation and uncontrolled discharges are avoided by decreasing the density of the gas. Practical issues arising from the first attempt of realization of the ALLS chamber, and the adopted solutions, are herewith presented.

Methods

The structure of the chamber showed mechanical instability while removing the air in the sensitive volume: a bending of the entrance window through the inside led to a significative change in the volume geometry and a non-homogeneous field in the gas. The adopted very low pressure led to an enhanced polarity effect due to the non-negligible diffusion of the primary electrons through the collecting electrode. Monte Carlo simulations are employed to optimize the geometry and the materials of the ALLS chamber.

Results

A new plastic material for the chamber body, TECAPEEK, was identified. Its high rigidity makes it suitable to support the mechanical stress. The polarity effect was reduced by metalizing thin layers of Aluminium on the chamber inner surfaces.

Conclusions

Technical modifications were applied to overcome the realization problems of the first prototype of the ALLS chamber. Further investigations and dosimetric measurements in UHDP conditions will be performed to optimize the chamber for FLASH absolute dosimetry.

Background and Aims

Background: Dosimetry uncertainties with preclinical radiobiological studies can affect the reproducibility and translation of new modalities, including FLASH radiotherapy, where Cherenkov emission imaging provides a potential solution.

Aim: A fast-imaging technique was developed for the first in vivo Cherenkov emission imaging from an ultra-high dose rate (UHDR) electron beam source at single pulse (360 Hz) and with at least millimeter resolution for real time monitoring of delivery.

Methods

Methods: A CMOS camera, gated to the UHDR LINAC, imaged the Cherenkov emission profiles pulse-by-pulse during irradiation of murine limbs and intestinal regions and a tissue equivalent phantom. An intensifier’s effect on image quality was investigated considering signal to noise and spatial resolution. Camera response and Cherenkov profiles from individual pulses were quantified spatiotemporally and compared to a dose rate independent EDGE diode detector to confirm beam output.

Results

Results: The intensifier improved the emission profile’s signal to noise ratio from 15 to 280, with reduced spatial resolution (2.8 to 1.0 line pairs/mm). The profile extended beyond the treatment field edge due to the lateral scattering of the electrons and optical photons in tissue. The intensified camera detected changes of ~3mm in Cherenkov emission profiles due to the murine respiration cycle. The camera resolved the LINAC’s variability in output agreeing with the diode to within 4%.

Conclusions

Conclusion: This fast-imaging technique can be utilized for in vivo intrafraction monitoring of FLASH patient treatments at single pulse resolution. It can display delivery differences during respiration, and variability in the delivered treatment’s surface profile, which may be perturbed from the intended UHDR FLASH treatment especially with pencil beam scanning systems. The technique may leverage the Cherenkov emission surface profile as a quality control and patient monitoring tool to document treatment under FLASH conditions while considering beam parameters such as beam’s per-pulse-output or spatial profile consistency during delivery.

Background and Aims

This work aims at investigating the usability of various luminescence materials to support dosimetry of electron beams delivered at ultra-high dose rates.

Methods

Three thermoluminescence detectors (TLDs; LiF:Mg,Ti, LiF:Mg,Cu,P, CaF₂:Tm) and one optically stimulated luminescence detector (OSLD; Al₂O₃:C) were studied. Another OSLD (BeO) was used as dose rate independent reference detector together with an Advanced Markus chamber, where the latter was corrected for recombination effects. The TLDs and OSLDs were read out using an automated reader and their response was normalized using the built-in source to reduce the inter-sample variability. Matrices of 24 samples per material were irradiated in a six by four grid with electrons in the (1 - 350) kGy/s dose rate range at the Federal Institute of Metrology (Switzerland).

Results

None of the materials shows dose rate effects for the investigated dose rates within ± 8 %. The response of the different materials mutually agree within their uncertainties (k = 2). The average deviation of the investigated detectors relative to the reference dose from an Advanced Markus chamber amounts to less than 3 %. Larger deviations (-7 % to +3 %) are observed at the highest dose rate, due to a slight non-uniformity of the electron beam which increases with the dose rate.

Conclusions

The results prove the usability of the investigated luminescence detectors for dosimetry in electron beams at ultra-high dose rate. The use of a matrix of TLD/OSL detectors and monitor detectors might support the characterization of ultra-high dose rate electron beams.