Supersonic Gas Curtain monitor technology extends beam monitoring capabilities for FLASH proton beam therapy
Proton beam therapy is already recognised for its ability to target tumours with high precision while sparing surrounding healthy tissue. FLASH therapy advances this approach by delivering radiation at ultra-high dose rates (>40 Gy/s). Preclinical studies suggest that such rapid delivery can destroy cancer cells as effectively as conventional treatment while substantially reducing side-effects in normal tissue.
Monitoring these extremely intense and short proton pulses, and the dose they deliver, remains a major technical challenge and requires new diagnostic techniques.
Researchers at the University of Liverpool from the QUASAR Group, led by OMA Project Coordinator Prof Carsten P Welsch, have been developing the Supersonic Gas Curtain Ionisation Profile Monitor (SGC-IPM) for real-time beam-profile and dose monitoring in FLASH therapy for the past two years. The device is a derived from the beam gas curtain (BGC) monitor installed in the Large Hadron Collider at CERN.
The new SGC-IPM was modified to use a different detection system called the Ionization Profile Monitor (IPM) for faster detection. It projects a thin gas curtain, generated from a supersonic jet in vacuum, across the path of the ion beam. As the proton beam pass through, they ionise the gas, and a detector captures the ions resulting in a signal which is used to reconstruct the beam’s two-dimensional shape, all without disturbing the beam itself, making the measurement completely non-invasive.
After achieving the first key milestone of testing the device for beam profile monitor in lab measurement, the second key milestone of the project was to extend the technique beyond beam profiling to also estimate the dose. The newly published study, led by Dr Milaan Patel, presents a model that calibrates the device response to the beam current and energy and can be used to measure the delivered dose and the beam profile simultaneously. This transforms the device from a beam-profile monitor into a dose-profile monitor.
The paper reports results from first proof-of-concept tests, carried out by measuring proton beams from the Pelletron accelerator at the Dalton Cumbrian Facility (UK) and later at the MC-40 cyclotron at the University of Birmingham. Data from both experiments were used to validate the calibration model, which showed a distinct but predictable response of the device for different beam energies and currents, while remaining independent of dose rate.
This behaviour allows the SGC-IPM to be calibrated to predict the dose using the same fundamental principles as conventional ionisation chambers (ICs), the current industry standard for radiotherapy, but without the drawbacks of the ion-recombination effects that limit IC performance at ultra-high dose rates. This capability enables the SGC-IPM to operate effectively across both conventional and FLASH dose regimes. Combined with its fast (≈ 100 ms) and non-invasive detection, the technology offers potential for in-vivo dose-profile monitoring for (future) clinical FLASH proton beam therapy.
Dr Patel commented: “The SGC-IPM is a complex device which requires interdisciplinary expertise in gas dynamics, electronics and control engineering, accelerator physics, and medical physics. This paper represents an important milestone in its development toward a practical dose-profile monitor. It would not have been possible without the experience gained over almost two decades of work on gas jet and ionisation-profile measurement technologies, and the support of our collaborators at the University of Birmingham and Imperial College Healthcare NHS Trust, as well as our colleagues from GSI, CERN and the LhARA collaboration.
While we have demonstrated that the technique works in principle, our next challenge is to improve dose-response sensitivity for faster detection, calibration, and testing at FLASH dose rates. There is also an engineering challenge to scale the device down for an even simpler and more robust design.”
With PhD students Mr William Butcher and Ms Farhana Thesni MP, the project has gained significant momentum, and ongoing developments are expected to further advance the SGC-IPM towards a fully functional dose-profile monitor. These efforts will bring the technology closer to clinical application and have good potential to enable next-generation cancer treatment modalities.
More information:
'Supersonic Gas Curtain Based Ionization Beam Profile Monitor for FLASH Proton Beam Therapy', Milaan Patel, et al., Front. Oncol., Sec. Radiation Oncology 15 - 2025 https://doi.org/10.3389/fonc.2025.1694310