Online Dose Verification in VHEE Radiotherapy Using Bremsstrahlung Radiation

Abstract

Very high energy electrons (VHEE) in the 50-250 MeV range, delivered in short pulses at ultra-high dose rates, are proposed for clinical FLASH radiotherapy (RT) targeting deep-seated tumors. The clinical implementation of VHEE-FLASH RT requires online verification to optimize dose delivery. In this study we propose a novel online dose verification technique based on the detection of bremsstrahlung photons during VHEE interactions with matter. A polymethyl methacrylate (PMMA) phantom was simulated to evaluate the dose deposited by a VHEE beam and to optimise the system design to detect the bremsstralung radiation. Experimental validation was performed at the Beam Test Facility at Laboratori Nazionali di Frascati (Isituto Nazionale di Fisica Nucleare-INFN). A deep learning pipeline was developed to reconstruct the dose distribution in the phantom based on the bremsstrahlung radiation profile. Experimental results demonstrated successful detection of bremsstrahlung radiation emitted orthogonally to the beam axis. The deep learning model achieved accurate dose reconstruction based on the bremsstrahlung radiation profile with a discrepancy of less than 2 percent compared to the simulated dose distribution. This study confirms that bremsstrahlung detection provides a viable online verification for VHEE RT.

Figures (10)

• Figure 1: Conceptual representation of the secondary radiation online monitoring system for VHEE radiotherapy. The system detects bremsstrahlung photons emitted orthogonally to the beam axis (in red). It samples the radiation profile along the beam axis, producing a histogram (in orange) that can be correlated with the dose distribution within the target area.
• Figure 2: Distribution of the emission angles of the photons produced in the phantom for $10^4$ simulated events.
• Figure 3: Simulated integrated depth dose and number of gamma rays emitted at 90° for 80 MeV electrons interacting in the PMMA phantom for $10^5$ simulated events, both normalized at the maximum value.
• Figure 4: Experimental setup at the BTF at LNF-INFN. (a) Phantom configuration with detectors at 90° to the beam. (b) With a 2 cm air gap. (c) Photograph of the first setup. (d) Photograph of the setup with a gap in the phantom.
• Figure 5: Normalized counts acquired during the 150 MeV electron beam scan. The experimental data (orange points) shows the detector counts at different positions along the phantom, while the blue dashed line represents the corresponding simulated bremsstrahlung profile.