Monitoring electron energies during FLASH irradiations

Abstract

When relativistic electrons are used to irradiate tissues, such as during FLASH pre-clinical irradiations, the electron beam energy is one of the critical parameters that determine the dose distribution. Moreover, during such irradiations, linear accelerators (linacs) usually operate with significant beam loading, where a small change in the accelerator output current can lead to beam energy reduction. Optimisation of the tuning of the accelerator’s radio frequency system is often required. We describe here a robust, easy-to-use device for non-interceptive monitoring of potential variations in the electron beam energy during every linac macro-pulse of an irradiation run. Our approach monitors the accelerated electron fringe beam using two unbiased aluminium annular charge collection plates, positioned in the beam path and with apertures (5 cm in diameter) for the central beam. These plates are complemented by two thin annular screening plates to eliminate crosstalk and equalise the capacitances of the charge collection plates. The ratio of the charge picked up on the downstream collection plate to the sum of charges picked up on the both plates is sensitive to the beam energy and to changes in the energy spectrum shape. The energy sensitivity range is optimised to the investigated beam by the choice of thickness of the first plate. We present simulation and measurement data using electrons generated by a nominal 6 MeV energy linac as well as information on the design, the practical implementation and the use of this monitor.

Implementation and validation of a beam‐current transformer on a medical pulsed electron beam LINAC for FLASH‐RT beam monitoring

“… 10 This need comes from the heavy saturation that conventional transmission chambers usually used in clinical practice experience in UHDR modes. Although preliminary results show that a linear relationship can be found between dose and monitoring units (MU) for UHDR modes 18 and a modified ionization chamber could be used as a potential monitoring device, 19 another approach is needed due to the lack of intra‐pulse monitoring and the lack of data done with beams with larger field sizes. Using beam current transformers (BCTs), the beam is monitored in real time without beam perturbation and without saturation effects.…”

FLASH irradiation induces lower levels of DNA damage ex vivo, an effect modulated by oxygen tension, dose, and dose rate

“…All irradiations were performed with a FLASH‐optimized in‐house developed linear accelerator (linac), which has been described in more detail elsewhere, 30 delivering electrons of 6 MeV nominal energy with a circular horizontal beam of 5 cm in diameter, with each of the tubes containing the slides placed in contact with the collimation system so that the blood/agarose gels were centred in and perpendicular to the beam ( Figure 1 ).…”

“…For online verification of the dose delivery, a toroidal beam charge monitor as well as a beam energy monitor was used. 30 The energy monitor was also used to verify that the electron beam energy was consistently 6 MeV. Our overall uncertainty in dosimetry was estimated to be 4%, including a measured output variation of our FLASH and CONV IR deliveries of within 2%.…”

Design and Test of C-band Linac Prototypes for Electron FLASH Radiotherapy

“…However, further research is needed to understand the benefits and limitations of this methodology fully and to determine the best use cases for its application in clinical settings. Currently, only a few dedicated electrons linacs are employed for experimental research [6,7,8,9,10,11,12,13], ongoing studies and technological advancements aim to make these linacs more compact and cost-effective for highenergy electrons. The ultimate goal is to use these linacs to treat deep tumors with FLASH radiotherapy in the future.…”