ESTRO 2022 - Abstract Book

S239

Abstract book

ESTRO 2022

the lack of a beam control system able to perform real time monitoring of beam intensity and direction since standard detectors (such as gas-filled ionization chambers) undergo saturation and discharges in FLASH regime. The Flash Detector beam Counter (FlashDC) project aims to develop an innovative and economical detection system capable of overcoming these limitations. Materials and Methods The FlashDC monitor is based on the physical principle of air fluorescence of electrons, never exploited before for this application. The main advantages of using fluorescence with respect to other luminescence-based phenomena are: a) a simple light collection system due to isotropic emission of photons; b) the absence of an energy threshold; c) photon yield per electron almost constant (4-5 fluorescence ph./m) in a wide kinetic energy range (10-1000 MeV), allowing for a linear response as a function of the beam intensity. So far two prototypes have been built and tested with two electron linacs provided by the SIT company (Aprilia, Italy), consisting of two cuboids (with different dimensions optimized for the machine characteristics) in PVC with black walls, filled with air, with light sensors on the opposite edges, assembled and positioned as shown in Fig. 1.

Results The first qualitative results in terms of beam position sensitivity and signal linearity as a function of the linac current are encouraging. The monitor with volume 7x7x90 cm 3 was able to discriminate the in-beam/off-beam configurations, and showed a variation of signal as the beam was shifted along its longitudinal direction, hinting at an intrinsic spatial resolution. The monitor with volume 2x2x60 cm 3 was irradiated with electron FLASH currents in the range 70-130 mA, and kept a remarkable signal linearity as the density of electrons traveling through the active volume increased (Fig. 2).

Conclusion The analysis of preliminary data taken with FLASH electron beams has provided an encouraging indication that the FlashDC monitor can fulfill the expected requirements. With the help of a FLUKA MC simulation the detector design, capable of making a 2D map of the beam intensity, will be finalized. Further measurements will be made to verify the possible installation and performances of SiPM devices in the light collection system.

OC-0280 Identification of dose rate transients at the start of every beam-on period for a gated MR-linac

M.F. Klavsen 1 , K. Boye 2 , R. Hvass Hansen 2 , I. R. Vogelius 2 , C. P. Behrens 3 , C. Ankjærgaard 1 , C. E. Andersen 1

1 Technical University of Denmark, Department of Health Technology, Roskilde, Denmark; 2 Copenhagen University Hospital – Rigshospitalet, Dept. of Oncology, Copenhagen , Denmark; 3 Copenhagen University Hospital – Herlev and Gentofte, Dept. of Oncology, Copenhagen , Denmark Purpose or Objective Some MR-linacs have the ability to perform gated treatments based on continuous tracking on 2D images acquired during dose delivery (ciné). This potentially reduces the treatment margins required for tumor coverage, but at the same time