ESTRO 2024 - Abstract Book

S3336

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

This contribution describes our first measurements with plastic scintillator sheets irradiated with low-energy electron FLASH beams and tested for use in quality assurance.

Material/Methods:

Two EJ212 (Eljen Technology) plastic scintillator sheets were used, with an area of 200 mm x 200 mm, and a thickness of 3 mm and 0.5 mm, respectively. These two values were tested to evaluate the scintillator performance as a compromise between spatial resolution and amount of scintillation light produced. The sheets were irradiated with 9-MeV electron Flash beams at the Pisa Center for Flash Radiotherapy (CPFR) and imaged with a scientific CCD camera coupled to an objective, placed orthogonal to the beam axis. Depending on the goal of the measurement, two configurations were used (Fig. 1). The setup on the left was used to: 1) verify the linearity of the scintillator response with increasing dose per pulse, 2) measure the spatial resolution of the two scintillators by means of a slanted edge measurement, 3) assess the capability to detect beam patterns (e.g. rows and point grids). The setup on the right served for a proof-of-concept measurement of a percentage depth dose curve. Although the CCD camera was shielded to suppress scattered electrons and Bremsstrahlung radiation, an algorithm to remove image noise had to be developed, based on a top hat filter.

Results:

Preliminary results are summarized in Fig. 2: a) the EJ212 response appears linear with the dose per pulse up to 5 Gy/p (intra-pulse dose rate of >10 6 Gy/s); b) the top-hat filter algorithm is efficient in removing both electron and X ray noise, as can be seen by comparing the unfiltered image and the one where the 3x3 filter kernel was applied; c) first qualitative images of the beam transversal profile were obtained, as in the shown example, where a template placed at the accelerator exit created spatial fractionation of the beam; d) the plastic scintillator sheets allow to obtain in a single acquisition a 2-D map of the dose distribution, as in the reported example of a PDD image; e) profiles at a given height can be extracted from the image and analyzed. A discrepancy between the measured PDD image and the reference one, obtained with a gafchromic film, was observed, and it is still under investigation, also with the aid of Geant4 Monte Carlo simulations. Moreover, the measured spatial resolutions were of the order of 1 mm and 2