ESTRO 2023 - Abstract Book

S776

Monday 15 May 2023

ESTRO 2023

Materials and Methods A 1-mm thick transparent scintillator (peak emission wavelength 490nm) sheet in the isocenter plane was imaged using a fast intensified CMOS camera, located 2m from the isocenter at 60° angle from the sheet (Fig. 1B). The sheet was imaged at 1kHz rate with 1 µ s dead time, which enabled imaging of individual spots. A 2x2 cm2 UHDR PBS field with 13 spots was delivered using a 250MeV proton beam and 99nA beam current (320Gy/s at the spot center). The raw images were background subtracted, flat field corrected, spatially filtered with a Gaussian filter and geometrically transformed from the camera view to a beams’ eye view. The profile of each spot was determined by averaging the intensity across all frames in a spot position. The measured total field intensity was found by summing the intensity of all frames. The summed images for both the field and single spots were calibrated with a PPC05 ionization chamber and compared to radiochromic film measurements as well as a commercial scintillation-based beam profiler.

Results The system resolved all individual beam spots temporally (Fig. 2A) with 0.1mm pixel and 0.6mm spatial resolution. The measured signal intensity fluctuated 3% between spots. The Gaussian width of individual spots in the scintillation images was 0.6mm smaller than the width measured with a radiochromic film (Fig. 2B), while it matched better with the reference scintillation-based profile (Fig. 2C), confirming the known film UHDR overresponse. For the scanning field (Fig. 2D), the spot center locations matched the plan within 0.3 and 0.1mm in x and y direction, respectively. The field at 50% isodose showed an average difference of 1.5mm between the scintillator and the film (Fig. 2E).