ESTRO 2024 - Abstract Book

S3343

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

In vivo dosimetry was performed with an in-house developed detector system consisting of three small scintillating crystals (ZnSe:O, <0.25mm3), each coupled to a silicon photomultiplier (SiPM) through optical fibres. A single scintillating crystal was taped to each of the mouse holders close to the irradiated leg. The signals from the SiPMs were read out with a 50kHz sampling rate, providing measurement of the dose rate for the individual beam spots. The measured dose rate was determined for each spot position as the average SiPM signal during the spot. The signal was transformed into dose rate in Gy/s using an established calibration method 1 . The quenching of the scintillating crystal was determined in a controlled measurement using a water phantom with a motorised stage. The dose received per energy layer and the total dose were then determined by summing the dose rate for each spot multiplied by the spot duration. The position of the detector relative to the spot pattern was determined in two steps. First, the horizontal (Y) and vertical (X) position of the detector in the plane perpendicular to the beam direction was determined for each layer. The measured dose rates as function of position was fitted to a 2D Gaussian as function of the planned spot positions. The top point of the Gaussian function represented the detector position within the spot pattern. Secondly, the position in depth of the detector (Z) was determined using a modified version of a Bragg Peak model derived by Bortfelt 2 . The model describes the dose distribution as function of both energy and depth, fig. 2A. A horizontal profile along the depth-axis gives the normal Bragg peak interpretation in depth space, fig. 2A. In our case, we used vertical profiles along the energy axis to determine the depth position of the detector. This was done by finding the depth, at which the dose distribution as function of beam energy fitted the measured doses per energy layer, fig. 2A.