ESTRO 2024 - Abstract Book

S3257

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Material/Methods:

The setup [Fig 1] allowed for the right hind leg of one mouse to be irradiated. A mouse holder was placed with brass shielding protecting the mouse body from radiation while the mouse leg to be irradiated was submerged in a water bath. To generate a 5cm SOBP from a monoenergetic 250 MeV proton beam, a 3D printed static range modulator was placed in front of the water bath and solid water was added to align the centre of the SOBP with the mouse leg.

Irradiations of 28 mice were analysed after FLASH treatments with doses of 30 – 65 Gy. The mouse legs were irradiated with a 5-by-7 spot pattern with 6mm spacing delivered with a field dose rate of 60 Gy/s.

For time-resolved in vivo dosimetry, three detector probes were placed at well-defined positions relative to the mouse leg targets: Probes 1 and 2 were positioned in guide tubes on the mouse holder next to the joint and foot, respectively. In contrast, Probe 3 was placed directly on the foot. The probes consisted of inorganic scintillating crystals (ZnSe:O) coupled to optic fibres. Silicon photomultipliers measured their light signal at a sampling rate of 50kHz, which provided the dose rate at the probe positions in sub-spot time resolution. As the first step in the data analysis, the mean signal for each delivered spot was determined from the detector signal [V]. The individual probes were calibrated for use in the SOBP by conducting reference measurements against an Advanced Markus ionisation chamber in a water bath. Therefore, the signal could be translated from V to Gy/s to yield the instantaneous dose rate at the probe position. For each mouse irradiation, the instantaneous dose rates measured from each spot were fitted to a 2D Gaussian distribution as function of the planned spot positions. This allowed for the determination of the probe position within the spot pattern as the maximum of the 2D Gaussian. The total time-integrated dose was determined for each probe and compared with the dose expected at its position based on the applied spot pattern and spot dose profiles.

Results:

During the mouse experiments, instantaneous dose rates of up to 300 Gy/s were recorded.