ESTRO 2024 - Abstract Book

S3240

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

The pO 2 in the sealed container was measured throughout the proton PBS delivery using an Oxyled phosphorometer (Oxygen Enterprises, Ltd.). The phosphorometer excited the PtG4 probe using 5µs pulses from a red LED with 250µs repetition rate and measured the phosphorescent light emitted from the probe after each excitation pulse. The LED light was directed to the target using an optical fiber (Figure 1, left). The phosphorescence light from the deexcitation of the PtG4 probe was directed through a second optical fiber to a photodetector (avalanche photodiode). For each sampling, the phosphorescence decays induced by 500 LED pulses were used to determine the phosphorescence decay time (Figure 1, right). Using the dependency of the decay time on the pO 2 level resulted in dynamic pO 2 measurements as a function of time with a sampling rate of 4.5Hz. After the experiments, the time-resolved cumulative dose in the sealed container was estimated from the point dose measurements by the scintillator detector to allow comparison with the pO 2 changes measured during the irradiations. First, the detector position within the spot pattern was determined by triangulation using the measured instantaneous dose rate of all spots and assuming a Gaussian spot dose profile. This also resulted in the Gaussian dose profile of the pencil beam, which allowed calculation of the mean dose to the sealed container from each spot by integrating the Gaussian dose of each spot over the container volume. Next, the cumulative dose curve was synchronized with the measured pO 2 curve, which yielded the start of irradiation in the pO 2 measurement. Furthermore, the logged spot durations were used to mark the start and end of each spot in the pO 2 signal. The rate of pO 2 change was found for each spot as the change in pO 2 level from the start to the end of each spot divided by the spot duration. The dose rate per spot was similarly found from the cumulative dose curve.