ESTRO 2021 Abstract Book

S389

ESTRO 2021

high temporal resolution is essential to validate that the treatment is delivered under FLASH conditions. In this project, a novel ultra-fast dosimetry system for pencil beam scanning (PBS) proton FLASH therapy has been developed. The system is used in pre-clinical FLASH mouse studies at our institution. Materials and Methods The dosimetry system uses fiber-coupled scintillating ZnSe:O crystals of sub-millimeter dimensions to sample the dose rate in single points at 50kHz. During PBS delivery (Fig. 1A), the sampled detector signal consists of distinct plateaus that correspond to the individual spots (Fig. 1B). The signal height depends on the instantaneous dose rate at the detector position and the length of each plateau represents the spot duration. The system was first employed in a phantom study to measure the individual spot durations for 250MeV single- layer FLASH fields with four detectors that covered the entire field area (Fig. 1A). Measurements included plans with sequences of 1, 2, 5, 10, 15 and 35 spots delivered sequentially without beam pauses. The measured times were compared with treatment log files. Next, the system was used for in vivo dosimetry in a pre-clinical FLASH mouse study. The dose rate was measured with a single detector placed on the exit side of the target (mouse leg). So far, measurements from 14 mice irradiations have been analyzed. The same field and requested nozzle beam current (215nA) were used in all irradiations. The field mean dose rate was determined as the integrated signal divided by the total irradiation time and compared with the log-file determined values.

Results The phantom time measurements showed that the total duration of an uninterrupted spot sequence was 0.251±0.003ms longer than the logged duration. This discrepancy was mainly due to too short logged duration for the first spot in the sequence. The durations of the subsequent spots were 0.015±0.1ms longer than the logged durations. The measured transition time between spots separated by 5mm was 0.20±0.04ms (horizontal scan) and 0.50±0.04ms (vertical scan). The field mean dose rates in the in vivo experiments varied from 73.1Gy/s to 87.1Gy/s across the mice (Fig 2B). The measured dose rates agreed with log files with an RMS difference of 0.92Gy/s. The difference in instantaneous dose rate between mice irradiations was clearly visible in a direct comparison (Fig. 2A).