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Alanine dosimetry relies on the Zeeman effect, which leads to a splitting of the energetic levels when an external magnetic field is applied. Optimization of alanine dosimetry was based on the acquisition of electron paramagnetic resonance (EPR) spectra with a Bruker spectrometer ( 1). For these reasons, we focused on the optimization of alanine dosimetry, which could combine both high accuracy and short reading time. TLDs have an uncertainty level that cannot be further optimized, and film dosimetry typically requires 24 h of stabilization before reading. Therefore, dosimetric preparations with passive dosimeters are necessary just before biological experiments with animals or for patient treatments in the future. Furthermore, the eRT6 linear accelerator was proven to have non-negligible day-to-day variations of the beam output in the FLASH irradiation mode ( 2). However, they require a significant time window between the irradiation and the dosimeter readout. Passive dosimeters such as alanine, thermoluminescent (TLD) and Gafchromic ™ films are suitable, as they are independent of the dose rate ( 8). However, biological pre-clinical irradiations, as well as the transfer to humans ( 7), requires an accurate and repeatable dosimetry. At such high-dose rates, ionization chambers experience saturation effects ( 6). Interest in FLASH-RT is based on the resulting enhanced differential effect between normal tissues and tumors, which ultimately widens the therapeutic index ( 4, 5). This enabled an accurate and fast dose determination for biological preparations as part of FLASH-beam irradiations.įLASH radiation therapy (FLASH-RT), a novel technique incorporating ultra-high dose-rate ( 2), was recently used for the first time in humans ( 3). For doses between 10 Gy and 100 Gy, the optimized acquisition parameters made it possible to keep the average differences between the reference and the measured doses below ☒%, for a reading time of 7.8 min. By increasing the number of measurements to five, the average difference to the reference dose was reduced to less than 5% with a total reading time increased to 13.0 min. For lower doses such as 4.9 Gy, three measurements led to a deviation greater than 5%. The total reading time for the three measurements was 7.8 min (3 × 2.6 min). Three measurements were enough to obtain a maximum dose deviation to the reference of 1.8% for the range of 10–100 Gy. That reduction was not at the cost of the SNR because it was kept comparable to the default parameters. The optimization of the Bruker default parameters made it possible to reduce the reading time for one measurement from 5.6 to 2.6 min. A low-dose alanine pellet of 4.9 Gy was also measured to evaluate the quality of the optimization for doses lower than 10 Gy. After optimizing the parameters, we compared the doses measured with alanine pellets up to 100 Gy with the reference doses, and then determined the number of measurements necessary to get a difference lower than ☒%. Reading parameters such as the conversion time, the number of scans, the time constant, the microwave power and the modulation amplitude of the magnetic field were optimized as a trade-off between the signal-to-noise ratio (SNR) and the reading time of one measurement using the reference 10.1 Gy alanine pellet. Optimization of alanine dosimetry was based on the acquisition of electron paramagnetic resonance (EPR) spectra with a Bruker spectrometer. Alanine dosimetry is accurate however, to be used for FLASH-RT in biological experiments and for clinical transfer to humans, the reading time needs to be reduced, while preserving a maximum deviation to the reference of ☒%. Until now, only passive dosimeters have provided the necessary dosimetric data. FLASH radiation therapy (FLASH-RT) reference dosimetry to obtain traceability, repeatability and stability of irradiations cannot be performed with conventional dosimetric methods, such as monitor chambers or ionization chambers.