Title: Temporal and dosimetric beam monitoring of individual pulses in FLASH Radiotherapy using Timepix3 pixelated detector placed out-of-field

Abstract: FLASH radiotherapy necessitates the development of advanced Quality Assurance methods and detectors for accurate and online monitoring of the radiation field. This study introduces enhanced time-resolution detection systems and methods tailored for single-pulse detection. The goal of this work was to measure the delivered number of pulses, investigate temporal structure of individual pulses, and to develop a method for dose-per-pulse (DPP) monitoring based on secondary radiation particles produced in the experimental room.
A 20 MeV electron beam generated from a linear accelerator (LINAC) was delivered to a water phantom. Ultra-high dose-per-pulse (UHDPP) electron beams were used with a dose per pulse ranging from 1 Gy to over 7 Gy. The pulse lengths ranged from 1.18 us to 2.88 us at a pulse rate frequency of 5 Hz. A semiconductor pixel detector Timepix3 (TPX3) was used to track direct interactions in the Silicon sensor created by single secondary particles. Measurements were performed in the air, while the detector was positioned out-of-field at a lateral distance of 200 cm parallel with the LINAC exit window. The dose deposited in the silicon was measured along with the pulse length and the nanostructure of the pulse.
Simultaneously deposited energy and time of arrival of single particles were measured with a precision of 1.56 ns. The measured pulse count agreed with the delivered values. A linear response (R^2 = 0.999) was established between the delivered beam current and the measured dose at the detector position (orders of nGy). The difference between the average measured and average delivered pulse length was 0.003(30) us.
This simple non-invasive method, exhibits no limitations on the delivered DPP within the range used during this investigation. It enhances the precision and real-time monitoring of FLASH treatment plans with nanosecond precision.