Author: Javier Chiasson, Dominique Guillet, Arthur Lalonde, Karim Zerouali 👨🔬
Affiliation: Universite de Montreal, Centre Hospitalier de l'Universite de Montreal 🌍
Purpose:
To validate a Monte Carlo (MC) model of a Ultra-High dose rate (UHDR) electron beam using a prototype flashdiamond (fD) detector coupled to an electrostatically shielded beam current transformer (BCT).
Methods:
Relative dosimetry measurements (percent depth dose (PDD), profiles and output factors (OF)) were taken using a prototype flashDiamond detector (PTW, Freiburg) in a water phantom for two UHDR energies (6 and 9 MeV) and three field sizes (FS) of 2.5, 4 and 6 cm, the latter being considered the reference field size. Electron fluence was monitored through an electrostatically shielded BCT to correct for output variation between each data point. The results were compared to the previously developed Monte Carlo model of a UHDR-capable intra-operative linear accelerator, the Mobetron (IntraOp, CA). The MC model geometry was built using data provided by the manufacturer and an in-house secondary collimation, while source parameters were optimized based on gafchromic film measurements. Surface and out-of-field fD dose measurements were validated using independent gafchromic film measurements, resulting to our knowledge in the first reported fD data in these two measurement conditions.
Results:
Measured R50 for all FS and energy show good agreement to the MC predicted PDD, within ± 0.5 mm. Surface dose measured with the fD corresponds to film measurements, within uncertainty, but showed a small discrepancy with the MC prediction. Differences in output factors did not exceed 1.6 % for 6 MeV and 4.2% for 9 MeV.
Conclusion:
fD is a versatile tool for relative dosimetry measurements if electron fluence is correctly monitored to account for variations. Also, considering that surface dose is a critical parameter for superficial Flash clinical applications, the MC model must be adjusted to reflect these specific measurements