Author: Javier Chiasson, Dominique Guillet, Arthur Lalonde, Bryan R. Muir, James Renaud, Karim Zerouali 👨🔬
Affiliation: Universite de Montreal, Centre Hospitalier de l'Universite de Montreal, National Research Council 🌍
Purpose: To develop, optimize and validate a novel applicator for ultra-high dose rate (UHDR) electron radiotherapy incorporating an electrostatically shielded beam current transformer (BCT) for real-time dose monitoring.
Methods: Monte Carlo (MC) simulations with EGSnrc user codes BEAMnrc and DOSRZnrc were used to optimize the dimensions of a novel 3D printed applicator for the Mobetron (IntraOp, USA). Applicator wall thickness was optimized to minimize out-of-field dose while its length was adjusted to balance dose-per-pulse and profile flatness across multiple field sizes at 6 MeV and 9 MeV. Additionally, four conductive materials were considered to achieve electrostatic shielding of the BCT while minimizing beam disruption. Once produced, the final applicator with the incorporated shielded BCT was calibrated in terms of absolute dose delivered using alanine dosimeters. The capacity of the final applicator design to accurately report the dose delivered in a preclinical setting was evaluated through an end-to-end test using a representative phantom (Polydimethylsiloxane micro-chip for tumor spheroid culture) with incorporated EBT4 films.
Results: MC simulations showed optimal results for a cylindrical applicator made of polylactic acid (PLA) with 2.5 cm wall thickness to minimize out-of-field dose. A length of 12.5 cm provided the best balance between dose per pulse and profile flatness across field sizes for both 6 MeV and 9 MeV UHDR beams. Ultra-thin conductive Kapton proved most effective for BCT shielding, reducing beam energy fluence by less than 0.15%. End-to-end validation demonstrated good agreement between measured and MC predicted doses in the preclinical phantom, with differences of 0.68% at 6 MeV and 3.62% at 9 MeV.
Conclusion: We successfully developed, optimize, and experimentally validated a novel Mobetron applicator for UHDR mode. The results confirm the applicator's capability to achieve optimal dosimetric properties while enabling reliable, real-time beam monitoring, making it suitable for both pre-clinical and clinical radiotherapy applications.