Author: Petr Bruza, David J. Gladstone, Lesley A Jarvis, Austin Sloop, Kevin J. Willy, Rongxiao Zhang 👨🔬
Affiliation: Thayer School of Engineering, Dartmouth College, Dartmouth Cancer Center, Dartmouth Health 🌍
Purpose:
Active beam monitoring and output-gated dosimetry are essential systems for safe and accurate radiation delivery. UHDR irradiators such as the Mobetron rely on pre-programmed regimes and the assumption that radiation will be delivered as planned, leaving only recorded data and retrospective analysis as the means of determining if a delivery was successful. Many monitoring systems are bypassed for UHDR operation, widely considered unacceptable for trials involving humans. Real-time control and adjustment of UHDR deliveries is essential to prevent errors when per-pulse output differs from what was calculated and intended.
Methods:
A system developed around a National Instruments FPGA platform was interfaced with an IntraOp Mobetron to regain the functionality of output control and beam-monitoring systems during UHDR operation. The per-pulse signal from Beam Current Transformers (BCT) are collected on a gated integrator and determine if a delivery should continue or be halted. Control of the HV electron gun system prevents additional output, and conjunctive manipulation of the door interlock allows for rapid beam interruption if any issue with the delivery is detected.
Results:
The dosimetric accuracy of the BCT system was validated with the standard deviation for per-pulse residuals under 1%. Measurement on the integrator was linear with R2 > 0.99, with an offset that is corrected in software. The system can prevent additional output during the inter-pulse period. Auxiliary monitoring systems can also trigger an interlock that halts additional pulses. Deliveries can be prescribed to a unit of fluence or dose with known output factors.
Conclusion:
This system provides essential monitoring and control for UHDR deliveries that current platforms lack. Additional monitoring of beam shape via scintillating fibers and patient alignment via optical means is under additional development and will provide an additional level of confidence in the safety and efficacy of UHDR deliveries.