Rapid Switching of a C-Series Linear Accelerator Between Conventional and Ultrahigh-Dose-Rate Research Mode With Beamline Modifications and Output Stabilization

Austin Sloop; M Ramish Ashraf; Mahbubur Rahman; Jacob Sunnerberg; Chad A Dexter; Lawrence Thompson; David J Gladstone; Brian W Pogue; Petr Bruza; Rongxiao Zhang

doi:10.1016/j.ijrobp.2024.01.215

Rapid Switching of a C-Series Linear Accelerator Between Conventional and Ultrahigh-Dose-Rate Research Mode With Beamline Modifications and Output Stabilization

Int J Radiat Oncol Biol Phys. 2024 Mar 28:S0360-3016(24)00299-2. doi: 10.1016/j.ijrobp.2024.01.215. Online ahead of print.

Authors

Affiliations

¹ Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire.
² Dartmouth Health, New Hampshire, Lebanon.
³ Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire; Dartmouth Health, New Hampshire, Lebanon; Geisel School of Medicine, Dartmouth College, Hanover, New Hampshire. Electronic address: David.J.Gladstone@dartmouth.edu.
⁴ Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire; Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin.
⁵ Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire; Geisel School of Medicine, Dartmouth College, Hanover, New Hampshire; Department of Radiation Medicine, New York Medical College, Valhalla, New York.

PMID: 38552990
DOI: 10.1016/j.ijrobp.2024.01.215

Abstract

Purpose: In this study, a C-series linear accelerator was configured to enable rapid and reliable conversion between the production of conventional electron beams and an ultrahigh-dose-rate (UHDR) electron beamline to the treatment room isocenter for FLASH radiation therapy. Efforts to tune the beam resulted in a consistent, stable UHDR beamline.

Methods and materials: The linear accelerator was configured to allow for efficient switching between conventional and modified electron output modes within 2 minutes. Additions to the air system allow for retraction of the x-ray target from the beamline when the 10 MV photon mode is selected. With the carousel set to an empty port, this grants access to the higher current pristine electron beam normally used to produce clinical photon fields. Monitoring signals related to the automatic frequency control system allows for tuning of the waveguide while the machine is in a hold state so a stable beam is produced from the initial pulse. A pulse counting system implemented on an field-programmable gate array-based controller platform controls the delivery to a desired number of pulses. Beam profiles were measured with Gafchromic film. Pulse-by-pulse dosimetry was measured using a custom electrometer designed around the EDGE diode.

Results: This method reliably produces a stable UHDR electron beam. Open-field measurements of the 16-cm full-width, half-maximum gaussian beam saw average dose rates of 432 Gy/s at treatment isocenter. Pulse overshoots were limited and ramp up was eliminated. Over the last year, there have been no recorded incidents that resulted in machine downtime due to the UHDR conversions.

Conclusions: Stable 10 MeV UHDR beams were generated to produce an average dose rate of 432 Gy/s at the treatment room isocenter. With a reliable pulse-counting beam control system, consistent doses can be delivered for FLASH experiments with the ability to accommodate a wide range of field sizes, source-to-surface distances, and other experimental apparatus that may be relevant for future clinical translation.