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Background: Ultra‐high dose rate radiation (UHDR) is being explored by researchers in promise of advancing radiation therapy treatments. Purpose: This work presents the commissioning of Varian's Flash Extension for research (FLEX) conversion of a Clinac to deliver UHDR electrons. Methods: A Varian Clinac iX with the FLEX conversion was commissioned for non‐clinical research use with 16 MeV UHDR (16H) energy. This involved addition of new hardware, optimizing the electron gun voltages, radiofrequency (RF) power, and steering coils in order to maximize the accelerated electron beam current, sending the beam through custom scattering foils to produce the UHDR with 16H beam. Profiles and percent depth dose (PDD) measurements for 16H were obtained using radiochromic film in a custom vertical film holder and were compared to 16 MeV conventional electrons (16C). Dose rate and dose per pulse (DPP) were calculated from measured dose in film. Linearity and stability were assessed using an Advanced Markus ionization chamber. Results: Energies for 16H and 16C had similar beam quality based on PDD measurements. Measurements at the head of the machine (61.3 cm SSD) with jaws set to 10×10 cm2 showed the FWHM of the profile as 7.2 cm, with 3.4 Gy as the maximum DPP and instantaneous dose rate of 8.1E5 Gy/s. Measurements at 100 cm SSD with 10 cm standard cone showed the full width at half max (FWHM) of the profile as 10.5 cm, 1.08 Gy as the maximum DPP and instantaneous dose rate of 2.E5 Gy/s. Machine output with number of pulses was linear (R = 1) from 1 to 99 delivered pulses. Output stability was measured within ±1% within the same session and within ±2% for daily variations. Conclusions: The FLEX conversion of the Clinac is able to generate UHDR electron beams which are reproducible with beam properties similar to clinically used electrons at 16 MeV. Having a platform which can quickly transition between UHDR and conventional modes (<1 min) can be advantageous for future research applications. [ABSTRACT FROM AUTHOR]

Purpose: Patient motion during radiation therapy treatment is a concern, especially for spine stereotactic body radiation therapy cases where the sharper dose gradient presents a toxicity threat to the spinal cord. Intrafraction motion review (IMR) is an application used to monitor patient position during treatment. The presence of spinal fixation hardware presents an opportunity for motion tracking to manually pause the beam., Methods and Materials: A cohort of 17 clinicians were shown a video of the imaging console during a simulated treatment. Participants decided after each triggered image if they would pause the treatment beam, indicating that they believed the phantom to have moved outside of clinical tolerance. A spine phantom with hardware intact was positioned on a motion platform, which was programmed to make shifts ranging in size from 0.5 to 1.5 mm. A 1-mm isotropic expansion contour from the hardware was overlayed on the triggered planar x-ray images using the IMR application., Results: User perception sensitivity did not exceed 0.5 until there was a physical shift of 1.4 mm, indicating that most users will not be able to reliably discriminate submillimeter shifts using contour-based shift identification., Conclusions: If adaptations to standard of care are implemented clinically, the proposed method should be evaluated and the role of training and education should be examined before implementation. However, contour-based IMR could still provide beneficial information for larger intrafraction motion during treatment and could be valuable for identifying gross anatomic motion during treatment., Competing Interests: Ashley Cetnar reports financial support by The Ohio State University, a relationship with the American Association of Physicists in Medicine that includes a board membership, and roles as associate senior editor of Advances in Radiation Oncology and an associate editor of the Journal of Applied Clinical Medical Physics. Erika A. Jank reports a relationship with the American Association of Physicists in Medicine that includes stipend funding., (© 2023 The Author(s).)