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Session: Clinical Dosimetry, Calibration, Shielding [Return to Session]

Temporally Stable UHDR LINAC Is Favorable for Clinical Translation of FLASH RT

M Rahman1, A Sloop2, J Sunnerberg3, D Gladstone4, C Dexter5, L Thompson6, P Bruza7, B Pogue8, R Zhang9*, (1) Dartmouth College, Hanover, NH, (2) Oregon Health and Science University, Lebanon, NH, (3) Dartmouth College, Lebanon, NH, (4) Dartmouth College, Hanover, New Hampshire, (5) Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire, (6) Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire, (7) Dartmouth College, Hanover, NH, (8) University of Wisconsin-Madison, Madison, WI, (9) Dartmouth-Hitchcock Med. Ctr., Lebanon, NH


TH-B-BRA-6 (Thursday, 7/14/2022) 8:30 AM - 9:30 AM [Eastern Time (GMT-4)]

Ballroom A

Purpose: This study is the first direct comparison of two converted clinical LINACs producing ultra-high dose rate (UHDR) electron beams with the same conversion methods. It sought to quantify the differences in the spatial and temporal properties of the beams to assess potential clinical impacts for FLASH radiotherapy.

Methods: UHDR electron beams were produced on Varian Trilogy and Varian 2100C/D linear accelerators by retracting the target, setting the carousel in an empty port and 10MV photon mode. Lateral profiles were measured by radiochromic film in solid water phantom at 2 cm depth with an open 40x40 cm^2 field. Percent-depth-dose (PDD) were measured along the central axis with film placed at varying depths. The temporal pulse structure and per-pulse output was recorded with a photomultiplier tube that amplified the Cherenkov emission signal from an optical fiber placed outside the beam, with film providing cumulative absolute dosimetry for the delivery.

Results: Spatial beam characteristics were similar between the two machines, with the Trilogy showing slightly wider profiles along inline and crossline directions. The mean dose-per-pulse on the Trilogy was 29% higher than the 2100C/D at 0.96 Gy/pulse (mean dose rate of 347 Gy/s vs. 268 Gy/s). Temporally, the Trilogy exhibited greater stability of the output (interquartile range of 0.02 vs. 0.25 Gy/pulse). The 2100C/D showed low-dose outliers in output, and variability during the initial 20 pulses, while the Trilogy output was nearly stable from the first pulse.

Conclusion: While both LINAC’s achieve UHDR in a large field, stability across initial pulses with high dose rates from the start are key to ensure the quality, repeatability, and safety of UHDR deliveries. The Trilogy demonstrated superiority in stability throughout the delivery, which is particularly favorable for the clinical translation of FLASH-RT as the entire treatment may consist of only a few pulses.


Electron Therapy, Radiation Dosimetry, Commissioning


TH- External Beam- Electrons: General (most aspects)

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