X Chen¹*, E Omari², A Tai³, H Jassar⁴, Y Zhang⁵, E Paulson⁶, E Ahunbay⁷, C Sandin⁸, M Kydonieos⁹, X Li¹⁰, (1) Medical College of Wisconsin, Milwaukee, WI, (2) Medical College of Wisconsin, Milwaukee, WI, (3) Medical College of Wisconsin, New Berlin, WI, (4) Medical College of Wisconsin, Milwaukee, WISCONSIN, (5) Medical College of Wisconsin, Milwaukee, WI, (6) Medical College of Wisconsin, Milwaukee, WI, (7) Medical College of Wisconsin, Milwaukee, WI, (8) Elekta Ltd, Crawley, WSX, GB, (9) Elekta Ltd, Crawley, WSX, GB, (10) Medical College of Wisconsin, Milwaukee, WI

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  X Chen

MO-G-BRC-4 (Monday, 7/11/2022) 2:45 PM - 3:45 PM [Eastern Time (GMT-4)]

Ballroom C

Purpose: Respiratory gated delivery with orthogonal real time cine MRI guidance is being developed on a 1.5T MR-Linac. To ensure dosimetry properties of gated beams remain the same as those of non-gated beams, this study reports, for the first time, a comprehensive dosimetry measurement of gated beams on the MR-Linac.

Methods: An investigational gating control software package was developed to interface to the service mode of the MR-Linac to trigger beam on and off. The package allowed to change intervals of beam-on and beam-off times with 0.01s increment. Using a Farmer type ion chamber (IC) and a 2D IC array dosimeter, the common dosimetry properties of the 7 FFF MV photon beams, including output, energy ratio, linearity and dose rate, tissue maximum ratios (TMR), and beam profiles, were measured under a variety of different gating conditions and beam settings, and compared with those measured under corresponding non-gated conditions. Spatial dose distributions for open and IMRT fields under different gated and non-gated conditions were also measured using a 3D diode detector array and IC and analyzed based on gamma criteria.

Results: The measurements of the machine outputs, energy ratio, linearity, and dose-rate, TMR, and profile under combinations of a wide range of beam-on [0.25s-20s] and beam-off times [0.5s-5s] agreed within 0.5% to those measured with the ungated beams. For example, the differences of the dosimetry properties between the commonly used respiratory gated conditions (beam on: 1s; beam off: 3s) and nongated conditions were within 0.4%. For the measurements of the open and IMRT fields with gated vs. nongated, the absolute point doses agreed within 0.7%, and all the 3D dose distributions achieved over 95% passing rates with clinically used gamma analysis criteria.

Conclusion: Respiratory gating does not alter beam dosimetry and can be safely and accurately delivered on the 1.5T MR-Linac.

Funding Support, Disclosures, and Conflict of Interest: Research partially funded by Elekta AB

Keywords

Not Applicable / None Entered.

Taxonomy

TH- External Beam- Photons: Motion management - intrafraction

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