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Dosimetry and Implementation of High-Dose Rate MLC-Based GRID Therapy Using a 6MV Flattened Filter Free (FFF) Photon Beam


K Dou

K Dou1, 2*, F Lerma1, 2, M Jacobs2, B Li1, 2, (1) RadAmerica, LLC--MedStar Health, Baltimore, MD, (2) Mercy Medical Center, Baltimore, MD

SU-E-T-327 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To dosimetrically characterize high-dose rate multileaf collimator (MLC)-based, spatially-fractionated GRID therapy (MLC GRID) using a 6MV flattened filter free (FFF) photon beam delivered from a Varian TrueBeam linear accelerator.

Methods:
Planning for MLC GRID therapy was done by a Varian Eclipse treatment planning system. The dose plane at depth of 5cm was exported for a dose comparison between the calculated and measured data. MLC GRID with the projected square openings of 10 x 10 mm at the isocenter was studied using 6MV FFF photons at a dose rate of 1400MU/min. Four separate MLC-shaped beams of each with two columns of GRID openings were automated for delivery by a Varian TrueBeam linear accelerator. MLC GRID was characterized with depth dose, profile and output using an IBA scanning system.

Results:
6MV FFF GRID patterns recorded using films and EPID were found to be consistent in both aperture sizes and alignment. A MapCheck was used to validate the 16 - 20 cm field of a 6MV FFF GRID with a pass rate of 97% at 3%/3mm criteria. The ratio of the openings to the blocked area was quantified to be 25% with a valley-to-peak ratio of about 16.5% from the beam profiles in the central aperture. GRID therapy is a technique to treat advanced bulky tumor in a single or a few fractions by delivering a dose of 12 to 20Gy to tumors that do not respond to conventional fractionation schemes. Four abutting GRID fields with 3000 MU to each field took only about 9 minutes using a 6MV FFF beam, but took more than 30 minutes for delivery at 400 MU/min.

Conclusion:
6MV FFF MLC-based GRID therapy aligned accurately with IGRT showed advantages in the higher dose delivery by more than 2 times reducing treatment time over a conventional delivery.

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