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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

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.

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.

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.

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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