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Implementation of High-Dose-Rate Grid Therapy Using a 6 MV Flattening-Filter-Free Photon Beam


Fritz Lerma

F Lerma*, K Dou, M Jacobs, B Li, B Lee, G Casados, S Arthur, B Lanahan, Mercy Medical Center and RadAmerica LLC--MedStar Health, BALTIMORE, MD

PO-BPC-Exhibit Hall-3 Saturday  Room: Exhibit Hall

Purpose: Implement High-Dose Rate Grid Therapy using a Flattening-Filter-Free 6 MV Beam to deliver single-fraction 12 - 20 Gy treatments to debulk tumors that do not respond to conventional fractionation schemes.

Methods: Grid therapy involving a 6 MV Flattening-Filter-Free (FFF) Photon Beam, shaped with 5 mm leaves of a millenium 120 multi-leaf collimator (MLC) into 1 cm x 1 cm beamlets spaced 1 cm apart, giving an effective blocking of 75% is planned on a Varian Medical Systems' Eclipse RTPS V10.0. The resulting plans are delivered on a TrueBeam linear accelerator at 1400 MU per minute. Quality Assurance is conducted using a comparison of computed dose planes in a flat, virtual water phantom against diode array measurements at 5 cm depth, 100 SAD in a MapCheck 2 detector, by Sun Nuclear Corporation. Patient treatments are comprised of one or more beams, 5 to 6 cm wide, and 10 to 16 cm long, where abutting fields are achieved by shifting the MLC pattern and the x-jaws on the linear accelerator. Image guidanced (IGRT) maximizes treatment setup accuracy and speed. Total treatment time, beam on time, and dose rate are minimized by the combination of image guidance, FFF beam operation and field abutment using jaw and MLC movements.

Results: 1500 to 4000 MU deliver 12 to 20 Gy using 1 - 3 abutting Grid therapy beams, in 5 to 12 minutes in contrast to extended treatment times of more than 30 minutes at 600 MU / min without IGRT or abutting field planning. Gamma analyses show 98 % pass rate at 2.2 mm and 2.2 % thresholds.

Conclusion: MLC-based, high-dose-rate FFF 6 MV beam Grid therapy under image guidance is planned, verified and delivered, thus demonstrating a sharp reduction in treatment times, and targeting accuracy in MLC-based Grid therapy methods.

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