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Investigations of Fidelity of Dose Delivery at High Dose Rate of Flattening Filter Free (FFF) Beams in VMAT On TrueBeam™

G Kalantzis

G Kalantzis*, J Qian, B Han, G Luxton, Stanford Univ School of Medicine, Stanford, CA

SU-E-T-338 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To assess dosimetric fidelity of Varian TrueBeam™ in flattening filter-free (FFF) modes by dose measurements comparing high versus low dose rate irradiation, particularly for volumetric-modulated arc therapy (VMAT) plans.

Methods: Measurements with a 22cm-diameter cylindrical PMMA phantom ScandiDos Delta⁴ multi-diode detector system and Farmer-type(0.6 cm³) and PinPoint™(0.015 cm³) cylindrical ionization chambers in rectangular Plastic Water™ phantoms were used to compare photon beam irradiation from 10MV(10XFFF) at 400 and 2400 MU/min and 6MV(6XFFF) at 400 and 1400 MU/min dose rates, respectively, on a Varian TrueBeam™. Two setups were studied: square fields and VMAT plans optimized for stereotactic body radiotherapy treatment of lung tumors. VMAT plans were tested both with and without periodic gating from the Varian RPM™ position-management gating system. Quantitative comparisons were based on mean and maximum relative dosimetric error (RDE) measured between the two dose rates.

Results: Reproducibility was very good for all measurements. For square fields, RDEs of 0.3-0.4% were found with ionization chamber measurements, with negligible contribution from recombination in the detectors. The Delta4 showed consistently higher mean RDE of ~0.8% for 10XFFF square fields. For non-gated VMAT plans, mean RDE was ~1.1% for 10XFFF. Error was independent of delivered dose for square fields. For VMAT plans, RDE increased as dose fraction size decreased. Finally, comparison between gated VMAT plans at high and low dose rates demonstrated a maximum error in low-dose regions, of 1.5% and 2.3% for 10XFFF and 6XFFF respectively.

Conclusions: There is a dosimetric error of the order of ~0.4 % for 10XFFF for TrueBeam at its highest dose rates. From measurements using the Delta⁴, VMAT treatment plan deliveries were found to be independent of dose rate to within ~0.5%. Overall, very good agreement was found between irradiation at high and low dose rates across the spectrum of our measurements.

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