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Dose Rate Constancy, MU Linearity and Profile Stability of the TrueBeam Radiotherapy System

D Jacqmin

D Jacqmin*, Z Labby , A Besemer , K Schaefer , University of Wisconsin, Madison, WI


SU-I-GPD-T-455 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To quantify the dose rate constancy, MU linearity and profile stability of the TrueBeam radiotherapy delivery system over its full range of available dose rates.

Methods: Dose rate constancy, MU linearity and profile stability were assessed for a TrueBeam with 6x, 10x, 15x, 6xFFF and 10xFFF photon beams. The dose rate constancy and MU linearity were determined using ion chamber measurements. A Farmer-type ionization chamber with a build-up cap was positioned in air at isocenter. Different combinations of MU and dose rate were delivered for each energy over a large range of MU (2-600 MU) and over all available dose rates. These measurements were compared to the 300 MU measurements at the maximum dose rate to determine dose rate constancy and MU linearity. The profile stability was measured using the IC Profiler ion chamber array. Profiles were measured at the minimum and maximum dose rate available for each energy using 1 MU and 100 MU. The IC Profiler software was used to compare flatness and symmetry for each set of four measurements.

Results: The dose rate constancy of the TrueBeam was better than 0.5% for all beam energies over the full range of dose rates. The MU linearity of the TrueBeam meets the tolerances in TG-142. The largest deviations were 1.0%, 0.8%, 2.3%, 3.5% and 3.1% for 6x, 10x, 15x, 6xFFF and 10xFFF, respectively, for a delivery of 2 MU. The IC Profiler measurements showed that the beam flatness and symmetry were similar for all four combinations of dose rate and MU for all energies.

Conclusion: The dose rate constancy, MU linearity and profile stability of the TrueBeam system meet accepted tolerances, allowing the TrueBeam to deliver very low dose rate treatments and fields with small MU-per-segment.

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