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A Comparison of Small MU Sub-Field Dosimetry for Step-And-Shoot IMRT Fields On Varian IX and Truebeam Machines

J Dolan*, R Ambrose, E Furhang, L Harrison, Continuum Cancer Centers of New York, New York, Ny

SU-E-T-368 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To compare measured and calculated dose distributions for step-and-shoot IMRT fields comprised of sub-fields with small MUs on Varian IX and Truebeam machines.
Methods: The Eclipse treatment planning system (v 8.9.9) was used to create step-and-shoot IMRT fields comprised of a sequence of rectangular sub-fields of decreasing size. MU settings of 1, 2 and 50 MU per sub-field and energies of 15x and 6x were investigated on both an IX (console v7.8.05) and a Truebeam (console v1.5.10.2). Dose rates of 300 MU/min and 600 MU/min were investigated. All fields were delivered with the sub-fields merged into a single field and with them unmerged as separate fields. The delivered dose distributions were measured and compared to the calculated distributions using the Mapcheck QA system(v 4.01.01).
Results: For unmerged fields, the measured and calculated dose distributions showed similar agreement for both machines. For the merged 1 and 2 MU per sub-field fields on IX, a previously described 'overshoot' phenomenon was readily apparent. In this phenomenon, a slight discrepancy in the beam off timing between sub-fields seems to introduce a systematic error in dose delivery. This phenomenon was not evident for the merged 50 MU per sub-field field on the IX or for any merged fields on the Truebeam. The magnitude of the overshoot phenomenon on the IX was found to increase with increasing dose rate.
Conclusions: The Truebeam appears to be able to deliver low MU step-and-shoot IMRT fields more accurately than the IX. On the Truebeam, neither field merging nor dose rate appear to affect the accuracy of step-and-shoot delivery. On the IX, merging fields introduces a small systematic error that may suggest the need for limiting dose rate and minimum sub-field MU in certain applications.

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