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Dosimetric Characteristics of TrueBeam STx System with High Definition Multi-Leaf Collimator: Commissioning of Stereotactic Radiosurgery


Z Chang

Z Chang*, Q Wu, H Yan, Y Yang, W Giles, C Busselberg, R Lipscomb, F Yin, Duke University Medical Center, Durham, NC

SU-E-T-143 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Introduction
Dosimetric characteristics of one TrueBeam STx treatment unit with a 120-leaf high definition multileaf collimator (HD120MLC) are systematically measured for commissioning of stereotactic radiosurgery (SRS). Due to limited data available, it is important to make the dosimetric data available for reference.

Methods
SRS commissioning data were acquired for two photon beam calculation algorithms of the treatment planning system iPlan (BrainLAB iPlan4.5): Clarkson method and Pencil Beam dose calculation (PBC) method. Major beam data required include percent depth dose (PDD), beam profile, and relative scatter factors for various field sizes. Generally, the PBC is used for HD120MLC based treatment planning, while Clarkson method is for cone-based planning.

Results
PDD, beam profile and relative scatter data for the circular cones with diameters of 4, 6.0, 7.5, 10, 12.5, 15 mm were measured for 6X flattening-filter-free (FFF) and 10X FFF. The relative scatter factors range from 1.12 to 1.45 for 6X FFF and 0.81 to 1.23 for 10X FFF. Beam penumbra of 6XFFF and 10XFFF of each the circular cone were less than 2.5 mm for the 20% to 80% criterion. PDD and relative scatter data for HD120MLC were measured for various MLC fields with corresponding jaw settings for various energy modes: 6X, 6X FFF, 10X, 10X FFF and 15X, with an MLC filed down to 5mmx5mm. The transverse and diagonal beam profiles for HD120MLC were measured at different depths to generate source function correction models in iPlan. HD120MLC leakage for open jaw and close jaw are ~1% and 0.0% for all the energy modes. The measured HD120MLC dynamic leaf shifts are 0.15, 0.11, 0.19, 0.18, 0.21 mm for 6X, 6XFFF, 10X, 10XFFF and 15X, less than 1/2 of those of the Novalis Tx system previous published.

Conclusion
The commissioning data provided us valuable insights and might be useful for future reference.


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