SU-E-T-324: Dosimetric Evaluation of Soft Docking Sensitivity On An INTRAOP Mobetron IORT Device

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Abstract

Purpose:

To assess Mobetron soft docking sensitivity thresholds by measuring dosimetric changes resulting from incremental displacements of isolated docking parameters.

Methods:

The Mobetron device uses a sensitive optical system to establish alignment of the collimator with a treatment cone. To assess the limits of this sensitivity, we isolated each docking parameter (transverse, tilt, and vertical) and varied each incrementally within the docking display range. Transverse displacements ranged in 1mm increments from 0.0cm to 0.4cm. Profiles were taken at dmax for each energy (6MeV, 9MeV and 12MeV) along the direction of displacement. Angular displacements ranged in 0.1° increments from 0.0° to 0.4°. Baseline corrected profiles were assessed with a point-by-point symmetry analysis and a max-min flatness metric (90%-1cm). Vertical displacements ranged from 0.0cm to 0.4cm in 1mm increments. Output factors were compared by % difference from baseline and PDDs were measured at each vertical displacement to assess changes in dmax and R50 values.

Results:

For 1mm lateral displacements, symmetry/flatness deviations for the 10cm cone were +0.48%/+0.38% at 6MeV, +0.95%/+1.04% at 9MeV, and +1.77%/+1.96% at 12MeV. Profiles in the direction orthogonal to gantry motion measured +0.99%/+1.29% symmetry/flatness changes at 4mm displacement. For 0.10 angular displacements, symmetry/flatness deviations for the 10cm cone were +0.07%/+0.05% at 6MeV +0.23%/+0.33% at 9MeV, and +0.34%/+0.59% at 12MeV. The 3cm and 6cm cones were less sensitive to transverse and angular displacements. At 4mm vertical displacement, the 3cm cone differed from baseline by 3.02%, 2.30%, and 2.42% (6MeV, 9MeV, and 12MeV respectively). Maximum combined deviation from baseline in quadrature for one extra docking light (half sensitivity) in each displacement vector was >2%.

Conclusion:

For transverse and vertical displacements, the docking sensitivity range is adequate to ensure flatness and symmetry differences <1.0%. However, for angular displacements, all energies with the 10cm cone deviated at or beyond this threshold.

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