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Experimental Characterization of Two-Dimensional Pencil Beam Scanning Proton Spot Profiles

L Lin

L Lin*, C Ainsley, J McDonough, University of Pennsylvania, Philadelphia, PA

SU-E-CAMPUS-T-5 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Dose calculation of pencil-beam scanning treatment plans relies on the accuracy of proton spot profiles; not only the primary component but also the broad tail components. A pair/magnification method is applied to EBT3 films to generate primary and tail components of two-dimensional spot profiles and dose kernels.

Methods: Four films are taken at multiple locations in air and multiple depths in Solidwater for six selected energies from the IBA universal nozzle. The films used for the primary component are exposed to 50-200 MU to avoid saturation. The films used for tail components are exposed to 800, 8000 and 80000 MU. By merging these data, dose kernels down to 10-4 of central spot dose can be generated. In-house programs are developed to calculate the total dose from multiple spots in each treatment field at all in-air locations and phantom depths based on the constructed dose kernels. Integration of these kernels from zero to 40 mm radius is used to evaluate the charge missed by the PTW Bragg peak chamber during integral depth-dose measurements.

Results: Measurement agrees within 1% of calculation for output versus field size using dose kernels. The asymmetric comet-shaped profile tails have a bigger impact at superficial depths and low energies. This output difference between two orientations at the surface of a rectangular field of 40 mm by 200 mm, a typical cranialspinal field, is about 2%, which persists until a depth of 50 mm and drops to 1% at the end of range. The charge deficit in the Bragg peak chamber varies < 2% from entrance to the end of Bragg peak for energies < 180 MeV, but exceeds 5% at 225 MeV.

Conclusion: Application of the pair/magnification method to EBT3 films shows the evolution of two-dimensional composite spot profiles in air and dose kernels in phantom.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-04-2-0022. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.

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