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SU-E-J-66: Evaluation of Proton Induced X-Ray Fluorescence from Gold Fiducial Markers for In-Vivo Determination of Proton Range and Energy.

PURPOSE
To evaluate a method for in-vivo determination of proton range and post-Bragg peak straggling by detection of proton induced x-ray fluorescence of markers placed at known locations.


METHODS
Therapeutic beams from the UF Proton Therapy Institute were used to excite proton-induced x-ray fluorescence emission (PIXE) from cylindrical pure gold fiducial markers. The markers were embedded in a homogeneous water phantom and PIXE was measured using NaI photodetectors with energy dispersive spectral analysis. The geometry of the phantom and marker placement was chosen to model parallel-opposed beam treatment of prostate cancer by proton therapy. The fluorescence yield from these markers was further modeled using the GEANT4 Monte-Carlo package with low-energy corrections. Gold K and L shell fluorescence yield as determined by the GEANT4 simulations was verified quantitatively by comparison to measured Au yield at energies from 1 MeV to 68 MeV, and to semiempirical model calculations covering the energy range from 1 MeV to 15 MeV.


RESULTS
The Au K-shell fluorescence cross section is significantly smaller thanthat of the L-shell, but the higher yield of the L-shell fluorescence isoffset by the larger absorption as the x-ray exits the phantom. The overallrelative detection efficiency of K and L shell fluorescence depends on thedetails of the shape of the phantom and location of the marker. Acharacteristic shape of fluorescence yield as it depends on proton range isfound, which can be used to extract an in-vivo PDD profile of a spread-outBragg peak (SOBP).


CONCLUSIONS
A combination of a specific protocol for delivering a SOBP, geometriclocation of fiducial markers from CT, and simultaneous detection of protoninduced x-ray fluorescence, can determine the depth range of a primary protonbeam in-vivo. The fluorescence yield as measured at the Proton Therapy Institute is easily distinguished from background radiation.