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Evaluation of Proton Induced X-Ray Fluorescence From Gold Fiducial Markers for In-Vivo Determination of Proton Range and Energy

B Tonner

Brian Tonner1*, Zuofeng Li2, Derek Tishler3, (1) Moffitt Cancer Center, Tampa, FL, (2) University of Florida, Jacksonville, FL, (3) University of Central Florida, Orlando, FL

SU-E-J-66 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

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.

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 semi-empirical model calculations covering the energy range from 1 MeV to 15 MeV.

The Au K-shell fluorescence cross section is significantly smaller than
that of the L-shell, but the higher yield of the L-shell fluorescence is
offset by the larger absorption as the x-ray exits the phantom. The overall
relative detection efficiency of K and L shell fluorescence depends on the
details of the shape of the phantom and location of the marker. A
characteristic shape of fluorescence yield as it depends on proton range is
found, which can be used to extract an in-vivo PDD profile of a spread-out
Bragg peak (SOBP).

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

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