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Energy Spectra Measurements of Therapeutic Proton Beams Using Proton-Copper Activation Leading to Multiple Positron Emitting Progenies

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P Nebah

P Nebah1*, Y Chen2 , M Prusator3 , J Bae4 , S Ahmad5 , H Jin6 , J Cho7 , (1) Oklahoma state university, Stillwater, Oklahoma,(2) University of Oklahoma Health Science Center, Oklahoma City, OK, (3) University of Oklahoma HSC, Oklahoma City, OK, (4) Oklahoma state university, Stillwater, Oklahoma, (5) Oklahoma Univ. Health Science Ctr., Oklahoma City, OK, (6) University of Oklahoma Health Science Center, Oklahoma City, OK, (7) Oklahoma State University, Stillwater, OK


SU-F-108-5 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 108

Purpose: To develop a novel method to measure the energy spectra of therapeutic proton beams. The developed method could potentially provide an alternative technique to estimate RBE that utilizes measured proton spectra instead of generated Monte Carlo data.

Methods: Activation of certain elements with distinctly different cross-sections were used to estimate the proton energy spectra. Three natural copper foils with the dimensions (50 mm × 50 mm × 0.1 mm) were placed at three different depths in a water equivalent phantom. The setup was irradiated with either a 15-cm range (90% dose) monoenergetic or a spread-out-bragg-peak (SOBP) proton beam. The activated copper foils created progeny radioisotopes, ⁶³Zn, ⁶¹Cu, ⁶²Cu, and ⁶⁴Cu which then decayed through positron emissions. Their time activity curves were recorded using time coincidence detectors and the relative fraction of each radioisotope was calculated using the least-squares method. The relative fraction of each radioisotope is proportional to the convolution of its proton-interaction cross-section and the proton energy spectrum. Our optimization code iteratively solved for the best spectrum shape and its position which resulted in the relative fraction of radioisotopes that matched closest with the actual measured radioisotope fractions.

Results: A quantitative evaluation was performed using the Chi-squared method. There was good agreement between the two spectra (Chi-square alpha of 0.05 (level of significance)).

Conclusion: The proposed method can be used for routine proton spectrum measurements in a clinical setting with either time-coincidence detectors or a PET scanner. Therefore, the Monte Carlo simulated proton spectrum can be validated with the measured proton spectrum. This has a potential to significantly improve the calculation of RBE.

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