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Relative Proton Stopping Power Ratio Database for Common Dosimetry Phantom Materials

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M Kerr

M Kerr*, S Dhanesar, F Guan, M Taylor, X Zhu, M Gillin, R Amos, N Sahoo, MD Anderson Cancer Ctr., Houston, TX

SU-E-T-48 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To create a database of relative proton stopping power ratios (RPSPRs)of various phantom materials, especially those that simulate different kinds of human tissue. More importantly, we intend to use these stopping powers to gain a better understanding of and more accurately estimate range uncertainties in heterogeneous phantoms and, ultimately, in patient treatment plans.

Methods: RPSPRs were determined using two independent techniques to measure the range shift produced by the various materials. The first technique used a PTW water tank scanning system with a PTW TN23343 Markus chamber to measure and compare the percent depth dose profiles of uniform proton beams with and without the various materials of interest blocking the beam. The second technique was similar to the first, except that an IBA Zebra, a water equivalent multi-layer ionization chamber system, was used to measure the percent depth dose profiles. Each material was evaluated using unmodulated proton beams of two different energies to examine possible stopping power energy dependencies.

Results: Currently, we have obtained RPSPR data for three materials that compose the CIRS Model 002LFC IMRT Thorax Phantom. We have also measured range shifts for Lucite and homogeneous rods from a Gammex RMI 467 Electron Density CT Phantom. The range shifts, based on the distal 90% ranges, measured with the water tank and Zebra agree within 0.45 mm, with a mean difference of 0.2 mm, and with no dependence on the energy of the incident proton beam. The difference in RPSPRs between the two measurement techniques was no more than 0.045, with a mean difference of 0.011.

Conclusion: The PTW water tank scans and Zebra measurements have allowed us to accurately determine the RPSPRs of different materials used to simulate human tissue. For the materials that we measured, the stopping powers had no apparent energy dependence.

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