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Design and Verification of a Heterogeneous Proton Equivalent Thorax Phantom for Use in End-To-End Assessment of Pencil Beam Proton Therapy

J Neihart

J Neihart*, N Sahoo, P Balter, P Summers, M Palmer, M Kerr, D Followill, UT MD Anderson Cancer Center, Houston, TX

TU-A-108-7 Tuesday 8:00AM - 9:55AM Room: 108

Purpose: Design and commission a dynamic proton-equivalent lung phantom for implementation as an end-to-end audit tool for credentialing proton therapy centers participating in proton lung clinical trials.

Methods: Phantom materials were tested and chosen with relative stopping powers (RSP) similar to those found on the Eclipse tissue calibration curve. One material was determined to be in good agreement with the Eclipse HU vs. RSP calibration curve to simulate bone. A phantom was designed with materials that could be imaged, planned, and treated without need for manual adjustment of HU. The lung target was designed to simulate 2cm respiratory motion. The phantom was simulated on a 4DCT and planned on the resulting average CT images. A proton pencil beam plan was generated to cover the ITV and spare other structures, and was verified by 2-D ion chamber array QA measurements. Image-guided spot scanned therapy was delivered to the phantom. The film and TLD within the moving dosimetry insert were read and registered to the planned 3D dose distribution. Using TLD for absolute dose and film for dose distribution, a gamma analysis was performed in the sagittal, coronal, and axial planes.

Results: A bone equivalent material was found with an agreement of 0.9% to Eclipse calculation. For the two initial spot scanned proton beam treatment deliveries, the Gamma index ranged from 86% to 73% at ±5%/5mm about an RPC/Institution ratio of 0.97 for all film planes. The regions of highest failure were found to be in the distal region beyond the target volume. The measured vs. calculated dose ratio ranged from 0.96-1.01 with an average of 0.98.

Conclusion: A proton-equivalent lung phantom, with a dynamic thoracic target, was designed and tested as an audit tool for credentialing proton therapy centers. Spot scanned therapy without motion management was determined inadequate for 2cm motion.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.

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