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  4. A Novel Multimodal Anthropomorphic Physical Pelvis Phantom for MRI-Based Radiotherapy

A Novel Multimodal Anthropomorphic Physical Pelvis Phantom for MRI-Based Radiotherapy

K Singhrao1*, J Fu1 , S Tenn1 , H Wu2 , P Hu2 , M Cao1 , A Kishan1 , R Chin1 , J Lewis1 , (1) Dept. of Radiation Oncology, University of California, Los Angeles, Los Angeles (2) Dept. of Radiology, University of California, Los Angeles, Los Angeles

Presentations

(Monday, 7/15/2019) 1:45 PM - 3:45 PM

Room: Stars at Night Ballroom 2-3

Purpose: Increasing MRI utilization in radiotherapy has caused a growing need for phantoms providing tissue-like contrast on both CT and MRI, so that MRI-based processes can be compared with CT-based clinical standards. Here, we develop, and demonstrate the clinical utility of a 3D-printed anthropomorphic pelvis phantom containing materials capable of excellent T1, T2 and electron density matching for bone, fat and most soft tissues.

Methods: Male pelvis images were used to design a phantom with defined tissue boundaries using Computer Aided Design (CAD). A slot and adapters were included to allow insertion of various dosimeters. Phantom cavities were filled with carrageenan-based materials and additives designed to match the T1, T2, and electron densities of the corresponding tissues. CT and MRI images were acquired on a Siemens Somatom and 3T Skyra, respectively, and used to compare phantom T1, T2, and electron density values to literature-reported values for human tissue. To demonstrate clinical utility, the phantom was used for end-to-end testing of an MRI-only treatment simulation and planning workflow. Based on T1- and T2- weighted MRI, a bulk density assignment synthetic CT and a VMAT prostate SBRT plan were created. The plan was delivered on a Truebeam STX, with film and 0.3cc ion chamber measurements acquired for verification. Doses calculated on the CT and synthetic CT were compared using typical OAR and target metrics.

Results: T1, T2 and electron density measurements for muscle, prostate and bone agreed within 1σ of literature-reported values. Film analysis resulted in a 99.0% gamma pass rate (2.0%, 2.0mm). The ion chamber-measured absolute dose discrepancy at the isocenter was 2.2%. All observed dose metric variations were under 3%.

Conclusion: The developed phantom provides tissue-like contrast on MRI and CT, and can be used to validate MRI-based processes through comparison with standard CT-based processes.

Funding Support, Disclosures, and Conflict of Interest: Varian Master Research Agreement

Keywords

Phantoms, MRI, Dosimetry

Taxonomy

IM/TH- MRI in Radiation Therapy: Development (new technology and techniques)

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