LAUSR

PURPOSE Increased utilization of MRI in radiotherapy has caused a growing need for phantoms that provide tissue-like contrast in both CT and MRI images. Such phantoms can be used to compare MRI-based processes with CT-based clinical standards. Here, we develop and demonstrate the clinical utility of a 3D-printed anthropomorphic pelvis phantom containing materials capable of T1 , T2 and electron density matching for a clinically relevant set of soft tissues and bone. METHODS The phantom design was based on a male pelvic anatomy template with thin boundaries separating tissue types. Slots were included to allow insertion of various dosimeters. The phantom structure was created using a 3D-printer. The tissue compartments were filled with carrageenan-based materials designed to match the T1 and T2 relaxation times and electron densities of the corresponding tissues. CT and MRI images of the phantom were acquired and used to compare phantom T1 and T2 relaxation times and electron densities 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 a T2 -weighted MRI image, synthetic CTs (sCTs) were created using a statistical decomposition algorithm (MRIPlanner, Spectronic Research AB, Sweden) and used for dose calculation during treatment planning of VMAT and 7-field IMRT prostate plans. The plans were delivered on a Truebeam STX (Varian Medical Systems, Palo Alto, CA), with film and a 0.3 cc ion chamber used to measure the delivered dose. Doses calculated on the CT and sCTs were compared using common dose volume histogram (DVH) metrics. RESULTS T1 and T2 relaxation time and electron density measurements for muscle, prostate and bone agreed well with literature-reported in vivo measurements. Film analysis resulted in a 99.7% gamma-pass-rate (3.0%, 3.0mm) for both plans. The ion chamber-measured dose discrepancies at the isocenter were 0.36% and 1.67% for the IMRT and VMAT plans respectively. The differences in PTV D98% and D95% between plans calculated on the CT and 1.5T/3.0T-derived sCT images 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.