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 T , T and electron density matching for a clinically relevant set of soft tissues and bone.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 T and T relaxation times and electron densities of the corresponding tissues. CT and MRI images of the phantom were acquired and used to compare phantom T and T 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 T -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.T and T 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%.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.