Radiation therapy aims to maximize dose delivery to areas of known tumor presence while minimizing the exposure to healthy tissue. Invariably, quality control is required to ensure that the delivered dose distributions closely match the calculated dose maps. The ability to directly and quantitatively visualize the 3D dose patterns, per patient, will form the foundation of this much needed quality control but is currently not technically feasible. Here, we propose the use of the inherent strength of MRI to perform patient-specific, non-invasive 3D visualization of soft tissues to visualize radiation dose. The multi-parametric MRI-based dose-imaging maps are quantitative, are easily fused with the CT-based planning images and can serve as the basis for assuring proper dose delivery. Specifically, a patient-specific anthropomorphic phantom of the head was 3D printed, based on the initial clinical CT images. The phantom was then filled with a proprietary MR-compatible gel (RTSafe) that changes its quantitative MR properties (T1, T2, MR Spectroscopy (MRS), Chemical Exchange Saturation Transfer (CEST)) upon Gamma Knife radiation. The phantom was scanned in the MR before and after radiation, whereby quantitative 3D T1 and T2 and CEST maps were generated. The change in relaxation parameters directly reflected the delivered dose, and allowed for quick, non-invasive 3D mapping of the dose painting. We expect this patient-specific quality assurance to lead to optimized dose delivery planning that may be especially important in hypo-fractionated treatments where the tumor anatomical boundaries may change along the treatment plan and where an ongoing re-evaluation of the 3D patterns of actual up-to-date accumulated treatment will be critical to efficient tumor eradication and healthy tissue sparing.An RT-Safe (Athens, Greece) anthropomorphic phantom was scanned at 3T (Philips Medical Systems) before and after Gamma Knife irradiation. The phantom consisted of polymeric gel made of 90% water, 5% gelatin, 1-Vinyl-2-pyrrolidone (VP), and N,N'-methylene-bis-acrylamid (BIS). This dosimetric material mimics the way that human soft tissue interacts with ionizing radiation and it fills the cranium in the 3D-printed phantom. Contoured ROIs (termed “lesions”) were irradiated using Gamma Knife (Perfexion, Elekta AB, Stockholm, Sweden) to a dose of 24Gy at 50% isodose line. These lesions were characterized by measuring their T1 (using a STEAM with a set of inversion recovery sequences: TR 10s, TE/TM 19/19ms, 12 log-spaced inversion recovery times 10/103/207/322/451/601/777/990/1261/1635/2238/4000 ms, NSA=2; See figure 1 for the placements of the ROI), T2 (using STEAM with a set of echo times with TR 10s, TE 20/37/56/77/100/127/157/193/237/292/370/500 ms, NSA=2), MR Spectroscopy (water-suppressed STEAM, 10x10x10 mm³, BW 2000 Hz, TR 3.5s, TE/TM 19/15ms, NSA 32), and CEST (done at 7T (Philips Medical Systems), single slice 2D FFE, TR/TE 2.9/1.53 ms, 1800ms saturation with a train of 1.6 uT, 50-ms Hanning pulses, 79 offsets from -6 to +6 ppm, TR 18s, NSA 1).Quantitative dose evaluation requires calibration curves to link radiation dose to percentage change in some MR parameters. Here we used a multi-parametric MRI approach, including T1, T2, MRS, and CEST to characterize a polymer gel dosimetry phantom. Figure 1A,B shows a typical T1-w image of the irradiated phantom, with the characteristic bright “lesions” formed by radiation. Fig.1C,D show a quantitative measurements of the lession’s T1 and T2. Compared to the normal, non-radiated tissue, these values were 29.4% and 41.1% shorter, respectively. Assuming linear relationship between dose and change in relaxation times1, this reflects a 0.0083 R₁^-1Gy^-1 and 0.102 R₂^-1Gy^-1, which closely match published values¹. Figure 1E shows the prominent spectroscopic signature of VP and BIS, and Figure 1F shows a quantitative MTR-asymmetry plot of the exchangeable protons associated with VP. Since the rate of exchange will depend highly on the degree of polymerization (and thus on the level of radiation), it is expected that CEST may provide an additional quantitative marker for quality assurance. We have shown that a multi-parametric MR quantitative evaluation of an antriopomorphic dosimetry gel produces several markers that can be used to both visualize and quantify the dose response to Gamma Knife irradiation.