Developing and characterizing MR/CT-visible materials used in QA phantoms for MRgRT systems:

Purpose: Synthetic tissue equivalent (STE) materials currently used to simulate tumor and surrounding tissues for IROC-Houston's anthropomorphic head and thorax QA phantoms cannot be visualized using magnetic resonance (MR) imaging. The purpose of this study was to characterize dual MR/CT-visible STE materials that can be used in an end-to-end QA phantom for MR-guided radiotherapy (MRgRT) modalities. Methods: Over 80 materials' MR, CT, and dosimetric STE properties were investigated for use in MRgRT QA phantoms. The materials tested included homogeneous and heterogeneous materials to simulate soft tissue/tumor and lung tissues. Materials were scanned on a Siemens' Magnetom Espree 1.5 T using four sequences, which showed the materials visual contrast between T1- and T2-weighted images. Each material's Hounsfield number and electron density data was collected using a GE's CT Lightspeed Simulator. Dosimetric properties were examined by constructing a 10 × 10 × 20 cm³ phantom of the selected STE materials that was divided into three sections: anterior, middle, and posterior. Anterior and posterior pieces were composed of polystyrene, whereas the middle section was substituted with the selected STE materials. EBT3 film was inserted into the phantom's midline and was irradiated using an Elekta's Versa 6 MV beam with a prescription of 6 Gy at 1.5 cm and varying field size of: 10 × 10 cm², 6 × 6 cm², and 3 × 3 cm². Measured film PDD curves were compared to planning system calculations and conventional STE materials' percent depth dose (PDD) curves. Results: The majority of the tested materials showed comparable CT attenuation properties to their respective organ site; however, most of the tested materials were not visible on either T1- or T2-weighted MR images. Silicone, hydrocarbon, synthetic gelatin, and liquid PVC plastic-based materials showed good MR image contrast. In-house lung equivalent materials made with either silicone- or hydrocarbon-based materials had HUs ranging from: -978 to -117 and -667 to -593, respectively. Synthetic gelatin and PVC plastic-based materials resembled soft tissue/tumor equivalent materials and had HUs of: -175 to -170 and -29 to 32, respectively. PDD curves of the selected MR/CT-visible materials were comparable to IROC-Houston's conventional phantom STE materials. The smallest field size showed the largest disagreements, where the average discrepancies between calculated and measured PDD curves were 1.8% and 5.9% for homogeneous and heterogeneous testing materials, respectively. Conclusions: Gelatin, liquid plastic, and hydrocarbon-based materials were determined as alternative STE substitutes for MRgRT QA phantoms.