Comparison of platinum and gold fiducial markers for prostate radiotherapy: a phantom study

Gold fiducial markers (GMs) are widely used in prostate radiotherapy; however, their magnetic susceptibility can reduce visibility on T2-weighted (T2w) MRI and may compromise CT-MRI registration. Using coil-shaped platinum fiducial markers (PMs) and GMs (5 mm in length; diameters 0.35-0.75 mm), we acquired 3 T T2w fast spin-echo MRI in an agar phantom and CT/cone-beam CT (CBCT) in a water-equivalent phantom under clinical settings. MRI visibility was quantified using the signal void area (SVA) and region-of-interest [ROI] value contrast (RVC), and CT/CBCT artifacts were quantified using an artifact index (AI). Additionally, to reflect clinical use, we evaluated CT-referenced registration estimation errors after automatic CT-MRI registration using a phantom containing a registration target. Registration errors were comparable at 0.35 mm, but were significantly smaller for PMs at 0.50 mm and 0.75 mm (p < 0.01). PMs may enable accurate T2w-based CT-MRI registration without increasing CT/CBCT artifacts.