MRI-based radiotherapy treatment planning for prostate cancer– Assessment of geometric distortion and detection of fiducial markers

Background The superior soft tissue contrast of magnetic resonance imaging (MRI) compared to computed tomography (CT) has urged the integration of MRI and exclusion of CT in prostate radiotherapy treatment, referred to as an MRI-only workflow. Such a workflow can eliminate image registration errors between the CTand MR-images but incorporates multiple challenges. MR-images lack electron density information, needed for absorbed dose calculation. MR-images are therefore converted to synthetic CT (sCT). Geometric distortion may exist in MR-images and could lead to dosimetric errors. Intra-prostatic gold fiducial markers (GFM) appear hyper-intense on CT-images and identification is straightforward. On MR-images the GFM appear as signal voids and identification is difficult. The need for dedicated MRI-sequence, development of quality assurance tools and clinical studies are therefore evident before an MRI-only workflow can be integrated as a clinical routine.