The target volume in prostate cancer radiotherapy typically consists of the entire prostate gland which is localized by contours drawn on axial computed tomography (CT) radiation planning images. Compared with CT, magnetic resonance (MR) can provide better definition of the prostate gland with respect to the surrounding tissues and the use of multi-plannar reconstruction avoids the problem of partial volume averaging inherent in CT. CT has been shown to significantly overestimate the volume of the gland using the MR-defined prostate volume as the gold standard. Co-registration of MR and CT datasets, matched on fixed bony landmarks, has enabled radiation planning using a MR-defined clinical target volumes, combined with CT-based electron density information necessary for radiation treatment planning.