CT is commonly used for anatomy delineation, based on early studies. Although the image quality has improved gradually, unfortunately, CT still severely overestimates the prostate volume. Furthermore, Teh et al found in 712 prostatectomy patients large mean differences between CT-based estimates of the GTV and PTV and Pathological Prostate Volume (PPV). Rasch et al found an average ratio between CT and MRI volumes of 1.4. Also the MRI technique improved and now produces a much better imaging quality, e.g. by increasing Tesla, by using the present phased array coils and by using thinner slice thickness. The accuracy of detecting extracapsular extension in prostate carcinoma with endorectal and phased-array coil MR imaging reaches 77% for experienced readers. Concluding, MRI is superior to CT in GTV contouring, but MRI alone (using combined T1 SE and T2 TSE) may not be sufficient for visualisation of the GTV. It is expected that biological imaging will improve GTV delineation further. Within the GTV a Biological Target Volume (BTV) will become visible, which may further improve the efficacy of cancer radiotherapy.