The purpose of external beam radiotherapy is to sterilize malignant tumours and at the same time to avoid complications by radiation injury to the surrounding healthy tissues. Modern radiation techniques in recent years have allowed to safely escalate the dose by approximately 10% for the treatment of prostate cancer, resulting in a disease control that is nowadays comparable to surgery or permanent seed implant brachytherapy. Two recent technical developments have dramatically increased the precision of radiation dose delivery: conformal radiotherapy and image guided radiotherapy (IGRT). Conformal radiotherapy aims to shape the dose distribution to the shape of the target. At least equally important as conformality is the accurate spatial delivery of the conformal dose distribution to the target. Conventional patient positioning by skin drawings and lasers is an imprecise way to target the prostate within the pelvis. The need for adequate patient/target setup led in recent years to the development of a variety of solutions. They bear in common that setup is no longer guided by skin marks but by some imaging modality. An ideal IGRT system would allow for daily prostate imaging without possible introduction of errors due to image-acquisition itself, do so within a reasonable time frame, without the necessity for implanted radio-opaque markers and preferentially without exposing the patient to radiation. A solution that combines all these features is inexistent so far.