The physicians then outline target volumes, GTVs and CTVs, as per ICRU 50 guidelines and outline critical structures such as the brain stem, spinal cord, salivary glands, optic apparatus, etc. After this has been done, the image and structure data is transferred to either of two treatment planning work stations: the NOMOS Corvus system running software version 5.0 or the ADAC Pinnacle RTP system running software version 6.2b. A planning target margin (PTV) is added to take into account organ motion and setup uncertainty. Lateral and Anterior-Posterior digitally reconstructed radiographs (DRRs) that demonstrate the location of isocenter are produced from the AcQSim system so that correct patient positioning can be confirmed prior to the commencement of treatment.