ConclusionIn summary, the surface guided Radiotherapy (SGRT) imaging has a significant correlation with CBCT imaging in detecting setup errors in the treatment for skull base and Head and neck cancer patients.The differences of Σ and δ between the two IGRT methods were less than 1mm and 0.5 o .Hence SGRT is feasible option to position the patient to reduce the setup errors as much and to reduce the frequency of CBCT to avoid unnecessary imaging radiation to patients OC-051To determine an imaging regime for Linac based VMAT SRS.Are we going overboard with image guidance?