Patient motion during radiotherapy or intra-fractional motion, is one of the major sources of uncertainty in dose application. 2D/3D registration is an intensity based method used successfully to track tumor motion with the potential to reduce uncertainty. Despite promising results, validation remains a problem due to the lack of a gold standard. In this paper, we demonstrate the use of clinical datasets acquired with the VERO LINAC to validate intensity based 2D/3D registration. The patient considered in this study had a gold marker implanted to enable live tumor tracking. We verify our results against the tracked gold marker position. Our results show that it is possible to accurately track the tumor using only intensity based 2D/3D registration. The mean error of our registration is of 1.6 ± 0.3mm and as low as 1.1 ± 0.5mm when looking only at the cranial-caudal direction. The results pave the way for a possible clinical application of the method.