Simulation is a critical preparation step before radiation treatment. It is performed at the simulation room equipped with a dedicated big-bore CT scanner. During the simulation, the treatment setup will be simulated by positioning the patient on the flat couch immobilized by specially designed devices. The patient will then be aligned to the reference low-energy lasers in the room and be marked on the skin with tattoos. Finally, a CT scan will be performed to acquire the anatomy involved in the treatment. This important CT scan will be used to identify the lesion(s) and surrounding normal critical organs for developing a treatment plan that will guide the treatment machine to target the lesion(s) accurately and spare critical organs as much as possible. The simulated setup will be exactly reproduced before each treatment by matching the reference lasers in the treatment room to the tattoos and comparing the 2D/3D on-board images with the simulation CT scan.