Dynamic X-ray Computed Tomography (CT) is an attractive imaging technology for the guidance in minimal invasive surgery. In this field, projection data simulation is an important tool to optimise scanner geometry and to validate reconstruction algorithms. A realistic simulation software, called "Sindbad" has been developed to compute 2D projections. It is based on an analytical model and allows simulating X-ray emission, attenuation through an examined object and photon detection. New functionality has been added to simulate a virtual scanner and to combine 2D projections from CAD phantoms with CT data volumes. Phantoms can be animated with independent motion and temporal evolution laws. CT data can be deformed over time by using a Free Form Deformation (FFD) technique. Encouraging results have been obtained for the simulation of a lung biopsy. To simulate breathing, CT lung data are animated by using a respiratory law. Biopsy needle was inserted along a straight line from an entry point to a target point at a regular speed. The guidance direction also varied with time according to the respiration law. Similar simulations are also used to validate dynamic reconstruction algorithm for radiotherapy planning.