A deliverable four dimensional (4D) intensity modulated radiation therapy (IMRT) planning method is disclosed, for delivery using a linear accelerator with a dynamic multi-leaf collimator (DMLC). A 4D computed tomography (CT) scan is used for segmenting tumor anatomy on a reference phase of periodic motion of the tumor. Deformable registration of the 4D CT data is used to generate corresponding anatomical structures on other phases. Preferably, the collimator for each beam position is aligned using the gross tumor volume (GTV) centroid motion corresponding to the periodic motion of the tumor, as determined from the two dimensional (2D) projection of a given beam position. A deliverable IMRT plan is created on the 4D CT image set in which the MLC leaf positions and beam on/off status can vary as a function of respiratory phase by solving a four dimensional optimization problem. The mechanical constraints of the MLC leaves are included in the optimization.
