1. A method comprising: receiving a movement model of a patient, the movement model generated prior to the start of a treatment session and comprising a plurality of deformation fields each associated with a different point in time; receiving a reference image based on the movement model; capturing a treatment image of the patient; calculating a model adaptation field representative of a difference between the reference image and the treatment image, the model adaptation field comprising a plurality of deformation vectors each representing change in location; and adapting the movement model using the model adaptation field.