1. A method comprising, comprising: segmenting a prostate in a 3D ultrasound image of a patient; fusing a reference patient-specific biomechanical model of the prostate extracted from a 3D planning image of the patient to a boundary of the segmented prostate in the 3D ultrasound image, resulting in a fused 3D biomechanical prostate model; in response to movement of an ultrasound probe to a new location, receiving a current 2D ultrasound image acquired by the ultrasound probe at the new location; and deforming the fused 3D biomechanical prostate model based on the current 2D ultrasound image to match a current deformation of the prostate due to the movement of the ultrasound probe to the new location.