A method and apparatus for tracking a pathological anatomy within a patient's body is described. A data model of a skin surface of the patient's body may be acquired using light reflected from the skin surface. The data model can be matched with skin surfaces reconstructed and/or interpolated from four-dimensional (4D) diagnostic imaging data, such as 4D CT data, to determine a temporal phase of the patient's respiratory motion. The identified temporal phase may then be used in conjunction with the diagnostic imaging data to identify a location of the pathological anatomy within the patient's body.