An ultrasound device (18) is arranged to acquire an ultrasound image (19) of a thoracic diaphragm. A current location (24) of a tumor is determined using the ultrasound image (19) of the thoracic diaphragm and a predetermined relationship (14) assigning tumor locations to a set of simulation phase ultrasound images of the thoracic diaphragm in different geometries, or using a predetermined relationship (114) assigning tumor locations to a set of meshes representing the thoracic diaphragm in different geometries. The tumor may, for example, be a lung tumor.
