Method and apparatus for real-time tracking of a target in a human body. In one embodiment of the invention, positron emission marker may be implanted into a target, the positron emission marker having a low activity positron isotope. In one embodiment, annihilation gamma rays associated with the low activity positron isotope may be detected using a plurality of position-sensitive detectors. In another embodiment, the target may be tracked in real-time based on a position of the positron emission marker.
