1. An apparatus comprising: a scanner that acquires projection data indicative of an object that includes a treatment target, wherein the projection data is list mode positron annihilation data and the target is subject to motion during a treatment session; a motion modeler that uses projection data acquired during the treatment session to model the motion of the target; a treatment device that treats the target during the treatment session, wherein the treatment device varies a spatial location of an applied treatment as a function of the model of the motion.