The invention comprises a method and apparatus for determining state of a positively charged particle, such as a proton, for use in imaging a tumor of a patient prior to and/or concurrent with cancer therapy. The imaging system comprises: (1) a beam transport path of the positively charged particle sequentially passing through a patient, through a first time of flight detector, and, after traversing a pathlength, at least into a second time of flight detector and (2) a beam state determination system using elapsed time between detection at the first and second time of flight detectors and the pathlength to determine a residual beam energy, which, when compared to a known incident beam energy, is used in generation of an image of the tumor. An optional beam energy degrading element increases time differences between the time of flight detectors.