Imaging positron emitting radionuclides generated during radiation therapy. 
In vivo generated positron emitting radioisotopes, primarily C-11 and N-13, have been documented following therapy with accelerators larger than 10 MeV. Six patients had positron emission tomography 15 to 25 minutes after radiation therapy with a 42 MeV accelerator.  Five patients had recurrent colorectal malignancy, and one required therapy for a carcinoma of the common bile duct.  We sought to determine whether state-of-the-art PET technology could be used to monitor the three-dimensional activity distribution of radiation-induced radioactivity.  At the time of the examination all six patients had sufficient concentrations of C-11 and N-13 activity in the irradiated volume to permit the evaluation of the activity distribution.  We found significant activity at the body surface, which permitted field delineation.  We conclude that the in vivo generated radioactivity can be monitored with PET.