In this proposal a research plan is being submitted which may lead to a practical method of localizing Bragg peak in a tumor volume. The plan is to use high-energy radioactive beams, and a positron-detection device. Recently, high-energy radioactive beams have become available from the BEVALAC at the Lawrence Berkeley Laboratory. The flux of these various beams (11C, 15O, 13N, and 19Ne) is sufficient to make the necessary diagnostic studies. All of these particles decay by positron emission. The positron detection device is also available, and can locate the end of the range with an accuracy of about 1 mm in the direction of the beam only. This device will be modified to do 3-dimensional reconstruction of the volume in which the beam particles stop. Experiments will be conducted with simple targets, phantoms, and animals. Once established, the technique will be used on patients subjected to Bragg peak therapy.