We propose the construction of an X-ray computerized tomography scanner for use in conjunction with the charged particle therapy program at the Lawrence Berkeley Laboratory. Major scanner components (gantry, detector, data acquisition system and x-ray equipment) will be purchased from commercial sources and interfaced with an existing computer and CT display system. Mechanical modifications will be made to permit scanning in both the patient vertical and recumbent positions. This will give the capability of scanning the patient in the actual treatment position. Existing reconstruction programs will be adapted to process scanner data. This scanner will be used as a research tool to investigate factors affecting tumor localization and the tissue density determination of inhomogeneities. Methods to convert CT data to electron density and to use this in treatment planning have already been developed, and will be utilized extensively in this research. The ultimate goal will be to provide accurate dose delivery in charged particle radiotherapy through the quantitative assessment of inhomogeneities through their accurate compensation.