A 4-MV CT scanner for radiation therapy: The prototype system

Abstract

A Varian 4-MV isocentric therapy accelerator was modified to perform also as a CT [computed tomography] scanner. The goal is to provide a computed tomography capability for use in radiotherapy at low cost. Three principal uses are envisioned for the system. These are to provide 2- and 3-dimensional maps of electron density distribution for CT-assisted therapy planning; to aid in patient set up by providing sectional views of the treatment volume and high contrast scout-mode verification images; and to provide a means for periodically checking the patients'' anatomical conformation against that which was used to generate the original therapy plan. The machine was modified by mounting an array of detectors on a frame that is bolted to the counterweight end of the gantry in such a manner as to define a 3rd generation CT scanner geometry. Details of the detectors and data acquisition electronics are provided. We present results of phantom studies which demonstrate that this prototype system has a spatial resolution of 4 mm and the ability to discriminate electron density differences of < 1%. The system is exactly linear over a wide range of electron densities corresponding to those found in body tissues.