A high count rate position-sensitive detector was developed in order to obtain CT information using a modified radiotherapy simulator. The detector is attached to one arm of the simulator gantry and acquires projection data from a fan beam of X-rays derived from the simulator X-ray tube. The scanner is designed principally for planning the dosimetry of breast cancer radiotherapy. During the collection of transmission projection data, suitable for the digital reconstruction of patient cross sections, the simulator gantry rotates continuously through about 360 degrees. A film cassette holder can be positioned above the detector to provide conventional simulator check films. Cross sections through patients receiving post-operative radiotherapy for breast cancer were reconstructed from projections acquired with the patient lying in the treatment position. These scans are an average over the breathing cycle. The outlines of internal anatomy and the patient external contour were used to produce treatment plans which include tissue inhomogeneity corrections. The data produced by this wide aperture CT scanner are suitable for planning those conditions in which radiotherapy is delivered to an organ such as the breast or the whole body, rather than to a localised lesion.