Non-standard CT scanners: their role in radiotherapy

In the past 10 years a number of groups worldwide have investigated the extent to which the imaging requirements for radiotherapy planning may be met by the development of non-standard computed tomography (CT) scanners. Some of these groups have constructed apparatus based on a radiotherapy simulator gantry, others around a linac or a special purpose gantry. The performance of these systems is reviewed and the extent to which they have justified their promise has been assessed. The major use of simulator-based machines has been in planning radio-therapy in the thorax where tissue inhomogeneity corrections are important. Linac-based machines yield images useful for checking the positioning of patients at the time of therapy. Interest at the Royal Marsden Hospital culminated in the construction of a simulator-based CT system which achieved its present form in late 1985. The importance of the subsequent clinical study lies in our conclusive evidence that planning conservative radiotherapy for the breast without multiple-level CT performed in the treatment position can have implications for local control of disease, pulmonary damage, and cosmesis.