The use of computed tomography numbers in dose calculations for radiation therapy.

Although corrections for 'beam hardening' and 'scattering' have been implemented in currently available CT scanners, systematic differences exist between a real CT image and an ideal, artefact-free and monochromatic image. The appearance and magnitude of these differences are discussed. Conversion to the ideal image, i.e. conversion from CT number to X-ray attenuation coefficient at diagnostic photon energies, turns out to be possible with an accuracy of 5 per cent. In order to use the CT 'density' information from patients, in clinical photon and electron beam dose calculations, conversions must be made from the X-ray attenuation coefficient at diagnostic energies to relevant high energy radiation interaction properties. These conversions turn out to be possible within an accuracy also of 5 per cent. These limited accuracies cause errors in the photon beam dose calculation of less than 1 per cent of the dose maximum and errors in electron beam dose calculations of less than 2 per cent of the dose maximum.