title: The use of computed tomography numbers in dose calculations for radiation therapy.
abstract: 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.
mesh_terms: D006801:Humans; D008433:Mathematics; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D014057:Tomography, X-Ray Computed / Q000379:methods*