Radiotherapy planning accuracy in terms of C.T. numbers and inhomogeneity correction techniques

A protocol is described for assessing the accuracy of radiotherapy plans produced using C.T. derived electron density information. Essentially there are two main sources of error, those coming from various CT related inaccuracies and those from inaccuracies in the radiotherapy dose calculation algorithms. The various subdivisions of these two sources of inaccuracy are discussed in detail. More specifically this paper deals with the effective depth method of inhomogeneity correction, but sufficient generality is applied to the discussion to enable the same analysis to be applied to other dose calculation algorithms. It would appear that with state of the art C.T. scanning that the area which introduces the greatest errors is now radiotherapy computing accuracy. Until convolution correction techniques become commercially available this will probably continue to be the case