Digital computers are now widely used for calculating absorbed dose distributions in radiotherapy and a number of commercial systems such as the RAD 8 (Digital Equipment Co., and SHM Nuclear Inc.), PC12 (Artronix Inc.) and Philips TPS (Philips Medical Systems) have been developed for this purpose. The greatest single problem encountered when implementing such systems is that of preparing beam data in a computer compatible form. In many cases, isodose curves are the only data available at the time a computer system is installed and the conversion of these to tabular form is both laborious and error prone. An alternative method of data preparation is therefore desirable and the two most viable solutions are: (1) calculation of beam-data tables using a computer program based upon a mathematical or physical model, (2) direct measurement of dose distributions in tabular form. Calculation methods, for example Cunningham (1972) can yield very good results, particularly when applied to 60Co dose distributions; ho...