A general purpose method for computer generation of isodose contours from external X-ray sources with applications to treatment planning

Abstract

A method has been developed for the generation of isodose contours from external 60Co sources which requires as input only sources to skin distance (SSD), field size (FS), trimmer setting, apparent source width and a field size dependent attenuation coefficient M. Fields are calculated directly, at any grid size and normalization desired, allowing continuous variation of beam-defining parameters. The penumbra is successfully approximated, and central axis data are within 3% of data tabulated at different SSD and FS. The same accuracy applies in the calculation of central axis data for X-rays in the energy range 2-31 MeV; thus, quite general applicability is indicated for many types of photon beams. Running time is about 5 s per symmetric 10*10 field on moderate computers. In this paper, a brief description of the method was presented, emphasis being placed subsequently on its application to treatment planning situations involving (1) split fields, (2) wedged fields, (3) oblique entry and (4) multiple ports, by making the coefficient M a function of position for a given FS. Finally, comparisons were made with hand-calculated results for multiple port, wedged field therapy in certain head and neck cases.