On the determination of spatial distributions of the total dose in combining telecobalt therapy with brachytherapy

The use of computers improves decisively the possibilities for individual determination of dose distribution in telecobalt therapy as well as in brachytherapy. The summation of single distributions obtained by percutaneous and intracavitary irradiation to a total distribution by means of computers is interesting and important for combination treatments. This important case of application is not yet available as standard software in all irradiation planning systems. It is described how this superposition is realized by the extension of a programme established for the calculation of the spatial dose distribution in telecobalt therapy only. The course of processing of the programme was altered. Input fields are not immediately processed cumulatively in a total matrix but they are available as single-field matrices and may be provided with charging factors and combined at choice. The input of externally determined (measured) fields and accordingly the incorporation of brachytherapy dose distributions into the spatial total dose distribution is possible. Applying the extended programme the example of irradiation of vaginal carcinoma by means of the combination of biaxial pendulum irradiation, ventral stationary inguinal fields and brachytherapy demonstrates the possibilities of a stepwise optimization. The determined total distributions in combination with exact topographic data allow certain statements about the homogeneity of distribution in the target volume and about the dose to especially interesting organs (e.g. bladder, rectum, Manchester points). (author)