TISSUE STANDARD RATIOS FOR IRREGULARLY SHAPED RADIOTHERAPY FIELDS

A method of calculating dose for a 6 MV X-ray beam using tissue standard ratios (TSR) is described. TSRs may be divided into a primary component and a scatter component, and the latter can then be further divided into contributions from elemental scattering volumes of 1 cm x 1 cm cross-sectional areas at 1 m from the source. The magnitude of the various components have been determined by analysing measurements of square and rectangular fields, involving the fitting of the scatter data to an exponential function of radial distance from the scatter element. The coefficient and exponent of the function were found to be functions of depth, and this variation is described. A method of utilizing reconstructed scatter data that is compatible with the motion of a multileaf collimator system is presented. Results obtained by re-combining the data for various non-standard geometries are compared with further experimental work.