CALCULATION MODELS FOR DETERMINING THE ABSORBED DOSE IN WATER PHANTOMS IN OFF-AXIS PLANES OF RECTANGULAR FIELDS OF OPEN AND WEDGED PHOTON BEAMS

Beam models are proposed for the calculation of the dose in off-axis p lanes of rectangular photon fields, when the data set used in the trea tment planning system is based on the simple storage model of Milan an d Bentley. For open beams the model separates the off-axis ratio into an envelope profile and two boundary profiles. The envelope profile gi ves the field intensity of the maximal position of the jaws and has ro tational symmetry. The boundary profiles describe the boundaries of th e field actually formed by the jaws. In the case of a wedged beam, the model also separates the off-axis ratio into envelope profiles and bo undary profiles. To determine these profiles for the non-wedge directi on from open beam profiles, the wedge thickness is converted to an equ ivalent water thickness. In the case of an asymmetric field, the bound ary profiles are shifted to the field centre. Results of calculation w ith these models have been compared with measurements and the simple m ultiplication of profiles, which has often been used with the Milan-Be ntley model. The new models agree within a few per cent with the measu rements and are a great improvement compared to the simple multiplicat ion of profiles.