Experimental method to obtain scattering contribution in portal dose images

A method for evaluating scattered dose contribution in portal images acquired under clinical conditions (phantom-device distance of 30 cm) is presented. This method is based on radiographic film and ionisation chamber measurements and is valid for homogenecus polystyrene phantoms and square fields of different size. The portal imaging device consisted of a radiographic film placed between slabs of polystyrene under full build-up conditions (1.5 cm for 6 MV beam and 3 cm for 18 MV and 1 cm of polystyrene backscatter material. First the primary dose image in the portal plane P(i,j) is obtained using a projection algorithm, then the scattered dose image S(i,j) is found by subtracting the primary dose image in the portal plane P(i,j) from the total dose image acqu red in the portal plane T(i,j). The ratio S(i,j)/T between the scattered dose distribution and the dose value measured on the bearn axis in the portal plane was found to be uniform within the radiation field for all the geometrical configuration of phantoms and fields studied. Under these conditions the mean value of the scatter fraction S/T evaluated within a ROI centred on the bearn axis accurately describes the scatter fraction distribution S(i,j)/T within the whole radiation field. S/T ranges from 7.4% to 31.1% in the 6 MV beam and from 8.9% to 30.8% in the 18 MV beam. Finally an analytical method to evaluate the ratio S/T has been developed from the experimental results. It comprises phantom, accelerator head and portal imaging device contributions and depends on field size and phantom thickness.