A CONVOLUTION MODEL TO CONVERT TRANSMISSION DOSE IMAGES TO EXIT DOSE DISTRIBUTIONS

The aim of this study is to develop a model which computes exit dose v alues from transmission dose data obtained during patient treatment wi th an electronic portal imaging device (EPID). The proposed model conv olves the primary dose distribution, derived from transmission dose di stributions at large air gaps, with a scatter kernel to obtain the exi t dose. The influence of inhomogeneities on the scatter contribution i s taken into account by using a radiological path length model. To det ermine the parameters of the model, an extensive set of transmission d ose measurements was performed behind various phantoms in an 8 MV beam using a liquid-filled EPID. The influence on the transmission dose of field size, phantom thickness, air gap between phantom and detector, and source-phantom distance was investigated. At air gaps larger than 50 cm the distribution of scattered dose is almost flat and its contri bution to the total dose is relatively small, thus allowing an accurat e separation of the primary and scattered dose by subtraction. Scatter ed dose distributions for air gaps smaller than 50 cm were obtained by subtracting the primary dose (corrected for divergence) from the meas ured total transmission dose. The resulting scattered dose distributio n behind homogeneous phantoms has a Gaussian shaped profile, which bec omes wider with increasing air gap. The relative contribution of scatt ered dose depends on the phantom thickness and is maximal for a thickn ess of about 10 cm. Using these results, the parameters of the convolu tion model (i.e., the shape of the scatter kernel) were determined. Wi th the model the absolute exit dose is predicted with an accuracy of a bout 2% (1 s.d.) within the entire radiation field for homogeneous pha ntoms. Inhomogeneities are taken into account by calculating the radio logical path length from the measured primary dose, i.e., without usin g CT data. By using the measured radiological path length the exit dos e can be determined for inhomogeneous phantoms with an accuracy of 2.5 %. It is concluded that, using our convolution model, EPID measurement s at large air gaps can be used to estimate absolute exit doses in an 8 MV beam with an accuracy of 2.5%. (C) 1997 American Association of P hysicists in Medicine.