WEDGE FACTOR CONSTITUENTS OF HIGH-ENERGY PHOTON BEAMS - HEAD AND PHANTOM SCATTER DOSE COMPONENTS

The head and phantom scatter contribution to the output of a treatment machine have been determined for open and wedged Co-60 gamma-ray beam s and 4, 8, 16 and 25 MV X-ray beams, using an extended and a small-si zed phantom. The wedge factor variation with field size and phantom de pth have been analysed as a function of both scatter components. For t he wedged beams a stronger increase of the head scatter contribution w ith field size, i.e. 4-9% for field sizes increasing from 5 cm x 5 cm to 20 cm x 20 cm, has been observed compared with open beams. This res ult indicates that the wedge factor variation with field size is relat ed to a change of the primary photon fluence. Our study shows that the ratio of the head and phantom scatter contribution for the wedged and open beams remains unchanged for all beams except the 4 and 25 MV X-r ay beam. This implies that, except for these latter energies, the vari ation of the wedge factor with phantom depth is determined by the wedg e-induced change of the primary photon energy fluence. For the 4 and 2 5 MV X-ray beam it is shown that the wedge factor is also influenced b y a change of the phantom scatter contribution. The wedge factor for t he 25 MV X-ray beam is strongly influenced by the electron contaminati on for phantom depths up to 6 cm. For the Co-60 and the 4 MV photon be am it is shown that the wedge factor decreases slightly with increasin g source-to-skin distance due to a reduced contribution to the total d ose from photons scattered in the wedge. For clinical use, an algorith m is given to calculate the wedge factor variation with field size and phantom depth.