WEDGE FACTOR CONSTITUENTS OF HIGH-ENERGY PHOTON BEAMS - FIELD SIZE AND DEPTH DEPENDENCE

Wedge factors have been determined as a function of field size and pha ntom depth for a Co-60 gamma-ray beam and X-ray beams in the range fro m 4 MV to 25 MV. The results show an increase of the wedge factor with field size, up to 9.1% for the 25 MV X-ray beam. The magnitude of thi s increase is a linear function of the product of that part of the irr adiated wedge volume that can be observed from the point of measuremen t, its mass energy-absorption coefficient and mass density. This relat ionship is independent of the photon beam energy, the type of wedge ma terial and the wedge angle. Differences in variation of the amount of backscatter to the monitor with field size for the open and wedged pho ton beam yielded only a minor influence, up to 0.7%. For the 4-16 MV X -ray beams the wedge factor increases linearly with phantom depth, alm ost independently of field size. For the Co-60 gamma-ray beam and the 25 MV X-ray beam the wedge factor variation is a more complicated func tion of phantom depth for a particular field size.