THE HEAD-SCATTER FACTOR FOR SMALL-FIELD SIZES

The head-scatter factors H were examined for four different linear accelerators and were found to be similar at field sizes larger than 3 X 3 cm(2). Sharply reduced values for small collimator openings were observed for all the accelerators. It is concluded that the head-scatter tot collimator-scatter) factor has two major components. Scatter of photons in various structures in the beam path, especially the flattening filter, causes a slow (about 10%) increase with increased collimator opening. Insertion of a built-in wedge may double this number. When the collimators are closed, they ultimately block photons from the periphery of the source. This may cause a considerable reduction of the primary photon fluence and typically affects fields smaller than 3 X 3 cm(2). The effect can be used to estimate the source size, with results that correlate with the design of the bending magnet.