COLLIMATOR (HEAD) SCATTER AT EXTENDED DISTANCES IN LINEAR ACCELERATOR-GENERATED PHOTON BEAMS

Purpose: A calculation formalism is proposed to predict variation of h ead scatter as a function of field size and treatment distance. Method s and Materials: Assuming that the tread scatter for the linear accele rator studied was contributed predominantly by the Battening filter, a formalism was devised to predict beam intensity as a function of dist ance from the target position, The method used the concept of an equiv alent collimator field in which a given field at any distance can be e quated to a field at the isocenter such that the extent of the Batteni ng filter seen at the two positions is the same. Results: The equation derived from the concept of equivalent collimator field size predicat ed change in head scatter with distance to within 0.5% for collimator field sizes ranging from 8 x 8 to 40 x 40 cm and distances up to 300 c m from the target. Conclusions: Considering flattening filter to be th e main source of head scatter, the observed deviation from inverse squ are law for extended treatment distances can be accounted for by an eq uivalent collimator field size, which sees the same extent of the flat tening filter at the isocenter as the field at the given distance.