Collimator (head) scatter at extended distances in linear accelerator-generated photon beams

Abstract Purpose: A calculation formalism is proposed to predict variation of head scatter as a function of field size and treatment distance. Methods and Materials: Assuming that the head scatter for the linear accelerator studied was contributed predominantly by the flattening filter, a formalism was devised to predict beam intensity as a function of distance from the target position. The method used the concept of an equivalent collimator field in which a given field at any distance can be equated to a field at the isocenter such that the extent of the flattening filter seen at the two positions is the same. Results: The equation derived from the concept of equivalent collimator field size predicated change in head scatter with distance to within 0.5% for collimator field sizes ranging from 8 × 8 to 40 × 40 cm and distances up to 300 cm from the target. Conclusions: Considering flattening filter to be the main source of head scatter, the observed deviation from inverse square law for extended treatment distances can be accounted for by an equivalent collimator field size, which sees the same extent of the flattening filter at the isocenter as the field at the given distance.