A dosimetric investigation of scatter conditions for dual asymmetric collimators in open fields.

Abstract

An investigation of the scattering effects of dual asymmetric collimators on open fields is presented. It has been determined that the output measured in air and in a phantom is radially symmetric and that the scatter as a function of field size is independent of field center offset, for a wide variety of square and rectangular field shapes. The analysis has led to a new and simplified method to represent primary off-center ratios (POCRs), which requires a minimum set of symmetric field data, including beam profiles, collimator and phantom scatter factors, and tissue maximum ratios. The field edge correction (FEC) method proposed has been shown to accurately model the POCRs to a distance of 17.5 cm from the collimator rotation axis in a plane perpendicular to the isocenter and at depths up to 30 cm without the need for extrapolation of the POCRs or special measurements setups, which may be required for POCR modeling techniques that measure narrow beam attenuation coefficients.