Collimator-produced energy degradation of megavoltage x rays

Monte Carlo computer calculations have been made to assess the effects of small collimators (i.e., output field sizes less than or equal to 1 cm) on megavoltage x-ray beams. Such collimators are often used in experiments to measure beam energy, half value layers, and other beam parameters. Our calculations demonstrate, however, that such procedures can introduce significant changes in these same parameters. Under certain conditions, more than 40% of the transmitted photons exiting the collimator do so with degraded energy. Optimum collimator design, however, can greatly reduce these effects. In particular, it is shown that the thickness of material required to effect a good collimator is not only dependent upon the photon energy, but also upon the size of the collimator hole.