A new description of the photon beam peak-depth profile as a function of field size

The dose profile at peak depth in water is described as the product of an apparatus function and a source function. In principle, the source function is the circularly symmetric profile which would be measured at peak depth without any collimation. In practice, the peak-depth profile in the diagonal plane, measured for the largest collimator setting, is used for this purpose. The apparatus function represents the collimator acting upon the source function, and is referred to as the collimator function. The collimator function for any field size can be developed from the ratio of the peak-depth profile for a single medium-sized field and the source function. The method has been tested for a set of irregularly flattened 4-MV x-ray beams as well as for practically flat 15-MV x-ray beams. The model requires as basic data only three peak-depth profiles: one in each principal plane of a medium-sized square field and the peak-depth profile in the diagonal plane for the largest field. It replaces the peak-depth transformation in the projective beam model.