High-energy linear accelerators with photon radiation are widely used in radiation oncology. In parallel with the technological developments, the design of collimators used in linear accelerators is also changing. The difference in the design of the collimators is directly related to the dose at the edge and outside of the field. Collimator designs become even more important in radiotherapy techniques where adjacent orthogonal fields are used. In this study, the linear accelerators used in radiation oncology and collimator designs were investigated by examining the existing literature on the advantages of these designs. At the conclusion of this study, the differences in the design of collimators of linear accelerators and the dose distribution at the edge of the field were seen to be important and effective in clinical applications and are discussed in detail.