A collimator assembly for use with radiation therapy systems, in particular stereotactic radiosurgery (“SRS”) systems, is provided. In general, the collimator assembly includes a collimator in which a plurality of concentric, conical slits is formed. Each conical slit is oriented along a different slit angle, such that radiation impinging on the top surface of the collimator is redirected along each conical slit towards a common target isocenter located at a distance away from the bottom surface of the collimator. The conical slits can be referred to as “Compton slits” because they are designed to increase the radiation output at the isocenter of the collimator by redirecting Compton scattered radiation towards the isocenter in the target region. The collimator assembly thus improves the efficiency of a radiation treatment system by utilizing Compton scattered radiation that would otherwise be lost.
