Embodiments disclose a radiotherapy apparatus comprising a source of radiation configured to emit a beam of radiation and a collimator structure configured to limit a lateral extent of the beam, the collimator structure including a primary collimator configured to shape the beam, a first collimator comprising a plurality of adjacent elongate leaves, the leaves being extendable into the beam in a first direction transverse to the beam, and a block collimator including an aperture configured to permit the beam to pass through, the block collimator being extendable into the beam in a second direction transverse to the beam and transverse to the first direction. In some embodiments, the aperture may be cone-shaped or a through-hole, which may be empty or filled with a radiotransparent material. In some embodiments, the block collimator may include a plurality of apertures, which may be of varying dimensions.